The fluorescence lifetimes were measured for six conformers of L-phenylalanine cooled in a supersonic jet. It was found that the S, state lifetimes differ by a factor of three among the conformers. Especially, the most stable conformer (intramolecular hydrogen-bonded form) in So had the shortest lifetime. Time-dependent DFT calculation suggested an importance of the mixing of the n pi* character to S-1(pi pi*) in this conformer dependent dynamics. (c) 2006 Elsevier B.V. All rights reserved.