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Article

Seven New Phenylhexanoids with Antioxidant Activity from Saxifraga umbellulata var. pectinata

1
West China School of Pharmacy, Sichuan University, Chengdu 610041, China
2
School of Ethnic Medicine, Chengdu University of Traditional Chinese Medicine, Chengdu 611137, China
*
Authors to whom correspondence should be addressed.
Molecules 2023, 28(9), 3928; https://doi.org/10.3390/molecules28093928
Submission received: 15 March 2023 / Revised: 21 April 2023 / Accepted: 29 April 2023 / Published: 6 May 2023
(This article belongs to the Special Issue Advances in Natural Products and Their Biological Activities)

Abstract

:
Seven new phenylhexanoids, (S)-(+)-3,4-dihydroxy-11-methoxyphenylhex-9-one (1), (E) 3,4-dihydroxy-phenylhex-10-en-9-one (2), (E)-4-hydroxyphenylhex-10-en-9-one (3), (R)-(−)-3,4,11-trihydroxyphenylhex-9-one 11-O-β-d-glucopyranoside (4), (R)-(−)-4,11-dihydroxyphenylhex-9-one 11-O-β-d-glucopyranoside (5), phenylhex-4,9,11-triol 11-O-β-d-glucopyranoside (6), and 9-O-acetyl-phenylhex-4,9,11-triol 11-O-β-d-glucopyranoside (7), were isolated and identified from Tibetan medicine Saxifraga umbellulata var. pectinate. The antioxidant activities of these compounds were evaluated using the DPPH and ABTS radical scavenging experiments. In the ABTS experiment, compounds 1 (IC50 13.99 ± 2.53 μM) and 2 (IC50 13.11 ± 0.94 μM) exhibited significantly better antioxidant activity than L-ascorbic acid (IC50 23.51 ± 0.44 μM).

1. Introduction

Saxifraga umbellulata var. pectinata, which belongs to the family Saxifragaceae, is a perennial herb mainly distributed on plateaus above 3000 m above sea level [1]. It is one of the major varieties of the traditional Tibetan medicinal herb called ‘Songdi’ [2]. Tibetan clinical medicine analyzes ‘Songdi’ in terms of the four qi and the five tastes. It is believed that ‘Songdi’ is cold, and the taste is bitter, and it is mostly used for the treatment of hepatobiliary (Tripa disease) and digestive diseases (Tripa enteropathy) [3]. ‘Songdi’ has a long history of medicinal use in Tibetan medicine, and is mostly used as the ruling medicine or in combination with other Tibetan medicines in Tibetan medical prescriptions. For example, the classical Tibetan prescription for hepatitis and cholecystitis, Ershiwuwei Songshi pills, uses ‘Songdi’ as one of the prescribed drugs [3].
Model pharmaceutical research has demonstrated the antibacterial activity of ethanol extract, as well as the inhibitory effects on the proliferation of liver cancer (HepG2) cells of diarylnonanes, and the protective effect on peroxidative damage in L02 hepatocytes of flavonoids from S. umbellulata var. pectinate [4,5,6]. Phytochemistry studies have revealed the presence of diarylnonanes, flavonoids, triterpenoids, polyphenols, organic acids, and sterols in the plant of S. umbellulata var. Pectinate [6,7,8,9,10,11].
To investigate potential active ingredients for the treatment of hepatitis, cholecystitis, and digestive system diseases, additional studies of the phytochemistry and biology of S. umbellulata var. pectinate were conducted.
As a result, seven novel phenylhexanoids (Figure 1) were isolated and identified from S. umbellulata var. pectinata. This paper reports on the isolation, structural identification, and antioxidant effect of these compounds.

2. Results and Discussion

2.1. Identification of Compounds 17

1 was obtained as a yellowish oil, and its molecular formula was analyzed as C13H18O4 by HR ESI MS (m/z 237.1132 [M-H], calculated 237.1127 for C13H17O4). This indicates five degrees of unsaturation.
The 1H NMR spectrum of 1 showed the following signals: a 1,2,4-trisubstituted benzene ring at δH 6.67 (1H, d, J = 8.0 Hz), 6.63 (1H, d, J = 2.1 Hz), and 6.50 (1H, dd, J = 8.0, 2.1 Hz), a methine group linking with oxygen at δH 3.78 (1H, dqd, J =7.5, 6.2, 5.2 Hz), three methylene groups at δH 2.72 (4H, m), 2.67 (1H, d, J = 15.9, 7.5 Hz), and 2.43 (1H, d, J = 15.9, 5.2 Hz), and a methyl group at δH 1.13 (3H, d, J = 6.2 Hz). In the 13C NMR spectrum of 1, the following signals were shown: six benzene carbon signals at δC 116.3, 116.5, 120.6, 134.0, 144.5, and 146.2, six aliphatic carbon signals at δC 19.4, 30.0, 46.3, 50.6, 74.6, and 211.3, and a methoxy carbon signal at δC 56.8.
The above 1H and 13C NMR data (Table 1) of 1 indicate that there was a phenylhexyl skeleton in 1, the same as in inonophenol A [12]. The molecular formula of 1 was C13H18O4 (m/z 238), having one more -CH2- group (m/z 14) than inonophenol A (C12H16O4, m/z 224). Comparing the 1H and 13C NMR data of 1 with the data for inonophenol A, most of the data are similar, except for the C10, C11, C12, and -OCH3 data of 1.
In 1, 1H and 13C NMR signals of an -OCH3H 3.27 (3H, s); δC 56.8)] were observed. However, in inonophenol A, there was no 1H and 13C NMR signal of -OCH3.
The C11 signal (δC 74.6) in 1 was down-shifted by 9.5 chemical shift units compared to that in inonophenol A. Meanwhile, the C10 and C12 signals (δC 50.6 and 19.4) in 1 were up-shifted by 2.1 and 3.8 chemical shift units, respectively, compared to those in inonophenol A. These pieces of evidence prove that 1 resulted from the substitution of the hydroxyl group in inonophenol A with a methoxy group. The long-range correlations (Figure 2) of δH 3.27 (3H, s, -OCH3) with δC 74.6 (C11) and δH 3.78 (1H, dqd, J =7.5, 6.2, 5.2 Hz, 11-H) with δC 56.8 (-OCH3) in the HMBC spectrum of 1 further support this inference.
Based on the above MS, 1H NMR, 13C NMR, and HMBC spectra, it was confirmed that 1 is 3,4-dihydroxy-11-methoxyphenylhex-9-one.
The absolute configuration of C11 in 1 was determined to be in the S-configuration based on the similarity of the rotation value of 1 ( [ α ] D 25 + 6.29) to that of inonophenol A ( [ α ] D 20 + 5.42) [12]. Therefore, 1 was identified as (S)-(+)-3,4-dihydroxy-11-methoxy-phenylhex-9-one.
2 was obtained as a yellow oil. Its molecular formula was determined to be C12H14O3 by HR ESI MS (m/z 205.0878 [M-H], calculated 205.0865 for C12H13O3), indicating six degrees of unsaturation.
The 1H NMR spectrum of 2 showed the following signals: a 1, 2, 4-trisubstituted benzene ring at δH 6.68 (1H, d, J = 8.0 Hz), 6.65 (1H, br.s), and 6.53 (1H, br.d, J = 8.0 Hz), a double bond with trans geometry at δH 6.94 (1H, dq, J = 15.7, 6.8 Hz) and 6.16 (1H, d, J = 15.7 Hz), two methylene groups at δH 2.85 (2H, t, J = 7.5 Hz) and 2.76 (2H, t, J = 7.5 Hz), and a methyl group at δH 1.91 (3H, d, J = 7.0 Hz). In the 13C NMR spectrum of 2, the following signals were observed: six benzene carbon signals at δC 116.3, 116.5, 120.6, 134.1, 144.5, and 146.2, and six aliphatic carbon signals at δC 18.4, 30.8, 42.6, 132.8, 145.2, and 202.6.
The above 1H and 13C NMR data (Table 1) of 2 indicated that there was also a phenylhexyl skeleton in 2, the same as in inonophenol A [12]. The molecular formula of 2 was C12H14O3 (m/z 206), which has one less -H2O group (m/z 18) than inonophenol A (C12H16O4, m/z 224). Comparing the 1H and 13C NMR data of 2 with the data for inonophenol A, most of the data are similar, except for the C8, C9, C10, C11, and C12 data of 2.
In 2, 1H and 13C NMR signals of a double bond with trans geometry [δH 6.94 (1H, dq, J = 15.7, 6.8 Hz) and 6.16 (1H, d, J = 15.7 Hz), δC 145.2 (C11) and 132.8 (C10)] were observed. However, in inonophenol A, there was no 1H and 13C NMR signal of a double bond. Instead, the NMR signals of a methylene group [δH 2.48 (1H, dd, J = 16.0, 4.9 Hz) and 2.56 (1H, dd, J = 1.60, 7.8 Hz), δC 52.7 (C10)] and a methine group linking with oxygen [δH 4.17 (1H, m), δC 65.1 (C11)] were observed in inonophenol A. In addition, meanwhile, the C8C 42.6), C9C 202.6), and C12C 18.4) signals in 2 were up-shifted by 3.7, 9.3, and 5.1 chemical shift units, respectively, compared to those in inonophenol A.
The HMBC spectrum of 2 showed the long-range correlations (Figure 2) of δH 6.16 (1H, d, J = 15.7 Hz, 10-H) with δC 18.4 (C12) and 42.6 (C8), and δH 6.94 (1H, d, J = 15.7 Hz, 6.8 Hz, 11-H) with δC 202.6 (C9). The above MS, 1H, 13C NMR, and HMBC spectra, 2 was a dehydration product of inonophenol A and identified as (E)3,4-dihydroxy-phenylhex-10-en-9-one.
3 was obtained as a yellow oil, and its molecular formula was analyzed as C12H14O3 by HR ESI MS (m/z 189.0933 [M-H], calculated 189.0916 for C12H13O3), indicating six degrees of unsaturation.
The 1H NMR spectrum of 3 showed the following signals: a p-substituted benzene ring at δH 6.68 and 7.00 (2H each, d, J = 8.5 Hz); a double bond with trans geometry at δH 6.90 (1H, dq, J = 15.8, 6.8 Hz) and 6.11 (1H, d, J = 15.8, 1.6 Hz), two methylene groups at δH 2.80 (4H, m), and a methyl group at δH 1.87 (3H, dd, J = 6.8, 1.6Hz). In the 13C NMR spectrum of 3, the following signals were shown: six benzene ring carbon signals at δC 116.1, 116.1, 130.3, 130.3, 133.2, and 156.6, and six aliphatic carbon signals at δC 18.4, 30.4, 42.6, 132.8, 145.2 and 202.5.
The above 1H and 13C NMR data (Table 1) of 3 indicated that there was also a phenylhexyl skeleton in 3, the same as in 2. The molecular formula of 3 was C12H14O3 (m/z 190), which was one less -O- atom (m/z 16) than 2 (C12H14O3, m/z 206). Comparing the 1H and 13C NMR data of 3 with the data for 2, most of the data are similar, except for the benzene data of 3. The 1H and 13C NMR signals of a p-substituted phenyl [δH 6.68, 7.00 (2H each, d, J = 8.5 Hz); δC 116.1–156.6 (C1–C6)] were observed in 3. However, in 2, there was no 1H and 13C NMR signal of p-substituted phenyl, instead, the NMR data of a 1, 2, 4-trisubstituted phenyl [δH 6.68 (1H, d, J = 8.0 Hz), 6.65 (1H, br.s), and 6.53 (1H, br.d, J = 8.0 Hz); δC 116.3–146.2 (C1–C6)] were observed. This evidence proved that 3 was a dehydroxyl product of 2. The long-range correlations (Figure 2) of δH 7.00 (2H, d, J = 8.5 Hz, 2-H/6-H) with δC 30.4 (C7) and 156.6 (C4), and δH 6.68 (2H, d, J = 8.5 Hz, 3-H/5-H) with δC 133.2 (C1) in the HMBC spectrum of 3 also proved the above inference.
Based on the above MS, 1H, 13C NMR, and HMBC spectra, it was confirmed that 3 was (E)-4-hydroxy-phenylhex-10-en-9-one.
4 was obtained as a white amorphous powder, and its molecular formula was analyzed as C18H26O9 by HR ESI MS (m/z 385.1515 [M-H], calculated 385.1499 for C18H25O9), indicating six degrees of unsaturation.
The 1H NMR spectrum of 4 showed the following signals: a 1, 2, 4-trisubstituted benzene ring at δH 6.67 (1H, d, J = 8.0 Hz), 6.64 (1H, d, J = 2.1 Hz), and 6.52 (1H, dd, J = 8.0, 2.1 Hz); a methine group linking with oxygen at δH 4.34 (1H, m); three methylene groups at δH 2.80 (2H, m), 2.72 (2H, m), 2.83 (1H, m) and 2.53 (1H, dd, J = 15.9, 5.4 Hz); and a methyl group at δH 1.20 (3H, d, J = 6.2 Hz). In the 13C NMR spectrum of 4, the following signals were shown: six benzene ring carbon signals at δC 116.3, 116.5, 120.5, 134.1, 144.4, and 146.1, and six aliphatic carbon signals at δC 20.3, 30.0, 46.2, 51.6, 72.4, and 211.9.
The above 1H and 13C NMR data (Table 2) of 4 indicate that there was a phenylhexyl skeleton in 4, the same as in inonophenol A [12]. The molecular formula of 4 was C18H26O9 (m/z 386), which was one more -C6H10O5- group (m/z 162) than inonophenol A (C12H16O4, m/z 224). Comparing the 1H and 13C NMR data of 4 with data for inonophenol A, most of the data are similar, except for the C10, C11, C12, and -C6H10O5- data of 4.
In 4, the 1H and 13C NMR signals of a monosaccharide moiety [δH 4.34 (1H, d, J = 7.2 Hz) and 3.12–3.84; δC 102.4 and 62.9–78.0 (C1′-C6′)] were observed. However, in inonophenol A, there were no 1H and 13C NMR signals of this.
The acid hydrolysis experiment on 4 afforded D-glucose, confirmed by TLC and a comparison of its NMR data with those of (5S)-1,7-bis-(3,4-dihydroxy-phenyl)-5-hydroxyheptan-3-one-5-O-β-d-glucopyranoside [13], and the relative configuration of the anomeric carbon to be β-configuration due to its large coupling constant. Based on the above evidence, the monosaccharide was determined to be β-d-glucopyranose.
The C11 signal (δC 72.4) in 4 was down-shifted by 7.3 chemical shift units compared to that in inonophenol A. Meanwhile, the C10 and C12 signals (δC 51.6 and 20.3) in 4 were up-shifted by 1.1 and 3.2 chemical shift units, respectively, compared to those in inonophenol A. This evidence proved that 4 was a glycation product of inonophenol A by a β-d-glucopyranose moiety. The long-range correlations (Figure 2) of δH 4.34 (1H, d, J = 7.2 Hz, 1′-H) with δC 72.4 (C11), and δH 4.34 (1H, m, 11-H) with δC 102.4 (C1′) in the HMBC spectrum of 4 further support this inference. Based on the above MS, 1H, 13C NMR, and HMBC spectra, it was confirmed that 4 is 3,4,11-trihydroxyphenylhex-9-one 11-O-β-d-glucopyranoside.
The absolute configuration of C11 in 4 was determined to be R-configuration based on the contrast of the rotation value of the hydrolyzed aglycone ( [ α ] D 25 − 4.56) of 4 with that of inonophenol A ( [ α ] D 20 + 5.42) [12]. Therefore, 4 was identified as (R)-(−)-3,4,11-trihydroxyphenylhex-9-one 11-O-β-d-glucopyranoside.
5 was obtained as a white amorphous powder, and its molecular formula was analyzed as C18H26O8 by HR ESI MS (m/z 369.1527 [M-H], calculated 369.1549 for C18H25O8), indicating six degrees of unsaturation.
The 1H NMR spectrum of 5 showed the following signals: a p-substituted benzene ring at δH 6.70 and 7.02 (2H each, d, J = 8.5 Hz); a methine group linking with oxygen at δH 4.34 (1H, m); three methylene groups at δH 2.80 (2H, m), 2.77 (2H, m), 2.84 (1H, dd, J = 15.9, 7.3 Hz) and 2.53 (1H, dd, J = 15.9, 5.4 Hz); a methyl group at δH 1.20 (3H, d, J = 6.2 Hz); an anomeric proton at δH 4.34 (1H, d, J = 7.7 Hz); and a typical sugar moiety proton at δH 3.12–3.84. In the 13C NMR spectrum of 5, the following signals were shown: six benzene ring carbon signals at δC 116.1, 116.1, 130.3, 130.3, 133.3, and 156.5, six aliphatic carbon signals at δC 20.3, 29.7, 49.2, 51.5, 72.3, and 211.9, and typical sugar moiety carbon signals at δC 62.8, 71.7, 75.0, 77.8, 78.0 and 102.3.
The above 1H and 13C NMR data (Table 2) of 5 indicated that there was also a phenylhexyl glycoside skeleton in 5, the same as in 4. The molecular formula of 5 was C18H26O8 (m/z 370), which was one less -O- atom (m/z 16) than 4 (C18H26O9, m/z 386). Comparing the 1H and 13C NMR data of 5 with the data for 4, most of the data are similar, except for the benzene data of 5. The 1H and 13C NMR signals of a p-substituted phenyl [δH 6.70, 7.02 (2H each, d, J = 8.0 Hz); δC 116.1–156.5 (C1–C6)] were observed in 5. However, in 4, there was no 1H and 13C NMR signal of p-substituted phenyl, instead, the NMR data of a 1, 2, 4-trisubstituted phenyl [δH 6.67 (1H, d, J = 8.0 Hz), 6.64 (1H, d, J = 2.1 Hz), and 6.52 (1H, dd, J = 8.0, 2.1 Hz); δC 116.3–146.1 (C1–C6)] were observed. This evidence proved that 5 was a dehydroxyl product of 4.
The long-range correlations (Figure 2) of δH 7.02 (2H, d, J = 8.5 Hz, 2-H/6-H) with δC 29.7 (C7) and 156.3 (C4), and δH 6.70 (2H, d, J = 8.5 Hz, 3-H/5-H) with δC 133.2 (C1) in the HMBC spectrum of 5 also proved the above inference.
Based on the above MS, 1H, 13C NMR, and HMBC spectra, 5 was identified as being 4,11-dihydroxyphenylhex-9-one 11-O-β-d-glucopyranoside.
The absolute configuration of C11 in 5 was determined to be R-configuration based on the similarity of the rotation value of 5 ( [ α ] D 25 − 19.6) to that of 4 ( [ α ] D 20 − 22.3). Therefore, 5 was identified as (R)-(−)-4,11-dihydroxy-phenylhex-9-one 11-O-β-d-glucopyranoside.
6 was afforded as a white amorphous powder, and its molecular formula was analyzed as C18H28O8 by HR ESI MS (m/z 371.1705 [M-H], calculated 371.1706 for C18H27O8), indicating five degrees of unsaturation.
The 1H NMR spectrum of 6 showed the following signals: a p-substituted benzene ring at δH 6.70 and 7.03 (2H each, d, J = 8.5 Hz); two methine groups linking with oxygen at δH 3.74 (1H, tt, J = 8.5, 4.3 Hz) and 4.10 (1H, m); three methylene groups at δH 2.62 (2H, m), 1.71 (2H, m), 1.84 (1H, m) and 1.58 (1H, m); a methyl group at δH 1.21 (3H, d, J = 6.1 Hz); an anomeric proton at δH 4.36 (1H, d, J = 7.8 Hz); and a typical sugar moiety proton at δH 3.15–3.87. In the 13C NMR spectrum of 6, the following signals were shown: six benzene ring carbon signals at δC 116.1, 116.1, 130.3, 130.3, 133.3, and 156.5, six aliphatic carbon signals at δC 20.1, 32.0, 40.8, 45.5, 70.1, and 74.3, and typical sugar moiety carbon signals at δC 62.9, 71.7, 75.1, 77.9, 78.0 and 102.3.
The above 1H and 13C NMR data (Table 2) of 6 indicated that there was also a phenylhexyl glycoside skeleton in 6, the same as in 5. The molecular formula of 6 was C18H28O8 (m/z 372), which was two -H- atoms (m/z 2) more than 5 (C18H26O8, m/z 370). Comparing the 1H and 13C NMR data for 6 with data for 5, most of the data are similar, except for the C8, C9, and C10 data of 6.
In 5, the 13C NMR signal [δC 211.9] of a C=O was observed. However, in 6, there was no 13C NMR signal of a C=O group. Instead, the 1H and 13C NMR signals [δH 3.74 (1H, tt, J = 8.5, 4.3 Hz); δC 70.1 (C9)] of one more methine group linking with oxygen were observed; meanwhile, the 13C signal of C9C 70.1) in 6 was up-shifted by 141.8 and the 13C signals of C8 and C10C 40.8 and 45.5) were down-shifted by 5.4 and 6.0 chemical shift units, respectively, compared to those in 5. This evidence proves that 6 should be the product of the reduction of the carbonyl group in 5. The long-range correlations (Figure 2) of δH 2.62 (2H, m, 7-H) and 4.10 (H, m, 11-H) with δC 70.1 (C9) in the HMBC spectrum of 6 further support this inference.
Based on the above MS, 1H, 13C NMR, and HMBC spectra, it was confirmed that 6 was phenylhex-4,9,11-triol 11-O-β-d-glucopyranoside. Due to technical limitations, the absolute configuration of 6 could not be determined.
7 was afforded as a white amorphous powder, and its molecular formula was analyzed as C20H30O9 by HR-ESI-MS (m/z 413.1804 [M-H], calculated 413.1812 for C20H29O9), indicating six degrees of unsaturation.
The 1H NMR spectrum of 1 showed the following signals: a p-substituted benzene ring at δH 7.01 and 6.69 (2H each, d, J = 8.0 Hz); two methine groups linking with oxygen at δH 5.08 (1H, m) and 3.97 (1H, dq, J = 7.8, 6.0 Hz); three methylene groups at δH 2.55 (2H, m), 1.97, 1.68 (1H each, m), 1.95, 1.84 (1H each, m); two methyl groups at δH 2.01 (3H, s) and 1.19 (3H, d, J = 6.0 Hz); an anomeric proton at δH 4.32 (1H, d, J = 7.7 Hz); and a typical sugar moiety proton at δH 3.14–3.79. In the 13C NMR spectrum of 7, the following signals were shown: six benzene ring carbon signals at δC 116.1, 116.1, 130.3, 130.3, 133.8, and 156.4, six aliphatic carbon signals at δC 20.0, 31.6, 37.0, 42.5, 72.7 and 73.3, and typical sugar moiety carbon signals at δC 62.9, 71.7, 75.1, 77.8, 78.0 and 101.8.
The 1H and 13C NMR data (Table 2) of 7 indicated that there was a phenylhexyl glycoside skeleton in 7, the same as in 6. The molecular formula of 7 was C20H30O9 (m/z 414), which was one more -COCH2- group (m/z 42) than 6 (C18H27O8, m/z 372). Comparing the 1H and 13C NMR data of 7 with data for 6, most of the data are similar, except for the C8, C9, C10, and -COCH3 data of 7.
In 7, the 1H and 13C NMR signals of a -COCH3H 2.01 (3H, s); δC 173.0 and 21.3] were observed. However, in 6, there was no 1H and 13C NMR signal of -COCH3.
The C8, C9, and C10 signals (δC 37.0, 72.7, and 42.5) in 7 were up-shifted 1.6, 3.8, and 3.0 chemical shift units, respectively, compared to those in 6. These pieces of evidence prove that 7 should be the substitution product of the C9-OH in 6 by C9-OCOCH3. The long-range correlations (Figure 2) of δH 2.01 (3H, s, 2″-H) with δC 173.0 (C1″) and δH 5.08 (1H, m, 9-H) with δC 31.6 (C7), 72.7 (C11), and 173.0 (C1″) in the HMBC spectrum of 7 further support this inference.
Based on the above MS, 1H, 13C NMR, and HMBC spectra, it was confirmed that 7 is 9-O-acetylphenylhex-4,9,11-triol 11-O-β-d-glucopyranoside. Due to technical limitations, the absolute configuration of 7 could not be determined.

2.2. The Antioxidant Activities of Compounds 17

Compounds 17 isolated from the title plant were tested for their antioxidant effects. The results of the antioxidant activity assays are listed in Table 3.
The 2,2′-azino-bis (3-ethylbenzthiazoline-6-sulphonic acid) ammonium salt (ABTS) radical scavenging effects of compounds 1 (IC50 13.99 ± 2.53 μM) and 2 (IC50 13.11 ± 0.94 μM) were more potent than the positive control, L-(+)-ascorbic acid (IC50 23.51 ± 0.44 μM) (p < 0.05), while the ABTS radical scavenging effects of compounds 4 (IC50 28.44 ± 3.86 μM) and 7 (IC50 27.03 ± 0.55 μM) were equivalent to L-(+)-ascorbic acid (p > 0.05).
The results of the DPPH and ABTS assays showed that the catechol groups of compounds are very important for enhancing activity. During the normal metabolic process of living organisms, more chemically active oxygen-containing substances, also known as reactive oxygen species (ROS), are produced [14]. Low levels of ROS are essential for a variety of biological functions, such as cell survival, growth, proliferation, differentiation and immune response [15]. When the generation of reactive oxygen radicals is higher than the antioxidant capacity, oxidative stress (OS) occurs. Excess ROS can cause damage to proteins, DNA and RNA, leading to genetic alterations in cells and promoting the development of disease or cell death [16]. Numerous studies have shown that cardiovascular diseases, inflammation, malignant tumors, diabetes, and atherosclerosis are all related to oxidative damage in the body, which is caused by excess free radicals or ROS generated during metabolic processes [17]. Therefore, drugs with the ability to scavenge reactive oxygen radicals have an important role in the pathogenesis of inflammation-related diseases due to oxidative stress caused by excess oxygen free radicals. 17 might be among the active constituents of S. umbellulata var. pectinata that play a role in the treatment of liver inflammation-associated diseases.

3. Materials and Methods

3.1. General Experimental Procedure

NMR spectra were obtained using an AC-E200 400 NMR spectrometer (Bruker Corporation, German) (1H at 400 MHz, 13C at 100 MHz) with CD3OD as the solvent at 25 °C, using TMS as the internal standard. The UV spectrum was obtained using a UV3600 spectrophotometer (Shanghai Pharmaceutical Machinery Co., Ltd., Shanghai, China). The IR absorption spectrum was recorded with a Nicolet 6700 spectrophotometer (Thermo Electron Co., Waltham, MA, USA). High-resolution electrospray ionization mass spectroscopy (HR ESI MS) was performed on a Waters Xevo G2-XS Q-TOF Premier mass spectrometer (Waters, Milford, MA, USA). The optical rotation value was tested at room temperature with a JASCO P-1020 polarimeter (Jasco Co., Tokyo, Japan). The microplate reader used in the antioxidant activity experiment was a SparkTM 10 M (Tecan Co., Männedorf, Switzerland).
Column chromatography (CC) was performed using silica gel (100–200 and 300–400 mesh; Qingdao Marine Chemical Factory, Qingdao, China), polyamide (60–90 mesh, Jiangsu Changfeng Chemical Industry Co., Yangzhou, China), RP-C18 silica gel (20–45 μm; Mitsubishi Chemical Co., Tokyo, Japan), and Sephadex LH-20 (40–70 μm; Amersham Pharmacia Biotech, Stockholm, Sweden). TLC was carried out using HPTLC Fertigplatten Kieselgel 60 F254 plates (Merck, Darmstadt, Germany), which were sprayed with the α-naphthol–sulfuric acid solution or 10% sulfuric acid–ethanolic solution and then baked for 3–5 min at a temperature of 105 °C. UV-vis absorbance was measured with a UV2700 spectrophotometer (Shimadzu, Kyoto, Japan). 2,2-Diphenyl-1-picrylhydrazyl (DPPH) was acquired from Macklin Biochemical Co., Ltd. (Shanghai, China). 2,2′-azinobis (3-ethylbenz thiazoline-6-sulphonic acid) ammonium salt (ABTS) was obtained from Aladdin Industrial Co., Ltd. (Shanghai, China).

3.2. Plant Material

The whole plant of S. umbellulata var. pectinate was collected from Tibet, China, in July 2020, and confirmed by Prof. Yi Zhang (School of Ethnic Medicine, Chengdu University of Traditional Chinese Medicine, Chengdu 611137, China). The specimen (No. BCHEC 20200912) was deposited in the School of Ethnic Medicine, Chengdu University of Traditional Chinese Medicine, Chengdu 611137, China.

3.3. Extraction and Isolation

The dried crude powder of the title plant (10 kg) was extracted with 95% ethanol (100 L) at room temperature three times (every 7 days). The ethanol extract was filtered and condensed in vacuo to yield ethanol extract (1.6 kg). The ethanol extract (1.6 kg) was mixed with silica gel (100–200 mesh) at a ratio of 1:1, then put into a continuous extractor and extracted with petroleum ether, dichloromethane, ethyl acetate, and methanol, respectively, and petroleum ether extract (245.6 g), dichloromethane extract (169.5 g), ethyl acetate extract (183.0 g), and methanol extract (1011.0 g) were obtained by depressurization and concentration.
The methanol extract (400 g) was separated on a silica gel column (CH2Cl2-MeOH 20:1–0:1) to obtain 8 fractions (Frs.1–8), according to the TLC analysis. Fr.2 (7.6 g) was separated on a silica gel column (CH2Cl2-MeOH, 120:1–65:1) to obtain 6 fractions (Frs.2–1 to 2–6). Fr.2–1 (218 mg) was purified using an RP-18 reverse-phase chromatography column (MeOH-H2O, 0:1–2:1) and a Sephadex LH-20 gel chromatography column (CH2Cl2-MeOH 1:1) to yield compound 3 (24 mg). Fr.2–4 (305 mg) was purified using an RP-18 reverse-phase chromatography column (MeOH-H2O, 1:4–3:1) and a Sephadex LH-20 gel chromatography column (CH2Cl2-MeOH 1:1) to yield compounds 1 (10 mg) and 2 (6 mg). Fr.4 (30 g) was separated using a polyamide chromatography column (EtOH-H2O, 0:10–4:1) to obtain 5 fractions (Frs.4–1 to 4–5). Fr.4–1 (8.0 g) was separated using a silica gel chromatography column (CH2Cl2-MeOH, 60:1–1:1) to obtain 5 fractions (Frs. 4–1-1 to 1–5). Fr.4–1-2 (110 mg) was purified using an RP-18 reverse-phase chromatography column (MeOH-H2O, 0:1–1:1) and a Sephadex LH-20 gel chromatography column (CH2Cl2-MeOH 1:1) to obtain compounds 5 (34 mg) and 7 (6 mg). Fr.4–1-4 (700 mg) was purified using an RP-18 reverse-phase chromatography column (MeOH-H2O, 0:1–1:2) and Sephadex LH-20 gel chromatography column (CH2Cl2-MeOH 1:1) to obtain compounds 4 (18 mg) and 6 (18 mg).
Compound 1: yellowish oil. [ α ] D 25 + 6.29 (c 0.03, MeOH). UV (MeOH) λmax (log ε): 284 (3.57) nm, IR (KBr) υmax: 3380, 2937, 1706, 1605, 1519, 1445 cm−1; 1H NMR (CD3OD, 400 MHz) and 13C NMR (CD3OD, 100 MHz) data, see Table 1; HR ESI MS m/z 237.1132 [M-H] (calculated for C13H17O4, 237.1127).
Compound 2: yellowish oil. UV (MeOH) λmax (log ε): 282 (3.50) nm, IR (KBr) υmax: 3381, 2927, 1658, 1628, 1520, 1443 cm−1; 1H NMR (CD3OD, 400 MHz) and 13C NMR (CD3OD, 100 MHz) data, see Table 1; HR ESI MS m/z 205.0878 [M-H] (calculated for C12H13O3, 205.0865).
Compound 3: yellowish oil. UV (MeOH) λmax (log ε): 282 (3.55) nm, IR (KBr) υmax: 3332, 1658, 1615, 1516, 1442 cm−1; 1H NMR (CD3OD, 400 MHz) and 13C NMR (CD3OD, 100 MHz) data, see Table 1; HR ESI MS m/z 189.0933 [M-H] (calculated for C12H13O2, 189.0916).
Compound 4: white amorphous powder. [ α ] D 25 −22.3 (c 0.06, MeOH). UV(MeOH) λmax (log ε): 284 (3.42) nm, IR (KBr) υmax: 3369, 2925, 1702, 1520, 1447 cm−1; 1H NMR (CD3OD, 400 MHz) and 13C NMR (CD3OD, 100 MHz) data, see Table 2; HR ESI MS m/z 385.1515 [M-H] (calculated for C18H25O9, 385.1499).
Compound 5: white amorphous powder. [ α ] D 25 −19.6 (c 0.11, MeOH). UV (MeOH) λmax (log ε): 280 (3.37) nm, IR (KBr) υmax: 3365, 2916, 1704, 1614, 1516, 1449 cm−1; 1H NMR (CD3OD, 400 MHz) and 13C NMR (CD3OD, 100 MHz) data, see Table 2; HR ESI MS m/z 369.1527 [M-H] (calculated for C18H25O8, 369.1549).
Compound 6: white amorphous powder. [ α ] D 25 −35.0 (c 0.02, MeOH). UV (MeOH) λmax (log ε): 280 (3.35) nm, IR (KBr) υmax: 3381, 2927, 1613, 1516, 1452 cm−1; 1H NMR (CD3OD, 400 MHz) and 13C NMR (CD3OD, 100 MHz) data, see Table 2; HR ESI MS m/z 371.1705 [M-H] (calculated for C18H27O8, 371.1706).
Compound 7: white amorphous powder. [ α ] D 25 −24.0 (c 0.06, MeOH). UV (MeOH) λmax (log ε): 280 (3.45), IR (KBr) υmax: 3363, 2920, 1711, 1516, 1451 cm−1; 1H NMR (CD3OD, 400 MHz) and 13C NMR (CD3OD, 100 MHz) data, see Table 2; HR ESI MS m/z 413.1804 [M-H] (calculated for C20H29O9, 413.1812).

3.4. Acid Hydrolysis of Compound 4

Dissolved 4 (4 mg) with 0.1 mL CH3OH was added to 4 mL of H2SO4 aqueous solution (1 mol/L) and kept at 90 °C for 3 h. Adjusted the reaction solution to pH neutral with sodium hydroxide solution (1 mol/L), and then ethyl acetate eluate was added to extract the solution 3 times. An ethyl acetate phase and an aqueous phase were obtained. The aqueous phase permeated and condensed, and monosaccharides in the concentrated solution were confirmed by TLC (CHCl3-CH3OH-H2O = 3:2:0.1) and D-glucose (standard sample) [18]. The Rf value of D-glucose was 0.6.

3.5. Determination of Antioxidant Activity

DPPH and ABTS radical scavenging experiments were performed to measure the antioxidant activity of compounds 17 [19,20].

3.5.1. DPPH Radical Scavenging Assay

A 100 μL volume of DPPH anhydrous ethanol solution (120 μM) was added to 100 μL anhydrous ethanol sample solution (12.5, 25, 50, 100, 200, and 400 μM) in a 96-well plate. The mixture was allowed to react at room temperature for 30 min in the dark, and then the absorbance of the mixture at a wavelength of 517 nm was measured with a microplate reader. Three parallel experiments were conducted. DPPH radical scavenging activity was calculated using the following formula: DPPH scavenging activity was calculated by the following formula: DPPH scavenging activity (%) = (AcontrolAsample)/Acontrol × 100%, where Acontrol is the absorbance of the anhydrous ethanol control without samples, and Asample is the absorbance of sample. L-ascorbic acid was used as a positive control in the experiment.

3.5.2. ABTS Radical Scavenging Assay

A 100 μL volume of ABTS anhydrous ethanol solution (140 μM) was added to 100 μL anhydrous ethanol sample solution (12.5, 25, 50, 100, 200, and 400 μM) in a 96-well plate. The mixture was reacted at room temperature for 5 min in the dark, and then the absorbance of the mixture at a wavelength of 734 nm was measured with a microplate reader. Three parallel experiments were performed. The ABTS radical scavenging activity was calculated by the following formula: ABTS scavenging activity (%) = (AcontrolAsample)/Acontrol × 100%, where Acontrol is the absorbance of anhydrous ethanol control without samples, and Asample is the absorbance of the sample. L-ascorbic acid was used as a positive control in the experiment.

3.6. Statistical Analyses

The statistical analyses were performed using GraphPad Prism 8.0. Every sample was analyzed in triplicate. The IC50 value of a compound (where half of DPPH and ABTS free radicals are cleared) was obtained by plotting the scavenging percentage of every sample of the compound against its concentration. The results are expressed as the mean ± standard deviation (SD). The difference in the means between compound and positive control was analyzed by one-way analysis of variance (ANOVA) using SPSS 25.0, to judge whether there was a statistically significant difference between the groups (p < 0.05).

4. Conclusions

To date, only three similar components (inositol A, inositol B, and hispolon) similar to the skeleton of the compounds reported in this paper have been reported to have been isolated from microorganisms [12,21]. However, the original literature defines them as being of the phenyl-substituted hexane type.
This paper reports these compounds from plants for the first time, and based on the skeleton naming rule of natural products, the skeleton type was denoted as phenylhexanoid. This name is more in line with its biosynthetic pathway.
Although compounds 17 showed some antioxidant activities, further research is needed to see if they have an effect when used for the treatment of hepatitis, cholecystitis, and digestive diseases.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28093928/s1, Figures S1–S7: 1H NMR, 13C NMR, 1H-1H COSY, HSQC, HMBC, HRESIMS, and IR spectra of compounds 1 (Figure S1), 2 (Figure S2), 3 (Figure S3), 4 (Figure S4), 5 (Figure S5), 6 (Figure S6), and 7 (Figure S7).

Author Contributions

Conceptualization, J.H. (Jiao Huang), Y.Y., M.L., Y.Z. and J.H. (Jing Huang); methodology, J.H. (Jing Huang) and D.C.; formal analysis, J.H. (Jiao Huang) and Y.Y.; investigation, J.H. (Jiao Huang) and M.L.; data curation, J.H. (Jiao Huang); writing—original draft preparation, J.H. (Jiao Huang); writing—review and editing, J.H. (Jing Huang); supervision, J.H. (Jing Huang); funding acquisition, Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the National Natural Science Foundation of China (no. 81973573).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available in the Supplementary Materials.

Acknowledgments

The authors are obliged to Fu Su, West China School of Pharmacy, Sichuan University, for measuring the NMR spectra and Zhao-Qing Pei, College of Ethnic Medicine, Chengdu University of Traditional Chinese Medicine, for measuring the HRESIMS.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Sample Availability

Samples of the compounds are not available from the authors.

Abbreviations

1H-1H COSY1H-1H homonuclear chemical shift correlation spectroscopy
ABTS2,2′-azino-bis (3-ethylbenzthiazoline-6-sulphonic acid) ammonium salt
ANOVAone-way analysis of variance
CD3ODmethanol-d4
DPPH2,2-diphenyl-1-picrylhydrazyl
EtOHethanol
HSQCheteronuclear single quantum coherence spectroscopy
HMBCheteronuclear multiple bond coherence spectroscopy
HR-ESI-MShigh-resolution electrospray ionization mass spectroscopy
IC50half inhibitory concentration
IRinfrared absorption spectrum
MeOHmethanol
NMRnuclear magnetic resonance
IRinfrared absorption spectrum
SDstandard deviation
TLCthin-layer chromatography

References

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Figure 1. Structures of compounds 17.
Figure 1. Structures of compounds 17.
Molecules 28 03928 g001
Figure 2. The key 1H-1H COSY (bold) and HMBC (H→C) correlations of 17.
Figure 2. The key 1H-1H COSY (bold) and HMBC (H→C) correlations of 17.
Molecules 28 03928 g002
Table 1. 1H NMR (400 MHz) and 13C NMR (100 MHz) data of 13 in CD3OD.
Table 1. 1H NMR (400 MHz) and 13C NMR (100 MHz) data of 13 in CD3OD.
Position123
δCδH (J in Hz)δCδH (J in Hz)δCδH (J in Hz)
1134.0 134.1 133.2
2116.36.63 d (2.1)116.36.65 br.s130.37.00 d (8.5)
3146.2 146.2 116.16.68 d (8.5)
4144.5 144.5 156.6
5116.56.67 d (8.0)116.56.68 d (8.0)116.16.68 d (8.5)
6120.66.50 dd (8.0, 2.1)120.66.53 br.d (8.0)130.37.00 d (8.5)
730.02.72 t (4.0)30.82.76 t (7.5)30.42.79 m
846.32.72 t (4.0)42.62.85 t (7.5)42.62.79 m
9211.3 202.6 202.5
1050.62.67 dd (15.9, 7.5)
2.43 dd (15.9, 5.2)
132.86.16 d (15.7)132.86.11 dq (15.8, 1.6)
1174.63.78 dqd (7.5, 6.2, 5.2)145.26.94 dq (14.1, 6.8)145.26.89 dq (15.8, 6.8)
1219.41.13 d (6.2)18.41.19 d (7.0)18.41.86 dd (6.8, 1.7)
-OCO356.83.27 s
Table 2. 1H NMR (400 MHz) and 13C NMR (100 MHz) data of 47 in CD3OD.
Table 2. 1H NMR (400 MHz) and 13C NMR (100 MHz) data of 47 in CD3OD.
Position4567
δCδH (J in Hz)δCδH (J in Hz)δCδH (J in Hz)δCδH (J in Hz)
1134.1 133.3 134.5 133.8
2116.36.64 d (2.0)130.37.02 d (8.0)130.37.03 d (8.5)130.37.01 d (8.0)
3146.1 116.16.70 d (8.0)116.76.70 d (8.5)116.16.69 d (8.0)
4144.4 156.5 156.3 156.4
5116.56.67 d (8.0)116.16.70 d (8.0)116.76.70 d (8.5)116.16.69 d (8.0)
6120.56.52 dd (8.0, 2.0)130.37.02 d (8.0)130.37.03 d (8.5)130.37.01 d (8.0)
730.02.72 m29.72.77 m32.02.62 m31.62.55 m
846.22.80 m46.22.80 m40.81.71 m37.01.95 m
1.84 m
9211.9 211.9 70.13.74 tt (8.5, 4.3)73.35.08 m
1051.62.53 dd (15.9, 5.4)
2.83 dd (15.9, 7.4)
51.52.53 dd (15.9, 5.4)
2.84 dd (15.9, 7.3)
45.51.84 m,
1.58 m
42.51.97 m
1.68 m
1172.44.34 m72.34.34 m74.34.10 m72.73.97 dq (7.8, 6.0)
1220.31.20 d (6.2)20.31.20 d (6.2 )20.11.21 d (6.0)20.01.19 d (6.0)
Glc-1′102.44.34 d (7.8)102.34.34 d (7.7)102.34.36 d (7.8)101.84.32 d (7.7)
Glc-2′75.03.12 m75.03.12 dd (9.2, 7.8)75.13.15 dd (9.1, 7.8)75.13.14 dd (8.8, 7.7)
Glc-3′77.83.25 m77.83.25 m77.93.28 m77.83.25 m
Glc-4′71.73.26 m71.73.26 m71.73.28 m71.73.30 m
Glc-5′78.03.35 m78.03.36 m78.03.37 m78.03.34 m
Glc-6′62.93.65 dd (11.9, 5.3)
3.84 dd (11.9, 1.9)
62.93.64 dd (11.9, 5.3)
3.84 dd (11.9, 1.9)
62.93.67 dd (11.8, 5.5)
3.87 dd (11.8, 1.7)
62.93.68 dd (11.8, 5.4)
3.79 dd (11.8, 2.3)
C=O 173.0
CH3 21.32.01 s
Table 3. Results of the antioxidant activity assays of compounds 17 from the title plant (mean ± SD, n = 3).
Table 3. Results of the antioxidant activity assays of compounds 17 from the title plant (mean ± SD, n = 3).
CompoundIC50 (μM)
DPPHABTS
148.66 ± 0.9413.99 ± 2.53 b
253.85 ± 1.1713.11 ± 0.94 b
3>10028.85 ± 0.18
443.95 ± 1.9128.44 ± 3.86 c
5>10033.04 ± 1.43
6>10038.10 ± 3.94
7>10027.03 ± 0.55 c
L-ascorbic acid a30.41 ± 1.4023.51 ± 0.44
a Positive control. b The DPPH and ABTS free radicals scavenging abilities of the compound are stronger than the positive control (p < 0.05). c The DPPH and ABTS free radical scavenging abilities of the compounds are equivalent to the positive control (p > 0.05).
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MDPI and ACS Style

Huang, J.; Chen, D.; Liu, M.; Yu, Y.; Zhang, Y.; Huang, J. Seven New Phenylhexanoids with Antioxidant Activity from Saxifraga umbellulata var. pectinata. Molecules 2023, 28, 3928. https://doi.org/10.3390/molecules28093928

AMA Style

Huang J, Chen D, Liu M, Yu Y, Zhang Y, Huang J. Seven New Phenylhexanoids with Antioxidant Activity from Saxifraga umbellulata var. pectinata. Molecules. 2023; 28(9):3928. https://doi.org/10.3390/molecules28093928

Chicago/Turabian Style

Huang, Jiao, Donglin Chen, Mengying Liu, Yarui Yu, Yi Zhang, and Jing Huang. 2023. "Seven New Phenylhexanoids with Antioxidant Activity from Saxifraga umbellulata var. pectinata" Molecules 28, no. 9: 3928. https://doi.org/10.3390/molecules28093928

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