PICOSECOND TIME-RESOLVED ABSORPTION AND FLUORESCENCE DYNAMICS IN THE ARTIFICIAL BACTERIORHODOPSIN PIGMENT BR6.11

The picosecond molecular dynamics in an artificial bacteriorhodopsin ( BR) pigment containing a structurally modified all-trans retinal chrom ophore with a six-membered ring bridging the C-11=C-12-C-13 positions (BR6.11) are measured by picosecond transient absorption and.picosecon d time-resolved fluorescence spectroscopy. Time-dependent intensity an d spectral changes in absorption in the 570-650-nm region are monitore d for delays as long as 5 ns after the 7-ps, 573-nm excitation of BR6. 11. Two intermediates, J6.11 and K6.11/1, both with enhanced absorptio n to the red (>600 nm) of the BR6.11 spectrum are observed within appr oximately 50 ps. The J6.11 intermediate decays with a time constant of 12 +/- 3 ps to form K6.11/1. The K6.11/1 intermediate decays with an approximately 100-ps time constant to form a third intermediate, K6.11 /2, which is observed through diminished 650-nm absorption (relative t o that of K6.11/1). No other transient absorption changes are found du ring the remainder of the initial 5-ns period of the BR6.11 photoreact ion. Fluorescence in the 650-900-nm region is observed from BR6.11, K6 .11/1, and K6.11/2, but no emission assignable to J6.11 is found. The BR6.11 fluorescence spectrum has a approximately 725-nm maximum which is blue-shifted by approximately 15 nm relative to that of native BR-5 70 and is 4.2 +/- 1.5 times larger in intensity (same sample optical d ensity). No differences in the profile of the fluorescence spectra of BR6.11 and the intermediates K6.11/1 and K6.11/2 are observed. Followi ng ground-state depletion of the BR6.11 population, the time-resolved fluorescence intensity monitored at 725 nm increases with two time con stants, 12 +/- 3 and approximately 100 ps, both of which correlate wel l with changes in the picosecond transient absorption data. The resona nce Raman spectrum of ground-state BR6.11, measured with low-energy, 5 60-nm excitation, is significantly different from the spectrum of nati ve BR-570, thus confirming that the picosecond transient absorption an d picosecond time-resolved fluorescence data are assignable to BR6.11 and its photoreaction alone and not to BR-570 reformed during the reco nstitution process (<5% of the BR6.11 sample could be attributed to na tive BR-570). The J6.11 and K6.11 absorption and fluorescence data pre sented here are generally analogous to those measured for native J-625 and K-590, respectively, and therefore, the primary events in the BR6 .11 photoreaction can be correlated with those in the native BR photoc ycle. The BR6.11 photoreaction, however, exhibits important difference s including slower formation rates for J and K intermediates as well a s the presence of a second K intermediate. These results demonstrate t hat the restricted motion in the C-11=C-12-C-13 region of retinal foun d in BR6.11 does not greatly change the overall photoreaction mechanis m, but does alter the rates at which processes occur.