Alanyl side chain folding in phenylalanine: Conformational assignments through ultraviolet rotational band contour analysis

Partially resolved ultraviolet rotational band contours associated with the S-1 <-- S-0 origin bands of the six most populated conformers of jet-cooled phenylalanine have been recorded via resonant two-photon ionization. The strong dependence of their transition moment orientation on the conformation of the alanyl side chain has facilitated their structural assignment through simulations based upon ab initio computation. The S-1 lifetimes of all six conformers, measured through pump-probe delayed ionization, reveal an efficient nonradiative decay pathway in the most stable conformer, which is stabilized through a chain of intramolecular hydrogen bonds linking the side chain to the benzene ring.