Conformational landscapes of aromatic amino acids in the gas phase: Infrared and ultraviolet ion dip spectroscopy of tryptophan

The conformational structures of tryptophan, isolated in the gas phase, have been assigned by combining the results of ultraviolet hole-burning and infrared ion dip spectroscopy with the predictions of ab initio calculations conducted at the MP2/6-311+G(d,p)//B3LYP/6-31+G(d) levels of theory. As in phenylalanine, the most strongly populated, and lowest energy conformer presents a folded alanyl side chain that is stabilised by a 'daisy chain' of hydrogen-bonded interactions. These link the acidic proton, the amino group and the indole ring. There is a further interaction between the carbonyl oxygen and the neighbouring CH group on the pyrrole ring. A quantitative evaluation of the dipole-dipole interactions between the alanyl side chain and the indole ring in the L-1(a) and L-1(b) electronic states does not support the suggestion of electronic state mixing. In particular it casts doubt on the assignment of the fluorescence of the most stable, 'special' conformer to emission from the L-1(a) state.