[MOS] March 6, 2012

[Zina M Queen]

Seminar on
Modern Optics and Spectroscopy
Ultrafast Dynamics in Photosynthetic Complexes and Components of DNA

Andrew Moran,
University of North Carolina at Chapel Hill
12:00 – 1:00 p.m.
Tuesday March 6, 2012
Transport processes and spectroscopic phenomena in light harvesting proteins are intimately connected to the delocalization of electronic states.  Decoherence imposed by low-frequency nuclear motion generally suppresses the delocalization of electronic states, whereas the Franck-Condon progressions of high-frequency intramolecular modes underpin a hierarchy of vibronic electrostatic interactions between pigments.  We use femtosecond laser spectroscopies to investigate the impact of vibronic couplings on the electronic structures and relaxation mechanisms of two cyanobacterial light harvesting proteins, allophycocyanin (APC) and c-phycocyanin (CPC).  Both APC and CPC possess three pairs of pigments (i.e., dimers), which undergo electronic relaxation on the sub-picosecond time scale.  Electronic relaxation is approximately 10 times faster in APC than in CPC despite the nearly identical structures of their pigment dimers.  We find that electronic relaxation in these closely related proteins is understood on the same footing only in a basis of joint electronic-nuclear states (i.e., vibronic excitons).
Intriguing fundamental physics surround photoinduced relaxation processes in DNA.  Internal conversion rapidly (<500fs) deactivates excited electronic states in the DNA bases, thereby suppressing the formation of lesions known to inhibit cellular function (e.g., thymine dimers).  We investigated these photoinduced relaxation processes in thymine and adenine families of systems at 100K and 300K using nonlinear laser spectroscopies.  Our data indicate that 14meV energy barriers reside between the Franck-Condon regions of the ππ* surfaces and the conical intersections leading back to the ground electronic states in both families of systems.  It follows that the internal conversion transitions responsible for DNA photoprotection are readily initiated by thermal fluctuations at equilibrium. This study is made possible by our extension of specialized femtosecond laser techniques to the UV spectral range.

Grier Room, MIT Bldg 34-401
Refreshments served after the lecture
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mailman.mit.edu/pipermail/mos/attachments/20120302/c33bb354/attachment.htm