Revealing the Structure of Tryptophan in Microhydrated Complexes by Cold Ion Spectroscopy

The geometry of biomolecules isolated in the gas phase usually differs substantially from their native structures in aqueous solution, which are the only ones truly relevant to life science. To connect the high resolution of cold ion spectroscopy that can be achieved in the gas phase and the key role of intermolecular hydrogen bonds that shape biomolecules in water, we study protonated tryptophan microhydrated by 1-6 water molecules. IR/UV spectra measured with the same instrument under similar conditions appear to be identical for the complexes of the same size produced by soft dehydration and cryogenic condensation methods. This observation points to the lack of kinetic trapping in the dehydration/rehydration processes. Quantum chemistry computations allow for the unambiguous assignment of the measured IR spectra to the most stable conformers of the complexes. The calculations reveal that retaining as few as four water molecules still conserves most of the TrpH⁺ native structural features.