ENERGY-FLOW FROM SOLUTE TO SOLVENT PROBED BY FEMTOSECOND IR SPECTROSCOPY - MALACHITE GREEN AND HEME PROTEIN SOLUTIONS

Femtosecond IR spectroscopy has been used to study the energy transfer from solute to solvent in various solutions by monitoring the change of solvent vibrational spectrum. A model system consisting of malachite green and D2O is used to study the heating of D2O. An increase in transmission in the 1800 cm(-1) region of the D2O spectrum after photoexcitation of the solute at 580 nm is explained by the heating of surrounding water which shifts the water infrared absorption bands. The response time for the water spectrum shift to the temperature change was measured to be 4 +/- 3 ps. The rise times of the heating signal in deoxyhemoglobin (Hb) and deoxymyoglobin (Mb) solutions are studied in detail to investigate the energy transport mechanisms in heme proteins. The kinetics of this increase of transmission is fitted to a model that consists of a fast and a slow component. The fast component is best fitted by a Gaussian rise function with time constants of 7.5 +/- 1.5 and 8.5 +/- 1.5 ps for Mb solution and Hb solutions, respectively. The slow component (ca. 20 ps), with 40% of the total amplitude, is attributed to energy transfer from heme to water through the protein via a classical diffusion process based on agreement between the measured time and that calculated with classical diffusion theory. The fast component, almost identical for both Hb and Mb, could not be described by classical diffusion theory and is suggested to proceed through collective motions of the protein.