| Abstract
| - Pump−probe and pump−dump probe experiments have been performed on several isolated modelchromophores of the photoactive yellow protein (PYP). The observed transient absorption spectra are discussedin terms of the spectral signatures ascribed to solvation, excited-state twisting, and vibrational relaxation. Itis observed that the protonation state has a profound effect on the excited-state lifetime of p-coumaric acid.Pigments with ester groups on the coumaryl tail end and charged phenolic moieties show dynamics that aresignificantly different from those of other pigments. Here, an unrelaxed ground-state intermediate could beobserved in pump−probe signals. A similar intermediate could be identified in the sinapinic acid and inisomerization-locked chromophores by means of pump−dump probe spectroscopy; however, in thesecompounds it is less pronounced and could be due to ground-state solvation and/or vibrational relaxation.Because of strong protonation-state dependencies and the effect of electron donor groups, it is argued thatcharge redistribution upon excitation determines the twisting reaction pathway, possibly through interactionwith the environment. It is suggested that the same pathway may be responsible for the initiation of thephotocycle in native PYP.
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