| Abstract
| - The picosecond molecular dynamics in an artificial bacteriorhodopsin (BR) pigment, BR6.11, are measuredby picosecond time-resolved coherent anti-Stokes Raman spectroscopy (PTR/CARS) and picosecond transientabsorption (PTA). The BR6.11 pigment contains a structurally modified retinal chromophore with a six-membered carbon ring bridging the C11C12−C13 bonds, which both locks the C11C12 bond in the transconfiguration and prevents rotation about the C12−C13 bond. The changes in the vibrational degrees of freedomof 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 BR6.11vibrational features are made through comparisons with the PR/CARS data from native BR-570 (previouslyanalyzed 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, revealdistinct retinal structures for each. The retinal in J6.11 contains delocalized (i.e., vibrational degrees of freedomnot well described by normal modes and spanning major regions of the retinal), out-of-plane motion, and ahighly twisted all-trans polyene configuration while K6.11 contains a retinal in which vibrational degrees offreedom have relaxed into well-defined, out-of-plane normal modes and a less twisted (though markedlynonplanar), 13-cis configuration. These vibrational data show that C13C14 isomerization is not the firststructural change to occur in the BR6.11 photocycle, but rather is the principal structural change during theJ6.11 to K6.11 transformation. The similarity of the retinal structural dynamics in the BR6.11 and nativeBR-570 photocycles demonstrates that subpicosecond, torsional motion within the polyene precedes C13C14 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 andground electronic states, respectively.
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