Electron Transfer
Our recent work has focused on two areas: metalloprotein electron transfer and, more recently, single electron transfer between radical cations and related neutral molecules.

Radical Cation-Neutral Electron Transfer
The work on radical cation–neutral electron transfer has involved 15 undergraduates (eight women and seven men) over the past 12 years. The results are summarized in publications in J. Org. Chem., J. Phys. Chem., J. Am. Chem. Soc. J. Chem. Soc. Perkin II, and an Accounts of Chemical Research review which is in press. This work has involved collaboration with the research group of Professor Stephen Nelsen at UW-Madison. The principal objectives of this project are to determine the intrinsic reactivity of a broad range of organic, organometallic, inorganic and biological molecules; to interpret the observed reactivity in terms of structure and bonding; and to test the application of modern electron transfer theory to the systems studied. The Accounts article summarizes our analysis of more than 160 reactions among 60 compounds. The couples studied vary widely – both in molecular and electronic structure as well as reactivity – and they span a range of ca. 1x1014 in self-exchange rate constants. The database of single electron transfer reactions is among the largest set of self-exchange rate constants for a related series of compounds. The reactions show surprisingly good agreement with classical Marcus theory in spite of almost certainly having different degrees of adiabaticity and the agreement is somewhat surprising in light of more modern vibronic coupling electron transfer theory.

These studies also have identified some interesting electronic effects for some of the couples studied that give convincing evidence for striking steric effects on intrinsic reactivity. We believe these effects originate in steric blocking of the approach of the electron donor to the electron acceptor. This steric blocking lowers the extent of HOMO-LUMO overlap near nitrogen atoms in the electron transfer transition state. For most of the nearly 60 compounds in the database electron transfer is carried by the minimal HOMO-LUMO overlap that is possible on the periphery of molecules and no steric or “orientation” effects are observed, however, when the alkyl groups get as small as methyl a profound increase in reactivity is observed that we believe has no other explanation. For the entire database of compounds the principal factor controlling activation energy is the bond reorganization energy associated with electron transfer (lambda-inner) and this should be nearly the same for Me2N)20/+ and Et2N)20/+ self-exchange. Moreover, solvent reorganization (lambda-outer) should be a smaller factor and, if anything, less for Et2N)20/+ than for Me2N)20/+. Nonetheless, Me2N)20/+ is observed to have a self-exchange rate constant more than 2000 times that of Et2N)20/+. The figure below, which is based on high-level DFT molecular orbital calculations, shows graphically why this is so. It also shows how clearly high-level calculations, and the related graphic displays, can provide a deeper understanding and more convincing rationale for these experimental results.

Figure Caption: HOMO electron density function mapped on the surface density function for Me2N)20 (lower left) and the LUMO mapped on the surface density function of Me2N)2+ (upper left). Blue indicates the region of highest density for the mapped function; red is zero density. The equivalent functions are displayed for the Et2N)20– Et2N)2+ reactant pair on the right. Geometry optimization and electron density function maps computed at the DFT B3LYP/6-311G* level using Spartan 02.

Metalloprotein Electron Transfer
The metalloprotein work has focused on the role of stereochemistry in the selectivity displayed by a metallprotein in its electron transfer reactions with coordination compounds. Our research has observed stereoselective electron transfer with three different proteins: horse cytochrome c, spinach plastocyanin, and human copper-zinc superoxide dismutase. For each protein studied, we have been able to assign the likely site of substrate binding prior to electron transfer and this has greatly enhanced our ability to interpret the observed stereoselectivity. The results on cytochrome c and plastocyanin have been published in J. Am. Chem. Soc. and Inorganic Chemistry and the superoxide dismutase study has been published in a special volume of Inorganica Chimica Acta devoted to electron transfer. This work has involved more than 40 undergraduate co-workers.

UW-Eau Claire Undergraduates
Most of the work described here has been done by UW-Eau Claire undergraduates. Nearly 60 undergraduates have been involved in my research to date and 75% of them have gone on to receive advanced degrees or have advanced degree work in progress. Many of have been coauthors on the papers listed in recent publications and have presented papers at national meetings of the American Chemical Society (ACS). For example, on the Accounts of Chemical Research paper described above 15 students over a ca. 12 year period contributed to the work and most of them have gone on to do advanced work in the chemical sciences (Craig Stolen, Ph.D. in Biochemistry UW-Madison; Molly Accola, Ph. D. Immunology-Pathology Harvard University; Yvonne Gindt, Ph.D. UC-Berkeley Biophysics; Steven Gullerud, Ph.D. Materials Science Stanford University; Troy Seehafer, chemical industry; Jobiah Sabelko, Ph.D. Chemistry University of Illinois; Jennifer Brandt, M.S. Chemistry University of Minnesota; Qinling Qu, M.S. Chemical Engineering Purdue University; Xi Chem, Ph. D. Biochemistry University of Rochester; Amy Odegard, Ph.D. in progress University of Wisconsin Madison and Harvard University; DeWayne Halfen, Ph.D. in progress University of Arizona; Katy Splan, Ph.D. in progress Northwestern University; Jamie Gengler accepted to Ph.D. program in Chemistry at Arizona State University; Teresa Jentzsch accepted to Ph.D. program in Chemistry at UC-Berkeley; Jessica O'Konec accepted to Ph.D. program in Pharmacology at University of Wisconsin-Madison). !2 of these students were coauthors of papers cited in the Accounts paper and two were coauthors on unrelated work.

Most recently Teresa Jentzsch (right) and Jessica O'Konec presented a paper at the National Meeting of the ACS in the spring of 2001. The picture below is from that presentation. Teresa and Jessica are also coauthors on our 2001 paper in J. Chem. Soc. Perkin 2.

John Pladziewicz
Department of Chemistry
(715) 836-4855
pladzijr@uwec.edu
updated: March, 2002
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