PhD Oral Exam - Alexander Levenberg, Physics

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

High-resolution frequency-domain optical spectroscopy methods such as single-molecule spectroscopy (SMS) and non-photochemical spectral hole burning (NPHB) have been extensively employed to study various amorphous systems including glasses, polymers and proteins, and in particular pigment-protein complexes involved in photosynthesis. In all these systems one can observe, even at cryogenic temperatures, the shifts of the zero-phonon lines. These shifts are attributed to small structural changes in the pigment’s environment that can be represented as switching between the minima of the energy landscape. They are observed directly in single-molecule spectroscopy and are the basis of spectral hole formation in NPHB. Understanding these shifts is important for proper interpretation of NPHB and SMS experiments in the context of photosynthesis research. NPHB also can be used to explore the protein energy landscapes. Joint analysis of the hole growth and recovery data for the same pigment-protein system may allow for determination of the molecular origins of the respective structural changes.

In this thesis I address several topics arising in NPHB experiments on pigment-protein complexes:

The mismatch between the widths of the distributions of the barrier heights and of the tunneling parameter was explained with the presence of at least two distinct NPHB mechanisms featuring overlapping tunneling parameter distributions that arise due to quite different combinations of barrier heights, widths and the masses of the moving/ tunneling entities.

The slowdown of NPHB with increased light intensity can be explained with the presence of triplet state(s), though strong solvent deuteration effect likely indicates local heating.

Differences in NPHB and hole recovery behavior of different pigments in the same protein pocket indicate that pigment molecules themselves likely contribute to the small structural changes behind NPHB. Alternatively, pigment molecules affect the relevant barriers by constraining the movements in their immediate protein environment.

I also describe successfully building and testing a closed-cycle optical cryosystem that will be used in future experiments exploring protein dynamics as well as energy and charge transfer in pigment protein complexes.