PICOSECOND TIME-RESOLVED RESONANCE RAMAN-SPECTRUM OF A K-INTERMEDIATE IN THE PHOTOREACTION OF THE ARTIFICIAL BACTERIORHODOPSIN PIGMENT BR6.11

The resonance Raman (RR) spectrum of a K-intermediate formed during the photoreaction of an artificial bacteriorhodopsin (BR) pigment (derived from E-11,20 ethanoretinal) containing a six-membered ring spanning the C-11=C-12-C-13 region of the retinal chromophore (BR6.11) is recorded by picosecond time-resolved resonance Raman (PTR3) spectroscopy. A recent study of the BR6.11 photoreaction utilizing picosecond transient absorption and picosecond time-resolved fluorescence measurements revealed that (i) a J-intermediate (J6.11) appears within <3 ps and decays to form a K-intermediate, K6.11/1, with a 12 +/- 3 ps time constant and (ii) a second K-intermediate, K6.11/2, is subsequently formed from K6.11/1 with an almost-equal-to 100-ps time constant. The RR spectrum of K6.11/2, measured here by PTR3 at a 300-ps delay, differs significantly from that of BR6.11, thereby demonstrating that structural changes in the retinal chromophore occur as K6.11/2 is formed. Given the location of the six-membered ring in BR6.11, these structural changes cannot involve isomerization and/or rotation in the C-11=C-12-C-13 region of retinal. Changes in other regions of the retinal and/or involving the surrounding protein must account for the differences observed in the RR spectra of BR6.11 and K6.11/2. A comparison of the RR spectra from native K-590 (50-ps delay) and K6.11/2, recorded by PTR3 spectroscopy under the same experimental conditions, reveals both similarities and differences (e.g., the most prominent features appear in the C-C and C=C stretching regions while the maxima positions and relative intensities are significantly different).