David Pratt

Chemical Reactivity, Especially of Biologically Relevant Systems, as Probed by High Resolution Electronic Spectroscopy in the Gas Phase

Structure dictates function. Nowhere is this more apparent than in biological systems. An enzyme functions in the way that it does because of the three-dimensional arrangement of atoms or groups of atoms in its vicinity. Much of our current understanding of the molecular basis of life is based in large part on recent determinations of the structures of proteins by X-ray crystallography and NMR techniques. Knowing how proteins are constructed has given us much insight into how they form, how they function, and how they evolve in time.

Our own goals are no less ambitious. We develop new high resolution spectroscopic techniques for the determination of the structures of molecules and their assemblies. A recent example is rotationally resolved electronic spectroscopy in molecular beams, in which the resolution approaches 1 part in 109. We use this technique to determine the moments of inertia of isolated molecules and complexes, from which we obtain information about their three-dimensional structures. We also use this technique to determine how the structures change when they absorb light, thereby modeling chemical reactions. We ask, and answer, questions such as: What kind of interactions exist between two or more species to promote their assembly? How do the electronic charge distributions of the species interact? How are the structures of the individual species modified when they interact? Do they react? Why? And how do the answers to these questions depend upon the relative orientations of the constituents?

In recent work, we have shown how the electronic distributions of two atoms change when they are brought together to form a diatomic molecule, how a solute molecule is distorted towards a zwitterionic form when a solvent molecule is attached, how the motion of a molecule in free space is affected when it is weakly bound to a surface, how the lone pairs of a Lewis base orient themselves towards an acid in a proton-transfer (neutralization) reaction, initiated by light, and how the vibrational motions of a molecule are affected by neighboring molecules, with which only weak, van der Waals forces exist.

Our future work will continue to focus on these questions. We plan to study the geometric constraints on intramolecular excimer formation, hydrogen atom transfer, and "water-assisted" tautomerization reactions. We are designing gas-phase mimics of function in several biological systems, such as the catalytic triad in chymotrypsin. And we are developing new techniques for the study of analogous processes in the condensed phase, including a near-field scanning optical microscope that will employ a new CW laser as the light source. Awards: Chancellor's Distinguished Teaching Award- 1994, Earle K. Plyler Award of the American Physical Society- 1999, Chancellor's Distinguished Research Award- 2001, Pittsburgh Award of the American Chemical Society- 2005.

Recent Updates:

Chemistry in the Natural Sciences