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 intermed iate (containing a C-13(14),(15) retinal), formed during the room-temp erature (RT) bacteriorhodopsin (BR) photocycle, are measured using pic osecond time-resolved coherent anti-Stokes Raman scattering (PTR/CARS) . Although time-resolved resonance Raman data have been published prev iously 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 la beling because motions around the C-13=C-14 and C-14-C-15 bonds are th ought to underlie the structural transformation from BR-570 to K-590 a nd, therefore, the energy storage and transduction mechanism in the RT /BR photocycle. These PTR/CARS data are recorded 50 ps after the BR ph otocycle 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 res ults from earlier resonance Raman (RR) studies of K-590 at low tempera ture (LT) reveal new temperature effects. Specifically, three CARS ban ds (1197, 1184, and 1167 cm(-1)) are observed from C-13(14,15) K-590 i n 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. Analog ous temperature-dependent differences are found in data measured from C-13(14,15) K-590 in D2O samples. Independently, PTR/CARS data at RT d emonstrate that deuteration of the Schiff-base nitrogen causes major c hanges in the fingerprint region: the 1197-cm(-1) band decreases to 11 93 cm(-1) while diminishing in intensity by half and a new band appear s at 1206 cm(-1). No such deuterium effect is observed in the LT/RR da ta 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 cou pled 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/CA RS data continues to support the 13-cis, 14-trans retinal structure in K-590 proposed from LT results.