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27 pages, 6030 KiB

Open AccessArticle

Electronic Structures of Radical-Pair-Forming Cofactors in a Heliobacterial Reaction Center

by Yunmi Kim, A. Alia, Patrick Kurle-Tucholski, Christian Wiebeler and Jörg Matysik

Molecules 2024, 29(5), 1021; https://doi.org/10.3390/molecules29051021 - 27 Feb 2024

Cited by 2 | Viewed by 1625

Photosynthetic reaction centers (RCs) are membrane proteins converting photonic excitations into electric gradients. The heliobacterial RCs (HbRCs) are assumed to be the precursors of all known RCs, making them a compelling subject for investigating structural and functional relationships. A comprehensive picture of the electronic structure of the HbRCs is still missing. In this work, the combination of selective isotope labelling of ¹³C and ¹⁵N nuclei and the utilization of photo-CIDNP MAS NMR (photochemically induced dynamic nuclear polarization magic-angle spinning nuclear magnetic resonance) allows for highly enhanced signals from the radical-pair-forming cofactors. The remarkable magnetic-field dependence of the solid-state photo-CIDNP effect allows for observation of positive signals of the electron donor cofactor at 4.7 T, which is interpreted in terms of a dominant contribution of the differential relaxation (DR) mechanism. Conversely, at 9.4 T, the emissive signals mainly originate from the electron acceptor, due to the strong activation of the three-spin mixing (TSM) mechanism. Consequently, we have utilized two-dimensional homonuclear photo-CIDNP MAS NMR at both 4.7 T and 9.4 T. These findings from experimental investigations are corroborated by calculations based on density functional theory (DFT). This allows us to present a comprehensive investigation of the electronic structure of the cofactors involved in electron transfer (ET). Full article

(This article belongs to the Special Issue NMR in Biochemical Research: From Small Molecules to Macromolecular Complexes)

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15 pages, 2144 KiB

Open AccessArticle

Shining a Spotlight on Methyl Groups: Photochemically Induced Dynamic Nuclear Polarization Spectroscopy of 5-Deazariboflavin and Its Nor Analogs

by Sabrina Panter, Audrey Ayekoi, Jannis Tesche, Jing Chen, Boris Illarionov, Adelbert Bacher, Markus Fischer and Stefan Weber

Int. J. Mol. Sci. 2024, 25(2), 848; https://doi.org/10.3390/ijms25020848 - 10 Jan 2024

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Abstract

5-Deazaflavins are analogs of naturally occurring flavin cofactors. They serve as substitutes for natural flavin cofactors to investigate and modify the reaction pathways of flavoproteins. Demethylated 5-deazaflavins are potential candidates for artificial cofactors, allowing us to fine-tune the reaction kinetics and absorption characteristics of flavoproteins. In this contribution, demethylated 5-deazariboflavin radicals are investigated (1) to assess the influence of the methyl groups on the electronic structure of the 5-deazaflavin radical and (2) to explore their photophysical properties with regard to their potential as artificial cofactors. We determined the proton hyperfine structure of demethylated 5-deazariboflavins using photochemically induced dynamic nuclear polarization (photo-CIDNP) spectroscopy, as well as density functional theory (DFT). To provide context, we compare our findings to a study of flavin mononucleotide (FMN) derivatives. We found a significant influence of the methylation pattern on the absorption properties, as well as on the proton hyperfine coupling ratios of the xylene moiety, which appears to be solvent-dependent. This effect is enhanced by the replacement of N5 by C5-H in 5-deazaflavin derivatives compared to their respective flavin counterparts. Full article

(This article belongs to the Special Issue Recent Advances in Free Radicals, Radical Ions and Radical Pairs)

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19 pages, 4263 KiB

Open AccessReview

Spin Dynamics of Flavoproteins

by Jörg Matysik, Luca Gerhards, Tobias Theiss, Lisa Timmermann, Patrick Kurle-Tucholski, Guzel Musabirova, Ruonan Qin, Frank Ortmann, Ilia A. Solov’yov and Tanja Gulder

Int. J. Mol. Sci. 2023, 24(9), 8218; https://doi.org/10.3390/ijms24098218 - 4 May 2023

Cited by 10 | Viewed by 3846

Abstract

This short review reports the surprising phenomenon of nuclear hyperpolarization occurring in chemical reactions, which is called CIDNP (chemically induced dynamic nuclear polarization) or photo-CIDNP if the chemical reaction is light-driven. The phenomenon occurs in both liquid and solid-state, and electron transfer systems, often carrying flavins as electron acceptors, are involved. Here, we explain the physical and chemical properties of flavins, their occurrence in spin-correlated radical pairs (SCRP) and the possible involvement of flavin-carrying SCRPs in animal magneto-reception at earth’s magnetic field. Full article

(This article belongs to the Special Issue Recent Advances in Free Radicals, Radical Ions and Radical Pairs)

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13 pages, 7139 KiB

Open AccessArticle

New Aspects of the Antioxidant Activity of Glycyrrhizin Revealed by the CIDNP Technique

by Aleksandra A. Ageeva, Alexander I. Kruppa, Ilya M. Magin, Simon V. Babenko, Tatyana V. Leshina and Nikolay E. Polyakov

Antioxidants 2022, 11(8), 1591; https://doi.org/10.3390/antiox11081591 - 17 Aug 2022

Cited by 21 | Viewed by 2679

Abstract

Electron transfer plays a crucial role in ROS generation in living systems. Molecular oxygen acts as the terminal electron acceptor in the respiratory chains of aerobic organisms. Two main mechanisms of antioxidant defense by exogenous antioxidants are usually considered. The first is the inhibition of ROS generation, and the second is the trapping of free radicals. In the present study, we have elucidated both these mechanisms of antioxidant activity of glycyrrhizin (GL), the main active component of licorice root, using the chemically induced dynamic nuclear polarization (CIDNP) technique. First, it was shown that GL is capable of capturing a solvated electron, thereby preventing its capture by molecular oxygen. Second, we studied the effect of glycyrrhizin on the behavior of free radicals generated by UV irradiation of xenobiotic, NSAID—naproxen in solution. The structure of the glycyrrhizin paramagnetic intermediates formed after the capture of a solvated electron was established from a photo-CIDNP study of the model system—the dianion of 5-sulfosalicylic acid and DFT calculations. Full article

(This article belongs to the Topic Antioxidant Activity of Natural Products)

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13 pages, 1169 KiB

Open AccessArticle

Insights Into the Micelle-Induced β-Hairpin-to-α-Helix Transition of a LytA-Derived Peptide by Photo-CIDNP Spectroscopy

by M. Victoria Gomez, Margarita Ruiz-Castañeda, Philipp Nitschke, Ruth M. Gschwind and M. Angeles Jiménez

Int. J. Mol. Sci. 2021, 22(13), 6666; https://doi.org/10.3390/ijms22136666 - 22 Jun 2021

Cited by 2 | Viewed by 2414

Abstract

A choline-binding module from pneumococcal LytA autolysin, LytA_239–252, was reported to have a highly stable nativelike β-hairpin in aqueous solution, which turns into a stable amphipathic α-helix in the presence of micelles. Here, we aim to obtain insights into this DPC-micelle triggered β-hairpin-to-α-helix conformational transition using photo-CIDNP NMR experiments. Our results illustrate the dependency between photo-CIDNP phenomena and the light intensity in the sample volume, showing that the use of smaller-diameter (2.5 mm) NMR tubes instead of the conventional 5 mm ones enables more efficient illumination for our laser-diode light setup. Photo-CIDNP experiments reveal different solvent accessibility for the two tyrosine residues, Y249 and Y250, the latter being less accessible to the solvent. The cross-polarization effects of these two tyrosine residues of LytA_239–252allow for deeper insights and evidence their different behavior, showing that the Y250 aromatic side chain is involved in a stronger interaction with DPC micelles than Y249 is. These results can be interpreted in terms of the DPC micelle disrupting the aromatic stacking between W241 and Y250 present in the nativelike β-hairpin, hence initiating conversion towards the α-helix structure. Our photo-CIDNP methodology represents a powerful tool for observing residue-level information in switch peptides that is difficult to obtain by other spectroscopic techniques. Full article

(This article belongs to the Special Issue Molecular and Ionic Dynamics by Means of Nuclear Magnetic Resonance)

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15 pages, 3892 KiB

Open AccessArticle

Stereoselectivity of Electron and Energy Transfer in the Quenching of (S/R)-Ketoprofen-(S)-Tryptophan Dyad Excited State

by Aleksandra A. Ageeva, Simon V. Babenko, Ilya M. Magin, Victor F. Plyusnin, Polina S. Kuznetsova, Alexander A. Stepanov, Sergey F. Vasilevsky, Nikolay E. Polyakov, Alexander B. Doktorov and Tatyana V. Leshina

Int. J. Mol. Sci. 2020, 21(15), 5370; https://doi.org/10.3390/ijms21155370 - 28 Jul 2020

Cited by 9 | Viewed by 2668

Abstract

Photoinduced elementary processes in chiral linked systems, consisting of drugs and tryptophan (Trp) residues, attract considerable attention due to several aspects. First of all, these are models that allow one to trace the full and partial charge transfer underlying the binding of drugs to enzymes and receptors. On the other hand, Trp fluorescence is widely used to establish the structure and conformational mobility of proteins due to its high sensitivity to the microenvironment. Therefore, the study of mechanisms of Trp fluorescence quenching in various systems has both fundamental and practical interest. An analysis of the photo-chemically induced dynamic nuclear polarization (CIDNP) and Trp fluorescence quenching in (R/S)-ketoprofen-(S)-tryptophan ((S/R)-KP-(S)-Trp) dyad carried out in this work allowed us to trace the intramolecular reversible electron transfer (ET) and obtain evidence in favor of the resonance energy transfer (RET). The fraction of dyad’s singlet excited state, quenched via ET, was shown to be 7.5 times greater for the (S,S)-diastereomer than for the (R,S) analog. At the same time, the ratio of the fluorescence quantum yields shows that quenching effectiveness of (S,S)-diastereomer to be 5.4 times lower than for the (R,S) analog. It means that the main mechanism of Trp fluorescence quenching in (S/R)-KP-(S)-Trp dyad is RET. Full article

(This article belongs to the Special Issue Kinetic Manifestation of Bimolecular Multistage Physicochemical Processes in Solutions)

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