Vibrational Spectrum of a Picosecond Intermediate in the Artificial BR5.12 Photoreaction: Picosecond Time-Resolved CARS of T5.12

Abstract

The vibrational spectrum of an intermediate, T5.12, in the photoreaction of an artificial bacteriorhodopsin (BR) pigment containing a five-membered carbon ring spanning the C12−C13C14 bonds (BR5.12) is measured by picosecond time-resolved coherent anti-Stokes Raman spectroscopy (PTR/CARS). Observed initially by picosecond transient absorption (PTA) measurements, T5.12 is the only intermediate in the BR5.12 photoreaction (i.e., T5.12 decays only to BR5.12). BR5.12 does not have a photocycle analogous to that in native BR, presumably because the five-membered ring blocks the reaction coordinate leading to C13C14 bond isomerization. Since T5.12 may therefore represent the molecular events (reaction coordinates) that precede C13C14 bond isomerization, its vibrational spectrum may aid in elucidating the primary reaction coordinate(s) in the BR photocycle. Although T5.12 is identified via a red-shifted absorption (660 nm maximum, <3 ps formation with 3 ps BR5.12 excitation and decay in 17 ± 1 ps), no spectrosco...