Carotenoid Radical Formation: Dependence on Conjugation Length

Abstract

The relative energy of carotenoid neutral radicals formed by proton loss from the radical cations of linear carotenoids has been examined as a function of conjugation length from n = 15 to n = 9. For a maximum conjugation length of n = 15 (bisdehydrolycopene, a symmetrical compound) proton loss can occur from any of the ten methyl groups, with proton loss from the methyl group at position C1 or C1' being the most favorable. In contrast, the most energetically favorable proton loss from the radical cations of lycopene, neurosporene, spheroidene, spheroidenone, spirilloxanthin and anhydrorhodovibrin occurs from methylene groups that extend the conjugated system. For example, decreasing the conjugation length to n = 11 (lycopene) by saturation of the double bonds C3–C4 and at C3'–C4' of bisdehydrolycopene favors proton loss at C4 or C4' methylene groups. Saturation at C7'–C8' in the case of neurosporene, spheroidene, spheroidenone (n = 9, 10, 11) favors formation of a neutral radical at the C8' methylene group. Saturation of C1–C2 by addition of a methoxy group to a dehydrolycopene-like structure with conjugation of n = 12 or 13 (anhydrorhodovibrin, spirilloxanthin) favors proton loss at the C2 methylene group. As a consequence of deprotonation of the radical cation, the unpaired electron spin distribution changes so that larger β-methyl proton couplings occur for the neutral radicals (13–16 MHz) than for the radical cation (7–10 MHz) providing a means to identify possible carotenoid radicals in biological systems by Mims ENDOR.

Chemistry of carotenoid neutral radicals

“…57 Further study 62 has shown that the electron is transferred from the carotenoid molecules adsorbed on the activated alumina or silica-alumina to the surface Al 3+ Lewis acid site, since 27 Al couplings were detected. Adsorption of carotenoids in activated silicaalumina 63,64 results in the formation of radical cations by electron transfer between carotenoid molecules and the Al 3+ electron acceptor. Subsequent loss of a proton from the weak acid forms the neutral radical.…”

“…63,64 EPR techniques that been used to characterize carotenoid radicals to resolve g-anisotropy (HF-EPR, located at the National High Magnets Field Lab at Florida State University), anisotropic coupling constants due to α-protons (CW, pulsed ENDOR, HYSCORE), isotropic coupling constants due to β-methyl protons (CW, pulsed ENDOR), to determine distances between carotenoid radicals and electron acceptor site (ESEEM, relaxation enhancement), etc., are listed with references in Table 1. 5,6,72,73,74,75,76,77 Measuring distances between carotenoid radicals and formed upon photo-irradiation of carotenoids adsorbed on silica-alumina 24,63,64,65,88 or on the molecular sieves MCM-41, 34,88 Ti-MCM-41 89 or Cu-MCM-41. 33,86 Davies and Mims ENDOR complement each other because hyperfine coupling are shown using one method but not in the other due to different pulse delay times.…”

“…The outer peaks due to neutral radical were simulated in a similar manner. Examples have been published for βcarotene radicals on silica-alumina, 63 zeaxanthin and violaxanthin on silica alumina, 24 for lutein radicals formed in Cu-MCM-41 matrices, 86 9′-cis neoxanthin radicals on MCM-41, 34 lycopene on activated silica-alumina, 64 and astaxanthin on silica-alumina, 87 MCM-41 and Ti-MCM-41. 88 α-Protons From HYSCORE Analysis of Powder Spectra α-Proton hyperfine couplings of carotenoid radicals can be determined from 2D-HYSCORE analysis of the contour line-shapes of the cross-peaks 91 , 92 , 93 which provide the principal components of the tensors.…”

“…Several papers have been published on the unpaired distribution that occurs for various carotenoids as a function of substituent, symmetry and function. 24,34,63,64,86 The unpaired spin density for the carotenoid neutral radicals increases at carbons along the chain distant from the position from where the proton was lost. As an example, in Figure 3 are shown the most energetically favorable neutral radicals of zeaxanthin, #Zea • (4) (lowest energy structure) and #Zea • (5), formed by proton loss at the C4(4′) and at C5(5′) positions at the terminal ends of zeaxanthin radical cation which extends conjugation.…”

Radicals formed from proton loss of carotenoid radical cations: A special form of carotenoid neutral radical occurring in photoprotection

“…41 and metal-substituted MCM-41. 2,3,4,5,6,7,8,9 EPR measurements show that Car •+ is formed in the absence of light by electron transfer to the Lewis acid sites of the matrix surface, and upon light irradiation, neutral radicals (Car • ) were detected. There was an order of magnitude increase in the concentration of carotenoid radicals, including both radical cations and neutral radicals, when a metal was present.…”