Primary picosecond molecular events in the photoreaction of the BR5.12 artificial bacteriorhodopsin pigment.

Abstract

The picosecond dynamics of the photoreaction of an artificial bacteriorhodopsin (BR) pigment containing a retinal in which a five-membered ring spans the C-12 to C-14 positions of the polyene chain (BR5.12) is examined by using time-resolved absorption and fluorescence and resonance Raman spectroscopy. The ring within the retinal chromophore of BR5.12 blocks the C-13 = C-14 isomerization proposed to be a primary step in the energy storage/transduction mechanism in the BR photocycle. Relative to the native BR pigment (BR-570), the absorption spectrum of BR5.12 is red-shifted by 8 nm. The fluorescence spectrum of BR5.12 closely resembles that of BR-570 although the relative fluorescence yield is higher (approximately 10-fold). Picosecond transient absorption (4-ps pulses, 568-662 nm) measurements reveal an intermediate absorbing to the red side of BR5.12. Kinetic fits show that the red-absorbing intermediate appears within < 3 ps and decays with a time constant of 17 +/- 1 ps to form only BR5.12. No emission in the 650- to 900-nm region can be attributed to the red-absorbing species. Since rotation around C-12 - C-13 and isomerization around C-13 = C-14 are prevented in BR5.12, these results demonstrate that motion in these regions of the retinal is (i) necessary to form the K-like intermediate observed in the native BR-570 photocycle and (ii) not necessary to form a red-absorbing intermediate that has spectral and kinetic properties analogous to those of J-625 in the native BR photocycle. Discussions of the excited and ground electronic state assignments for the intermediate observed in the BR5.12 photoreaction are presented.