Two-Photon Excitation Spectrum of Fluorescence of the Light-Harvesting Complex B800-850 from Allochromatium minutissimum within 1200-1500 (600-750) nm Spectral Range is not Carotenoid Mediated

Two types of peripheral light-harvesting complexes LH2 (B800-850) from photosynthetic purple bacterium Allochromatium minutissimum were studied. First type containing carotenoids was prepared from wild type cells. The other one was obtained from carotenoid-depleted cells grown with diphenylamine. We have shown that under laser femtosecond excitation within absorption wavelength range of 1200-1500 nm the two-photon excitation of LH2 complexes takes place. This excitation is a result of fluorescence of bacteriochlorophyll (BChl) spectral form B850 (BChl molecules of circular aggregate with strong exaction interaction in 850 urn spectral domain). LH2 fluorescence excitation spectra under two-photon excitation are the same for carotenoid-containing and carotenoidless preparations. In both cases the broad band with peak near 1350 (675) nm (FWHM similar to 240 (120) nm) was found. It is concluded that the broad band with peak near 1350 (675) nm in two-photon excitation spectra of LH2 complexes from Allochromatium minutissimum can not be interpreted as two-photon excitation band of the optically forbidden S-0 -> S-1 transition of carotenoids (rhodopin). Possible nature of this band is discussed.