STARK SPECTRA OF CHLOROPHYLLS AND CAROTENOIDS IN ANTENNA PIGMENT-PROTEINS LHC-II AND CP-II

The band structure of low-temperature absorption and Stark spectra of light-harvesting chlorophyll a/b protein complexes LHC-II and CP-II is analysed. In the red spectral region, the Stark spectrum serves as an independent experimental reference allowing for the resolution of eight bands at 639, 642-3, 649, 656, 662, 665, 671 and 678 nm, consistently with the gaussian deconvolution of the absorption spectrum. Different parts of the Stark spectrum are fitted with combinations of the derivatives of either the total absorption spectrum or of individual gaussian components. Two electronic transitions at 671 nm and 678 nm and two others at 639 nm and 642-3 nm are found to excite, respectively, localized chlorophyll a and chlorophyll b states. Increased changes in permanent dipole moment, DELTAmu congruent-to 2.2 Debye units, corresponding to the transitions at 656 nm and at 665 nm, and an increase in polarizability congruent-to 70 angstrom3 corresponding to the first of them indicate that these transitions originate from strong exciton coupling. In the violet, the Stark spectra are dominated by the contribution from xanthophylls. A value of DELTAmu greater-than-or-equal-to 10 D was estimated for xanthophyll bands at 484 nm and at 511-513 nm. The mechanism of the Stark effect in xanthophylls is briefly discussed.