Donatas Zigmantas

We report on ultrafast spectroscopy study of beta-crustacyanin, the carotenoprotein responsible for the coloration of the lobster shell. beta-Crustacyanin is formed by two closely positioned astaxanthin molecules encapsulated in protein. The 2D electronic spectroscopy together with two-color pump-probe was applied to investigate the electronic structure, the excited-state dynamics, and the influence of the excitonic interaction between the two carotenoids in beta-crustacyanin. By using the similar to 20 Is laser pulses tuned to absorption bands of the S-0-S-2 and S-1-S-n transitions of carotenoids, we were able to trace full excitation relaxation dynamics, starting with S-2-S-1 relaxation on the similar to 30 fs time scale and finishing with the ground-state recovery of 3.2 ps. Superimposed on the relaxation dynamics in the 2D spectra, we observed long-lived beating signals at the characteristic frequencies of astaxanthin vibrational modes. We assign these oscillations to the ground-state vibrational wavepacket dynamics. All major features of the 2ll spectra, including amplitude and phase maps of the long-lived oscillations, were reproduced by employing the exciton-vibronic model. Consistent modeling of all optical properties of beta-crustacyanin (including absorption and circular dichroism spectra) points to the relatively weak coupling between the two astaxanthin molecules (similar to 250 cm(-1)). This implies that the excitonic coupling provides insignificant contribution to the bathochromic shift in beta-crustacyanin. We discuss the origin of the shift and propose that it is caused by two major effects: conformational changes of astaxanthin molecules (increase in effective conjugation length) together with increased charge-transfer character of the S-2 state. We put the bathochromic shift in the broad perspective of other "blue" carotenoids properties.