Journal of Physical Chemistry, Vol.100, No.29, 12066-12075, 1996
Vibrational-Spectrum of K-590 Containing C-13(14,15) Retinal - Picosecond Time-Resolved Coherent Anti-Stokes-Raman Spectroscopy of the Room-Temperature Bacteriorhodopsin Photocycle
The C-C stretching vibrations (1100-1400 cm(-1)) of the K-590 intermediate (containing a C-13(14),(15) retinal), formed during the room-temperature (RT) bacteriorhodopsin (BR) photocycle, are measured using picosecond time-resolved coherent anti-Stokes Raman scattering (PTR/CARS). Although time-resolved resonance Raman data have been published previously for intermediates in the room-temperature BR photocycle, these PTR/ CARS data are the first time-resolved vibrational spectra from a picosecond BR intermediate at RT containing an isotopically-labeled (C-13) retinal. The C-14 and C-15 positions are selected for isotopic labeling because motions around the C-13=C-14 and C-14-C-15 bonds are thought to underlie the structural transformation from BR-570 to K-590 and, therefore, the energy storage and transduction mechanism in the RT/BR photocycle. These PTR/CARS data are recorded 50 ps after the BR photocycle is initiated with 570-nm (5 ps, fwhm) excitation and are fit to within <1 cm(-1) via third-order nonlinear susceptibility (chi((3))) relationships. Comparisons of these PTR/CARS data at RT with the results from earlier resonance Raman (RR) studies of K-590 at low temperature (LT) reveal new temperature effects. Specifically, three CARS bands (1197, 1184, and 1167 cm(-1)) are observed from C-13(14,15) K-590 in H2O samples via PTR/CARS at RT, while only two bands (1189 and 1170 cm(-1)) are found in LT/RR measurements from C-13(14,15) K-625. Analogous temperature-dependent differences are found in data measured from C-13(14,15) K-590 in D2O samples. Independently, PTR/CARS data at RT demonstrate that deuteration of the Schiff-base nitrogen causes major changes in the fingerprint region : the 1197-cm(-1) band decreases to 1193 cm(-1) while diminishing in intensity by half and a new band appears at 1206 cm(-1). No such deuterium effect is observed in the LT/RR data from C-13(14,15) K-590. The previously unrecognized sensitivity of fingerprint bands to deuteration of the Schiff-base nitrogen suggests that the C-C stretching modes are highly mixed with each other and coupled to the N-H(D) retinal rocking mode. Although the temperature and Schiff-base deuteration effects reported here had not been previously identified in LT vibrational data from K-625, an analysis of the RT/CARS data continues to support the 13-cis, 14-trans retinal structure in K-590 proposed from LT results.
Keywords:3RD-ORDER SUSCEPTIBILITY ANALYSIS;DARK-ADAPTED BACTERIORHODOPSIN;FTIR DIFFERENCE SPECTROSCOPY;DRIVEN PROTON PUMP;RESONANCE RAMAN;CHROMOPHORE STRUCTURE;PHOTOCHEMICAL CYCLE;LIGHT;ISOMERIZATION;RHODOPSIN