Cation radicals of xanthophylls

Abstract

Carotenes and xanthophylls are well known to act as electron donors in redox processes. This ability is thought to be associated with the inhibition of oxidative reactions in reaction centers and light-harvesting pigment-protein complexes of photosystem II (PSII). In this work, cation radicals of neoxanthin, violaxanthin, lutein, zeaxanthin, beta-cryptoxanthin, beta-carotene, and lycopene were generated in solution using ferric chloride as an oxidant and then studied by absorption spectroscopy. The investigation provides a view toward understanding the molecular features that determine the spectral properties of cation radicals of carotenoids. The absorption spectral data reveal a shift to longer wavelength with increasing pi-chain length. However, zeaxanthin and beta-cryptoxanthin exhibit cation radical spectra blue-shifted compared to that of beta-carotene, despite all of these molecules having 11 conjugated carbon-carbon double bonds. CIS molecular orbital theory quantum computations interpret this effect as due to the hydroxyl groups in the terminal rings selectively stabilizing the highest occupied molecular orbitals of preferentially populated s-trans-isomers. The data are expected to be useful in the analysis of spectral results from PSII pigment-protein complexes seeking to understand the role of carotene and xanthophyll cation radicals in regulating excited state energy flow, in protecting PSII reaction centers against photoinhibition, and in dissipating excess light energy absorbed by photosynthetic organisms but not used for photosynthesis.

Lutein Accumulation in the Absence of Zeaxanthin Restores Nonphotochemical Quenching in the Arabidopsis thaliana npq1 Mutant

“…Indeed, NIR TA traces revealed a carotenoid radical cation in szl1 npq1 by probing at 950 nm ( Figure 9B) but not at 1000 nm ( Figure 9C), in contrast with wild-type thylakoids (Holt et al, 2005). The TA spectrum of the suppressor thylakoids showed a maximum at ;920 nm ( Figure 9A), which is blue-shifted relative to the spectrum of a b-carotene or zeaxanthin radical cation and is in agreement with the reported spectrum of a lutein radical cation (Mortensen and Skibsted, 1997;Edge et al, 1998;Galinato et al, 2007). Similarly, the DA 535 leaf absorbance change associated with zeaxanthin-dependent qE was missing in npq1 and replaced with a blue-shifted absorbance change in szl1 npq1 ( Figure 11A).…”

“…The broad reconstructed spectrum provided evidence of CT quenching in szl1 npq1, but the spectrum was maximized at ;920 nm, which is substantially blue-shifted relative to the spectrum of a b-carotene cation radical (blue dotted line) or a zeaxanthin radical cation with a broad spectrum centered ;980 to 1000 nm (Holt et al, 2005;Amarie et al, 2007). Instead, the spectrum observed in szl1 npq1 was consistent with the reported absorption spectrum of a lutein radical cation, which was centered at ;920 to 950 nm depending on the solvent used (Mortensen and Skibsted, 1997;Edge et al, 1998;Galinato et al, 2007).…”

Zeaxanthin Radical Cation Formation in Minor Light-harvesting Complexes of Higher Plant Antenna

“…Nonetheless, since the MLHC-V and MLHC-Z samples contain 11.5 lutein molecules/100 chlorophylls (Table 1), an alternative interpretation is that the observed Car ⅐ ϩ signal is a Lut ⅐ ϩ . The spectrum of the Lut ⅐ ϩ peaks at ϳ920 and ϳ950 nm depending upon the solvent used (25)(26)(27), and has recently also been reported to peak at 880 nm (15), significantly blue-shifted relative to the peak of the spectrum in Fig. 4.…”

Lutein Can Act as a Switchable Charge Transfer Quencher in the CP26 Light-harvesting Complex

“…Variable Car ⅐ ϩ Formation in CP24 and CP26 ComplexesThe Car radical cation (Car ⅐ ϩ ) species exhibits strong absorption in the NIR (28). Shown in Fig.…”

“…Although these combined results are consistent with the occurrence of CT quenching involving a Z ⅐ ϩ species in the L2 domain in all three minor complexes, as has been previously suggested (17,19), the two peaks observed in the CP26 TA spectrum are consistent with the transient formation of multiple Car ⅐ ϩ species, possibly indicative of an auxiliary CT site. Recall that the L1 site of CP26 Z is occupied by Lut, hinting that the alternative CT site may involve a Lut ⅐ ϩ , previously shown to absorb at 940 -950 nm (28). The area under the ϳ940 nm band is ϳ28% of the total absorbance, suggesting a somewhat lower yield of Lut cation, because Lut and Z bind to CP26 in a 1:1 ratio (Table 1), whereas the extinction coefficient of Lut ⅐ ϩ is 70% that of Z ⅐ ϩ (30,31).…”