Transient Raman observations of heme electronic and vibrational photodynamics in deoxyhemoglobin

Transient resonance Raman spectroscopy has been used to probe the vibrational dynamics of the heme active site of deoxyhemoglobin during photoexcitation. Near UV pulses of approximately 35 ps in duration were used to both excite the sample and generate resonance Raman spectra of the heme during its rapid electronic and vibrational relaxation. The behavior of the Stokes and anti-Stokes transitions as a function of incident laser flux directly reflects net heme vibrational populations and permits the isolation and characterization of ground and excited electronic state phenomena. Scattering from excited electronic states significantly influences the spectra only at the highest excitation fluxes used in this study. A simple model that accounts for the flux-dependent 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 vibrational energy distribution associated with the ground electronic state. Stokes and anti-Stokes scattering from the prominent ν₄ and ν₇ modes reflect a heme with a non-Boltzmann vibrational population distribution, even at relatively modest excitation intensities. The ν₄ mode appears to act as a 'bottleneck' vibrational state, while the ν₇ mode couples quite effectively to the bath degrees of freedom. The potential origins and ramifications of the creation and maintenance of such relatively long-lived, nonstatistical vibrational population distributions in the heme also are addressed.