Development of Multidimensional Resonance Raman Spectroscopies for Studies of Photochemical Dynamics

In this project funded by the Chemical Measurement and Imaging Program, Professor Andrew Moran of the University of North Carolina is developing laser-based experimental methods for studies of photodissociation and electron transfer dynamics in heme proteins. The heme proteins targeted in this research project are of interest, because they mediate electron and oxygen transport in living systems. For example, hemoglobin is responsible for transporting oxygen in blood, whereas cytochrome plays a central role in the electron transport chain in mitochondria. This project focuses on the femtosecond time scale in which elementary nuclear motions of these systems take place. Pulsed laser techniques are uniquely equipped to resolve such fast dynamics. In addition to advancing the understanding of these particular proteins, the experimental methods developed in this project can be adapted by other researchers for applications to a much wider range of systems and processes. The graduate and undergraduate students who conduct this research develop a wide range of technical skills (e.g., optics, instrument development, machining, computer programming).

This project involves the development of two-dimensional resonance Raman (2DRR) spectroscopies in which five noncollinear laser beams induce light emission from a chemical system. Reaction mechanisms are exposed by separating resonances of the reactants and products in separate spectroscopic dimensions, which is not possible in traditional resonance Raman spectroscopies. The background-free nature of these techniques facilitates fast data acquisition rates and enhances sensitivity to weak signals. Two-dimensional Raman spectroscopies are challenged by undesired nonlinearities known as cascades in off-resonant systems (i.e., systems that do not absorb the incoming laser beams). It is generally understood that 'forbidden' steps in the desired nonlinear optical processes are the origin of the problems encountered under off-resonant conditions. In contrast, the experiments developed in this proposal are carried out under resonant conditions, where such unfortunate selection rules do not apply. A variety of control experiments are used to rule out contributions from cascades.