Formation dynamics and nature of tryptophan's primary photoproduct in aqueous solution

The excited state quenching and photoproduct formation of tryptophan in water is studied by femtosecond transient absorption experiments covering the near-UV and Vis range of wavelengths. The quenching of the excited state absorption occurs simultaneously with the rise of a photoproduct characterized by two absorption bands at 350 nm and 425 nm. Both processes are characterized by the same biexponential kinetics, and the time constants found are in excellent agreement with previous time-resolved fluorescence measurements. By varying the pH and comparing with the transient spectra of Trp incorporated in a peptide where electron transfer is the dominant quenching mechanism, we suggest that the photoproduct is a zwitterionic form of Trp with the indole moiety protonated, formed via excited state proton transfer from the side chain amine group, in agreement with conclusions drawn from nanosecond experiments. The present work thus fills the gap between ultrafast fluorescence decay and nanosecond flash photolysis experiments, and pinpoints Trp's fluorescence quenching mechanism at acidic and neutral pH.