Journal of Physical Chemistry B, Vol.114, No.8, 3038-3045, 2010
Photoselective Ultrafast Investigation of Xanthorhodopsin and Its Carotenoid Antenna Salinixanthin
Excited-state dynamics of xanthorhodopsin (XR) and of salinixanthin (SX) in ethanol were investigated by ultrafast pump-hyperspectral probe spectroscopy. Following excitation to the strongly allowed S-2 state of the SX chromophore, transient spectra were recorded photoselectively in the range 430-850 nm. Global kinetic analysis or these data shows the following. (1) Efficient energy transfer from S-2 of the SX in XR to its retinal moiety is verified here. The lifetime of S-2 in SX is, however, determined to be similar to 20 fs, much shorter than previously reported. (2) Branching ratios of excitation transfer from S-2 to S-1, to S*, and to retinal in XR are measured leading to species associated difference spectra (SADS) for all the states involved. Strong protein effects are detected on these branching probabilities. (3) S-1 and S* absorption bands in both systems exhibit anisotropy well below the expected r = 0.4, indicating an angle of similar to 25 degrees between the S-0 -> S-2 and S-1 -> S-n/S* -> S-n transition dipoles. The latter allows confident assignment of the debated S* absorption band to an excited state of SX, and not to "hot" S-0. In light of the extremely fast IC from S-2 to lower excited singlets, possible involvement of ballistic IC in SX, and of coherent energy transfer in XR, are discussed.