PICOSECOND TIME-RESOLVED ABSORPTION DYNAMICS IN THE ARTIFICIAL BACTERIORHODOPSIN PIGMENT BR6.9

The picosecond molecular dynamics in an artificial bacteriorhodopsin ( BR) pigment containing a structurally-modified retinal chromophore wit h a six-membered ring beginning at C-9 to bridge the C-9=C-10-C-11 reg ion of the polyene chain (BR6.9) are measured by picosecond transient absorption (PTA). Time-dependent absorption intensity and spectral cha nges in the 560-670 nm region are monitored for delays as long as 54 n s after the 4-ps, 573-nm excitation of BR6.9. Within <50 ps, two inter mediates are observed, both of which have enhanced absorption to the r ed (>600 nm) of the BR6.9 spectrum. The first intermediate decays with a time constant of 5 +/- 1 ps to form the second, but no other absorp tion changes are found during the remainder of the initial 54-ns perio d of the BR6.9 photoreaction. Since these PTA properties are generally analogous to those measured in the native BR photocycle for J-625 and K-590, the two BR6.9 intermediates are denoted J6.9 and K6.9, respect ively. The low-power energy, resonance Raman (RR) spectrum of ground-s tate BR6.9 is significantly different from that of native BR-570, ther eby confirming that these PTA data are assignable to BR6.9 and its pho toreaction alone and not to native BR species (BR-570 could remain in the reconstituted sample as a contaminant). The C-C stretching band st ructure in the RR spectrum of BR6.9 is similar to that of another of t he artificial BR pigments in which the six-membered ring is incorporat ed to bridge the C-11=C-12-C-13 bonds, namely BR6.11. Mechanistically, these results demonstrate that restricted motion in the C-9=C-10-C-11 region does not change that part of the BR6.9 photoreaction involved in forming J6.9 and K6.9 but does alter the rate at which the J to K t ransformation occurs. A molecular model correlating the primary events and their rates in the native BR photocycle with those appearing in t he BR6.9 photoreaction, as well as with other artificial BR pigments c ontaining carbon rings, is presented.