title: Origin of the bathochromic shift of astaxanthin in lobster protein: 2D electronic spectroscopy investigation of β-crustacyanin.
abstract: We report on ultrafast spectroscopy study of β-crustacyanin, the carotenoprotein responsible for the coloration of the lobster shell. β-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 β-crustacyanin. By using the ∼20 fs laser pulses tuned to absorption bands of the S0-S2 and S1-Sn transitions of carotenoids, we were able to trace full excitation relaxation dynamics, starting with S2-S1 relaxation on the ∼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 2D 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 β-crustacyanin (including absorption and circular dichroism spectra) points to the relatively weak coupling between the two astaxanthin molecules (∼250 cm(-1)). This implies that the excitonic coupling provides insignificant contribution to the bathochromic shift in β-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 S2 state. We put the bathochromic shift in the broad perspective of other "blue" carotenoids properties.
authors.affiliation: Faculty of Physics, University of Vienna , Strudlhofgasse 4, 1090 Vienna, Austria.
authors.forename: Niklas
authors.initials: N
authors.lastname: Christensson
authors.forename: Karel
authors.initials: K
authors.lastname: Žídek
authors.forename: Nikki Cecil M
authors.initials: NC
authors.lastname: Magdaong
authors.forename: Amy M
authors.initials: AM
authors.lastname: LaFountain
authors.forename: Harry A
authors.initials: HA
authors.lastname: Frank
authors.forename: Donatas
authors.initials: D
authors.lastname: Zigmantas
mesh_terms: D000818:Animals; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism; D002942:Circular Dichroism; D008121:Nephropidae / Q000378:metabolism*; D056951:Photoelectron Spectroscopy; D011789:Quantum Theory; D013997:Time Factors; D024341:Xanthophylls / Q000737:chemistry
chemical_list: D002352:Carrier Proteins; D024341:Xanthophylls; C018494:crustacyanins; C005948:astaxanthine