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Deinoxanthin (DX) is a unique carotenoid synthesized by

_{T1}) and homolytic O-H bond dissociation enthalpy (BDE), were estimated. The theoretical results further our understanding of the higher ROS-scavenging activities of DX compared to BC and ZX.

The structures of DX, BC and ZX were fully optimized by hybrid density functional theory (DFT) [_{T1}s) of DX, BC and ZX were calculated by time-dependent DFT (TD-DFT) formalism [_{r} + _{h} – _{p} [_{r} is the enthalpy of radical generated through H-abstraction reaction, _{h} is the enthalpy of H-atom, −0.49765 hartree, and _{p} is the enthalpy of parent molecule.

All calculations were performed using Gaussian 03 package of programs [

Carotenoids are efficient singlet oxygen (^{1}O_{2}) quenchers. Owing to the rather low E_{T1} of carotenoids, ^{1}O_{2} can be quenched through energy transfer (^{3}O_{2}).

The ^{1}O_{2} quenching capability is a good indicator of the E_{T1}. _{T1} of DX, BC and ZX. The theoretically predicted E_{T1}s of BC and ZX are close to the experimental value [

The E_{T1}s of the three carotenoids are lower than the deactivation energy of ^{1}O_{2} (0.97 eV), which indicates that they are ^{1}O_{2} quenchers. Moreover, the E_{T1} of DX is approximately 0.1 eV lower relative to those of BC and ZX. The lower E_{T1} of DX will make the energy transfer process more favorable energetically, relative to the other two carotenoids, which is consistent with the experimental finding that DX possesses stronger ^{1}O_{2} quenching ability than BC and ZX [^{1}O_{2} quenching enhanced as the number of conjugated double bonds in the polyene chain of carotenoids increased, by examining various naturally occurring carotenoids [^{1}O_{2} quenching ability of DX than that of other carotenoids should mainly arise from its extended conjugated double bonds system.

The direct H-atom transfer is one of the most important radical-scavenging processes [

Among the three carotenoids, DX and ZX possess hydroxyls as potential H-atom donors in their structures. O-H BDE acts as an appropriate parameter to characterize the H-atom donating ability [

Collectively, the extended conjugated double bonds system of DX seems crucial for its strong ROS-scavenging activity. The larger number of conjugated double bonds means DX possesses lower E_{T1}, and thus higher ^{1}O_{2} quenching potential through energy transfer, in comparison with BC and ZX. The strong ROS scavenging ability of DX renders this unique carotenoid great potential in antioxidant therapy application.

This work was supported in part by the National Natural Science Foundation of China (Grant No.30700113).

_{1}and T

_{1}energy levels of all-trans-.-carotene

Chemical structures of deinoxanthin, β-carotene and zeaxanthin.

Theoretically estimated lowest triplet excitation energies (E_{T1}) of deinoxanthin (DX), β-carotene (BC) and zeaxanthin (ZX) (in eV).

Theoretical data | 0.69 | 0.80 | 0.80 |

Experimental data | - | 0.88 [ |
0.87 [ |