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Physical Seminar: Professor Delmar Larsen, Univ. of CA, Davis

October 08, 2018
04:00 PM - 05:00 PM
Hutchison Hall 473

Professor Delmar Larsen
University of California, Davis
Department of Chemistry

Title:  "Non-Canonical Photocycle Initiation Dynamics of the Photoactive Yellow Protein: Shunting, Skipping, and Not Following the Rules”

Non-Canonical Photocycle Initiation Dynamics of the Photoactive Yellow Protein: Shunting, Skipping, and Not Following the Rules 

Photoactive Yellow Proteins (PYP) are a diverse class of blue-light absorbing bacterial photoreceptors. Electronically excitation of the p-coumaric acid chromophore covalently bound within PYP results in tri-phasic quenching kinetics, however, the molecular basis of this behavior remains unresolved. Here we explore this question by examining and the excitation wavelength dependence of the photodynamics of the PYP from Halorhodospira halophila via a combined theoretical and experimental approach. Conformational heterogeneity was identified as the most probably model, which model was supported with molecular mechanics simulations that identifies two levels of inhomogeneity involving the orientation of the R52 residue and different hydrogen bonding networks to the p-coumaric acid chromophore. Quantum calculations were used to confirm that these inhomogeneities track to altered spectral properties consistent with the experimental results. Although, over 100 other PYP systems have been identified in nature. We demonstrate that the photactivity of Halorhospira halophila does not simply extend to the activity of other PYPs including both excited-state and primary ground-state dynamics of the PYP from Leptospira biflexa. We demonstrate that the ultrafast dynamics of the PYP domain from the Ppr protein of Rhodospirillum centenum (Rcen) joins Leptospira biflexa as non-Hhal PYPs that have had their primary dynamics characterized. Of these three proteins, Rcen PYP is unique in that no I0 population, the primary photoproduce in the photocycle initiation dynamics, is observed. Based on previous knowledge of the mechanism of pCA photoisomerization and the crystal structures, we attribute these non-canonical initiation dynamics of Rcen PYP to the altered placement of the bulkier Phe98 side-chain in the chromophore pocket, which prefers a concerted two-bond hula-twist over a simple single-bond flip isomerization mechanism. This molecular-level description of protein structural photodynamics demonstrates that I0 is not a necessary for initiating of productive PYP photocycles. Moreover, our studies indicate that the dynamics of the ubiquitously studied Hhal PYP is not representative of the photodynamics of the broader PYP family- a topic of great interest with many studies including transient crystallography, optogenetics, protein dynamics, and quantum simulations.

Category: Seminars