Topic: Watching Molecules Move: Femtosecond Dynamics of Cobalamins
Speaker: Professor James E. Penner-Hahn, Department of Chemistry and Biophysics, University of Michigan (UMich)
Date and time: July 17, 10:00–11:30
Venue: Room 122, #4 Building of SPST
Host: Weng Tsu-Chien
Abstract:
The motion of atoms and electrons in a molecule is central to understanding its photophysical and photochemical properties. Although a variety of methods have been used to characterize excited state structure, X-ray spectroscopies (both absorption—XAS, and emission—XES) have proven to be uniquely useful in understanding excited state dynamics due to their ability to directly probe the local structural environment of metal ions. We will describe the use of these techniques to characterize a variety of vitamin B12 (cobalamin) congeners in order to investigate the structural changes that accompany the redistribution of energy following photo-excitation and to characterize how these differ as a function of axial ligation. We show how, by controlling the relative polarization of the UV-visible pump and the X-ray probe beams, it is possible to measure polarized XAS even for molecules in isotropic solution. We demonstrate that the structural evolution of the excited molecule is best described by a coherent ballistic trajectory on the excited state potential energy surface in which prompt expansion of the Co cavity by Ca. 0.03 Å is followed by significant elongation of the axial bonds (>0.25 Å) over the first 190 fs. Subsequent contraction of the Co cavity in both axial and equatorial directions results in the relaxed S1 excited state structure within 500 fs of excitation. In contrast to K-edge XAS, which is primarily sensitive to molecular structure, XES and L-edge XAS are primarily sensitive to electronic structure. We show that the combination of all 3 X-ray methods together with UV-visible spectroscopy can provide a uniquely detailed description of the excited state evolution.
Biography:
Professor James E. Penner-Hahn is an esteemed figure in the world of bioinorganic chemistry, renowned for his pivotal research on the structure and function of metalloenzymes. With a distinguished career at the University of Michigan, he has contributed significantly to our understanding of how enzymes that incorporate metal ions such as manganese and zinc operate within biological systems. His work has been instrumental in elucidating the complex mechanisms behind the photosynthetic oxygen evolving complex, offering insights into the bioinorganic chemistry of manganese and its indispensable role in oxygen production.
His dedication to the field has been recognized with several awards and honors, reflecting his contribution to the advancement of chemical sciences and education. Beyond his research, Professor Penner-Hahn is committed to teaching and mentoring, having developed innovative curricular material that integrates bioinorganic examples into chemistry education, thereby enhancing the learning experience for students across disciplines.