Many complex molecular systems absorb light in the UV spectral range, including those of paramount biological importance, like DNA bases or proteins. The excited states created by UV absorption are endowed with mechanisms of deactivation which are of major importance for the photochemical stability of these species. The majority of these processes are ultrafast and provide a rapid and efficient way to dissipate the electronic energy into vibration, thus avoiding photochemical reactions.
In proteins, for instance, absorption in the near UV range is mainly due to the aromatic side chain of the tryptophan, tyrosine and phenylalanine amino acids and both spectroscopic and dynamic properties of these chromophores generally depend on their immediate environment, i.e., on the local conformation of the protein. In this context, synergetic theoretical/experimental studies of gas phase model peptides as proteins building blocks should lead to better understanding the photophysical phenomena involved in the relaxation mechanisms of proteins. Such an approach has been developed by a collaboration between the SBM team of LFP (CEA-CNRS URA2453), a theoretical team of the Ruđer Bošković Institut (Zagreb, Croatia) and two experimentalists of CLUPS (Paris Sud University, Orsay) in order to characterize the excited states of the stable conformers of a model peptide and establish the nonradiative relaxation mechanisms.1