PRIMARY PICOSECOND MOLECULAR EVENTS IN THE PHOTOREACTION OF THE BR5.12 ARTIFICIAL BACTERIORHODOPSIN PIGMENT

The picosecond dynamics of the photoreaction of an artificial bacterio rhodopsin (BR) pigment containing a retinal in which a five-membered r ing spans the C-12 to C-14 positions of the polyene chain (BR5.12) is examined by using time-resolved absorption and fluorescence and resona nce Raman spectroscopy. The ring within the retinal chromophore of BR5 .12 blocks the C-13=C-14 isomerization proposed to be a primary step i n the energy storage/transduction mechanism in the BR photocycle. Rela tive 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 closel y resembles that of BR-570 although the relative fluorescence yield is higher (approximate to 10-fold). Picosecond transient absorption (4-p s pulses, 568-662 nm measurements reveal an intermediate absorbing to the red side of BR5.12. Kinetic fits show that the red-absorbing inter mediate 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 ca n be attributed to the red-absorbing species, Since rotation around C- 12-C-13 and isomerization around C-13-C-14 are pre vented in BR5.12, t hese results demonstrate that motion in these regions of the retinal i s (i) necessary to form the K-like intermediate observed in the native BR-570 photocycle and (ii) not necessary to form a red-absorbing inte rmediate that has spectral and kinetic properties analogous to those o f J-625 in the native BR photocycle. Discussions of the excited and gr ound electronic state assignments for the intermediate observed in the BR5.12 photoreaction are presented,