title: The use of tryptophan mutants in identifying the 296 nm transient absorbing species during the photocycle of bacteriorhodopsin.
abstract: The transient absorption at 296 nm was part of the spectroscopic evidence that initiated the proposal that tyrosinate (Tyr-) is formed during, and important to, the photocycle of bacteriorhodopsin (bR). Recent evidence against such a proposal comes from the results of NMR, UV Raman as well as electron cryo-microscopic structural studies. This makes it credible to assign this absorption to a charge perturbation of the lowest energy absorption of one of the tryptophan (Trp) residues in bR. The transient absorption at 296 nm is examined for each of 8 tryptophan mutants in which Trp is substituted by phenylalanine or cysteine, which absorb at shorter wavelength. It is shown that while all go through the photocycle, all but Trp-182 mutant show this transient absorption. This strongly suggests the assignment of this absorption to a charge perturbaton of the lowest energy absorption of Trp-182 during the photocycle. The chemical identity of the perturbing charge(s) is briefly discussed.
authors.affiliation: Department of Chemistry and Biochemistry, University of California-Los Angeles 90024.
authors.forename: S G
authors.initials: SG
authors.lastname: Wu
authors.forename: D J
authors.initials: DJ
authors.lastname: Jang
authors.forename: M A
authors.initials: MA
authors.lastname: el-Sayed
authors.forename: T
authors.initials: T
authors.lastname: Marti
authors.forename: T
authors.initials: T
authors.lastname: Mogi
authors.forename: H
authors.initials: H
authors.lastname: Gobind Khorana
mesh_terms: D001436:Bacteriorhodopsins / Q000378:metabolism*; D003545:Cysteine / Q000737:chemistry; D007700:Kinetics; D010649:Phenylalanine / Q000737:chemistry; D013057:Spectrum Analysis; D014364:Tryptophan / Q000737:chemistry / Q000235:genetics*; D014443:Tyrosine / Q000378:metabolism*
chemical_list: D014443:Tyrosine; D010649:Phenylalanine; D001436:Bacteriorhodopsins; D014364:Tryptophan; D003545:Cysteine