TRANSIENT RAMAN OBSERVATIONS OF HEME ELECTRONIC AND VIBRATIONAL PHOTODYNAMICS IN DEOXYHEMOGLOBIN

Transient resonance Raman spectroscopy has been used to probe the vibr ational dynamics of the heme active site of deoxyhemoglobin during pho toexcitation. Near UV pulses of approximately 35 ps in duration were u sed to both excite the sample and generate resonance Raman spectra of the heme during its rapid electronic and vibrational relaxation. The b ehavior of the Stokes and anti-Stokes transitions as a function of inc ident laser flux directly reflects net heme vibrational populations an d permits the isolation and characterization of ground and excited ele ctronic state phenomena. Scattering from excited electronic states sig nificantly influences the spectra only at the highest excitation fluxe s used in this study. A simple model that accounts for the flux-depend ent manifestations of the electronic and vibrational contributions to the heme transient resonance Raman spectra is discussed. In addition, the data presented here clearly show mode selectivity in the vibration al energy distribution associated with the ground electronic state. St okes and anti-Stokes scattering from the prominent nu(4) and nu(7) mod es reflect a heme with a non-Boltzmann vibrational population distribu tion, even at relatively modest excitation intensities. The Yq mode ap pears to act as a ''bottleneck'' vibrational state, while the nu(7) mo de couples quite effectively to the bath degrees of freedom. The poten tial origins and ramifications of the creation and maintenance of such relatively long-lived, nonstatistical vibrational population distribu tions in the heme also are addressed.