Density Functional Theory Study of the β-Carotene Radical Cation and Deprotonated Radicals

Abstract

The beta-carotene radical cation and deprotonated neutral radicals were studied at the density functional theory (DFT) level using different density functionals and basis sets: B3LYP/3-21G, SVWN5/6-31G*, BPW91/DGDZVP2, and B3LYP/6-31G**. The geometries, total energies, spin distributions, and isotropic and anisotropic hyperfine coupling constants of these species were calculated. Deprotonation of the methyl group at the double bond of the cyclohexene ring of the carotenoid radical cation at 5 or 5' produces the most stable neutral radical because of retention of the pi-conjugated system while less stable deprotonation at 9 or 9' and 13 or 13' of the chain methyl groups causes significant distortion of the conjugation. The predicted methyl hyperfine coupling constants of 13-16 MHz of the neutral radicals are in good agreement with the previous electron nuclear double resonance (ENDOR) spectrum of photolyzed beta-carotene on a solid support. DFT calculations on the beta-carotene radical cation in a polar water environment showed that the polar environment does not cause significant changes in the proton hyperfine constants from those in the isolated gas-phase molecule. DFT calculated methyl proton hyperfine coupling constants of less than 7.2 MHz are in agreement with those reported for the radical cation in photosystem II (PS II) and those found in the absence of UV light for the radical cation on a silica alumina matrix.

Theoretical investigation of carotenoid ultraviolet spectra

“…Therefore, CAC bond distances using B3LYP for the trans-␤-carotene were also in accordance with the experimental values [15]. CAC double bond values were approximately the same found for ethylene ((CH 2 ) 2 ; 1.336 Å) and they are in accordance with the results obtained by Bernardi et al, using B3LYP level [42,43]. The length of calculated COC single bond was shorter than the value of 1.53 Å found for normal alkane, consistent with delocalization of the electrons.…”

Influence of pH on the decay of β-carotene radical cation in aqueous Triton X-100: A laser flash photolysis study

“…1 and Eq. (4)) [24,46]. Similar results were reported for proton loss from the zeaxanthin radical cation (Chart S1) [47].…”

“…both pathways will form carotenoid neutral radicals (b-CAR Å and HO-b-CAR-H Å ) (Scheme 3). Recently, Kispert et al [24,[46][47][48][49][50] have extensively investigated the stabilities of different carotenoid neutral radicals formed via proton loss from various positions. For example; based on theoretical calculations and experimental results, the most energetically favorable positions for proton loss from b-CAR-H Å+ , to form b-CAR Å , are 4, 4 0 and methyl groups at 5 and 5 0 positions ( Fig.…”

EPR Study of the Astaxanthin n-Octanoic Acid Monoester and Diester Radicals on Silica–Alumina

“…6,22 The proton hyperfine couplings calculated with this method are within 0.5 MHz of the experimental couplings, whereas other levels of theory give values that differ by as much as a factor of 2. 22 The isotropic and anisotropic proton coupling tensors for the three carotenoids studied are given in the Supporting Information in Tables S1−S24. The unpaired spin densities defined as the difference in the α and β spin densities were obtained using the AGUI interface (Semichem, Inc.) 25 from the wave functions and spin densities produced by Gaussian 09.…”

“…All DFT calculations were done with the Gaussian 03 18 and Gaussian 09 19 program packages on the Cray XD1 computer at the Alabama Supercomputer Center. Geometries were optimized at the B3LYP/6-31G** level, 20,21 which we have previously shown 22 to be suitable for predicting the geometry of β-carotene based radicals. Single-point calculations at these geometries were used to predict ENDOR hyperfine couplings at the B3LYP level with the TZP basis set from Ahlrich's group.…”