Abstract
In order to better understand the early photophysics of PYP, this chapter compares three model chromophores, the deprotonated trans-p-coumaxic acid (pCA2-) and its amide (pCM-) and phenyl thioester (pCT-) analogues, in aqueous solution. The excited-state relaxation dynamics is followed by subpicosecond transient absorption and gain spectroscopy. The excited-state deactivation pathway of PYP model chromophores is found to depend strongly on the substituent adjacent to the carbonyl group. The photoisomerization reaction of the deprotonated p-coumaric acid (pCA2-) and of its amide analogue (pCM-) in solution does not show any spectroscopically detectable intermediate, which is quite different from PYP. On the contrary, the phenyl thioester derivative pCT- exhibits a photo-physical behavior in solution surprisingly close to that of the protein during its initial deactivation step. This chapter highlights the determining role of the thioester bond in the primary molecular events in PYP.
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