Proton Transfer in Biomolecules

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The Organic Dynamics Program is supporting Jacob W. Petrich, Mark S. Gordon, and George A. Kraus of the Department of Chemistry of Iowa State University in their collaborative studies of excited-state proton transfer in biomolecules. Through a combination of ultrafast spectroscopy, ab initio quantum chemical calculations, and synthetic organic chemistry, they will develop an understanding of excited-state proton transfer reactions. Two model systems, 7-azaindole and hypericin, form the focus of these studies. Although excited-state proton transfer plays a key role in each of these systems, they represent two very different cases, in that the proton transfer is strongly mediated by solvent in 7-azaindole, but only negligibly influenced by solvent in hypericin. The interplay between theory and ultrafast spectroscopy will lead to revelation of the relative contributions of intra- and intermolecular interactions in these proton transfer reactions. Movement of positively-charged hydrogen ions (protons) from one atom to another represents one of the simplest chemical reactions, yet such reactions lie at the heart of an astounding variety of chemical and biological processes. In some cases, proton transfer may be initiated through the action of light, which adds energy to (`excites`) the molecule giving up the proton. Despite their conceptual simplicity, proton transfer reactions may in fact occur through rather complex molecular-level processes. These collaborators will examine two classes of compounds displaying very different types of excited-state proton transfer reactions. One group of compounds, frequently used as probes of enzyme structure and reaction chemistry, is influenced strongly by the nature of the solvent in which the molecule is dissolved, while the other, typified by hypericin, an intriguing compound which displays anti-viral action when activated by light, is barely affected by the solvent. These studies will help to develop a molecular-level understanding of these critical proton transfer processes.