Vibrational Coherence and Tunneling in Proteins

This chapter discusses the use of vibrational coherence and ultrafast wide-dynamic-range population kinetics to probe biological molecules. We show how impulsive stimulated Raman scattering can be used to develop the method of vibrational coherence spectroscopy, which reveals both the structural and functional aspects of the difficult to detect low-frequency modes ( \(h\nu \lesssim {k}_{B}T\) ) in proteins. Studies of electron tunneling in cytochrome c as well as the kinetics of the methionine-heme binding reaction are emphasized. Several ultrafast kinetic studies of heme proteins are used to infer the adiabaticity of ligand-heme binding reactions as well as the potential role of heavy atom tunneling (at temperatures below ~60 K). We also examine vibrational coherence and its potential participation in the excited state proton transfer of green fluorescent protein (GFP). We compare three independent observations of vibrational coherence in GFP and conclude that coherent motion does not affect the excited state proton transfer rate that occurs on the ps timescale. For the ground state proton back-transfer reaction, we find that (incoherent) vibrationally assisted proton tunneling is the dominant transport channel and that the tunneling rate is ~400 ps at room temperature. These studies suggest how serine and/or threonine residues may play an important role in controlling biological proton transport along water-based proton wires.