Physics Colloquium, November 18, 2003

Exploring Low Frequency Modes and Rebinding Dynamics in Heme Proteins Using Femtosecond Coherence Spectroscopy

Paul M. Champion

Physics Department
Northeastern University

Femtosecond coherence spectroscopy (FCS) can be used to prepare and monitor coherent states of a biological sample [1]. Experiments will be described that focus on the coherent nuclear motions of a class of biomolecules known as heme proteins. This class of highly evolved proteins includes myoglobin, hemoglobin, and cytochrome c, and is responsible for a wide variety of fundamental electron transport, chemical signaling, and respiration processes within living systems. Following laser pulse induced photolysis of one of the ligands of the heme group, the (initially planar) heme group is left far from its final product state equilibrium geometry. This leads to coherent oscillations of those modes composing the reaction coordinate for ligand binding and dissociation. Analysis of the phase of these oscillations can help to determine the timescale for the appearance of the electronic ground state. These studies, along with “white light” continuum measurements of the spectral dynamics [2], suggest that this timescale must be much shorter than the 150fs period of the Fe-Histidine vibration. Overall, the results indicate that traditional models invoking a relatively long-lived cascade through multiple excited electronic states, may not offer the correct description of the underlying photophysics in these systems. Recent investigations on the effect of temperature and sample condition on the coherent motions and on the ultrafast geminate rebinding of the diatomic (NO) ligand will also be described.

[1] F. Rosca, A. Kumar, D. Ionascu, X. Ye, A. Demidov, T. Sjodin, D. Wharton, D. Barrick, S. Sligar, T. Yonetani, and P.M. Champion, “Investigations of Anharmonic Low-Frequency Oscillations in Heme Proteins”, J. Phys. Chem. A 106, 3540 (2002).

[2] X. Ye, A. Demidov, and P.M. Champion, “Measurements of the Photodissociation Quantum Yields of MbNO and MbO2 and the Vibrational Relaxation of the Six-coordinate Heme Species”, J. Am. Chem. Soc. 124 5914 (2002).

3.30 p.m., Smith Laboratory, Room 1005

Refreshments served in Smith 1094 at 3:00 p.m.