Picosecond Time-Resolved Coherent Anti-Stokes Raman Spectroscopy Of The Artificial Bacteriorhodopsin Pigment, Br6.11

The picosecond molecular dynamics in an artificial bacteriorhodopsin (BR) pigment, BR6.11, are measured by picosecond time-resolved coherent anti-Stokes Raman spectroscopy (PTR/CARS) and picosecond transient absorption (PTA). The BR6.11 pigment contains a structurally modified retinal chromophore with a six-membered carbon ring bridging the C-11=C-12-C-13 bonds, which both locks the C-11=C-12 bond in the trans configuration and prevents rotation about the C-12-C-13 bond. The changes in the vibrational degrees of freedom of the retinal attributable to the six-membered carbon ring are found in the picosecond resonance CARS (PR/CARS) spectrum of ground-state BR6.11. Normal mode assignments for more than forty BR6.11 vibrational features are made through comparisons with the PR/CARS data from native BR-570 (previously analyzed in terms of the selective isotopic substitution of the retinal). PTR/CARS spectra from two intermediates (J6.11 and K6.11), observed by PTA to appear during the initial 200 ps of the BR6.11 photocycle, reveal distinct retinal structures for each. The retinal in J6.11 contains delocalized (i.e., vibrational degrees of freedom not well described by normal modes and spanning major regions of the retinal), out-of-plane motion, and a highly twisted all-trans polyene configuration while K6.11 contains a retinal in which vibrational degrees of freedom have relaxed into well-defined, out-of-plane normal modes and a less twisted (though markedly nonplanar), 13-cis configuration. These vibrational data show that C-13=C-14 isomerization is not the first structural change to occur in the BR6.11 photocycle, but rather is the principal structural change during the J6.11 to K6.11 transformation. The similarity of the retinal structural dynamics in the BR6.11 and native BR-570 photocycles demonstrates that subpicosecond, torsional motion within the polyene precedes C-13 = C-14 isomerization and is critical for initiating the BR photoreactivity underlying their biochemical function. The structural dynamics comprising the initial 200 ps of the BR6.11 photocycle, as derived from these PTR/CARS data, are described well by a three-state model in which J6.11 and K6.11 are assigned as excited and ground electronic states, respectively.