期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2009
卷号：106
期号：19
页码：7718-7723
DOI：10.1073/pnas.0812877106
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Ultrafast transient absorption spectroscopy of wild-type bacteriorhodopsin (WT bR) and 2 tryptophan mutants (W86F and W182F) is performed with visible light excitation (pump) and UV probe. The aim is to investigate the photoinduced change in the charge distribution with 50-fs time resolution by probing the effects on the tryptophan absorption bands. A systematic, quantitative comparison of the transient absorption of the 3 samples is carried out. The main result is the absence in the W86F mutant of a transient induced absorption band observed at {approx}300-310 nm in WT bR and W182F. A simple model describing the dipolar interaction of the retinal moiety with the 2 tryptophan residues of interest allows us to reproduce the dominant features of the transient signals observed in the 3 samples at ultrashort pump-probe delays. In particular, we show that Trp86 undergoes a significant Stark shift induced by the transient retinal dipole moment. The corresponding transient signal can be isolated by direct subtraction of experimental data obtained for WT bR and W86F. It shows an instantaneous rise, followed by a decay over {approx}500 fs corresponding to the isomerization time. Interestingly, it does not decay back to zero, thus revealing a change in the local electrostatic environment that remains long after isomerization, in the K intermediate state of the protein cycle. The comparison of WT bR and W86F also leads to a revised interpretation of the overall transient UV absorption of bR.
