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

The geometry of biomolecules isolated in the gas phaseusuallydiffers substantially from their native structures in aqueous solution,which are the only ones truly relevant to life science. To connectthe high resolution of cold ion spectroscopy that can be achievedin the gas phase and the key role of intermolecular hydrogen bondsthat shape biomolecules in water, we study protonated tryptophan microhydratedby 1-6 water molecules. IR/UV spectra measured with the sameinstrument under similar conditions appear to be identical for thecomplexes of the same size produced by soft dehydration and cryogeniccondensation methods. This observation points to the lack of kinetictrapping in the dehydration/rehydration processes. Quantum chemistrycomputations allow for the unambiguous assignment of the measuredIR spectra to the most stable conformers of the complexes. The calculationsreveal that retaining as few as four water molecules still conservesmost of the TrpH(+) native structural features.