Dynamics and Retinal Structural Changes in the Photocycle of the Artificial Bacteriorhodopsin Pigment BR6.9

Abstract

The molecular mechanism describing the initial 200 ps of the room-temperature photocycle in the artificial bacteriorhodopsin (BR) pigment, BR6.9, is examined by both absorption and vibrational spectroscopy. The BR6.9 pigment contains a structurally modified retinal chromophore (retinal 6.9) having a six-membered carbon ring bridging the C9C10−C11 bonds. Picosecond transient absorption (PTA) data show that the initial 200-ps interval of the BR6.9 photocycle contains two intermediates: J6.9 formed with a <3-ps time constant and decaying to K6.9 with a 5-ps time constant (K6.9 has a >5-ns lifetime). Resonantly enhanced vibrational spectra from the light- and dark-adapted ground states of BR6.9 are measured using picosecond resonance coherent anti-Stokes Raman scattering (PR/CARS). Each of these PR/CARS spectra (800−1700 cm-1) contains 33 features assignable to the vibrational degrees of freedom in the retinal chromophore. CARS spectra from the K6.9, obtained from picosecond time-resolved CARS (PTR/CARS) data using 10-ps, 50-ps, 100-ps, and 200-ps time delays following the 570-nm initiation of the BR6.9 photocycle, contain a comparable number of features assignable to the retinal in K6.9. The vibrational spectrum of J6.9 can be tentatively characterized by two bands observed in the 1120−1200-cm-1 region from the analysis of the 10-ps PTR/CARS data. Comparisons involving these PTA and CARS data from BR6.9, as well as analogous results obtained from the ground states and photocycle intermediates of native BR and other artificial BR pigments, demonstrate that restricting retinal motion at the C9C10−C11 bonds does not generally change the initial 200-ps photocycle mechanism, but does alter the rates at which specific molecular processes occur. These vibrational CARS spectra show that the retinal structures in K6.9 and in both light- and dark-adapted BR6.9 are all distinct. However, the specific mechanistic role, if any, of C13C14 isomerization cannot be directly identified from CARS data recorded from BR6.9 and its photocycle intermediates. Even though C13C14 isomerization has been widely considered the primary retinal structural change underlying the proton-pumping mechanism in BR pigments, these results leave open the question of whether C13C14 isomerization is required as a mechanistic precursor for biochemical activity in BR pigments such as BR6.9.