Gas-phase photochemistry of the photoactive yellow protein chromophore trans-p-coumaric acid.
The photoisomerization of trans-p-coumaric acid (trans-CA) triggers a photocycle in photoactive yellow protein that ultimately mediates a phototactic response to blue light in certain purple bacteria. We have used fluorescence excitation and dispersed emission methods in a supersonic jet to investigate the nature of the electronic excited states involved in the initial photoexcitation and subsequent photoisomerization of trans-CA. We observed three distinct regions in the fluorescence excitation spectrum of trans-CA. Region I is characterized by sharp features that upon excitation exhibit trans-CA S(1) emission. In region II, features increase in width and decrease in intensity with increasing excitation energy. Upon excitation, we observed dual emission from the S(1) state of trans-CA and what may be the S(1) state of cis-CA. The onset of dual emission corresponds to an isomerization barrier of about 3.4 kcal/mol. Finally, the extremely broad absorption feature in region III is excitation to the S(2) electronic excited state and excitation results in trans-CA S(1) emission. Furthermore, we collected CA from the molecular beam after laser excitation in each of the three regions as further evidence of the photoisomerization process. The relative amounts of trans- and cis-CA in the collected molecules were measured with high-pressure liquid chromatography. Although trans-CA was excited in all three regions, a significant cis-CA peak appeared only in region II, though a small cis peak was observed in region III.
Ryan
Wendy L
WL
Department of Chemistry, University of Chicago, Chicago, Illinois 60637, USA. wlryan@midway.uchicago.edu
Gordon
David J
DJ
Levy
Donald H
DH
D001426:Bacterial Proteins / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D003373:Coumaric Acids / Q000737:chemistry*; D005740:Gases; D010777:Photochemistry; D020283:Photoreceptors, Microbial; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence
D001426:Bacterial Proteins; D003373:Coumaric Acids; D005740:Gases; D020283:Photoreceptors, Microbial; C050162:photoactive yellow protein, Bacteria