Microsolvation and Protonation Effects on Geometric and Electronic Structures of Tryptophan and Tryptophan-Containing Dipeptides

Photodissociation spectroscopy of solvated clusters of protonated tryptophan (TrpH(+)) and dipeptides containing tryptophan (Val-TrpH(+), Ala-TrpH(+), and Gly-TrpH(+)) has been carried out at low temperature to investigate the protonation and solvation effects on the electronic spectrum. For the protonated dipeptides, the S-1-S-0 transition exhibits a substantial red shift due to the stronger interaction between the NH3+ group and the indole pi ring. The S-1-S-0 spectra of TrpH(+)(CH3OH)(n) clusters exhibit a drastic change with the number of methanol molecules. This behavior is interpreted in terms of the decrease in the interaction between the pi pi* and the repulsive pi sigma* states. Ala-TrpH(+) and Gly-TrpH(+) exhibit all extensive spectral change with addition of two methanol molecules. This change is ascribed to a conformational change, which is induced by the insertion of solvent molecule in between the NH3+ group and the indole pi ring.