Structural Analysis of an Isolated Cyclic Tetrapeptide and its Monohydrate by Combined IR/UV Spectroscopy

Cyclopeptides are an important class of substances in nature, and their physiological effects are frequently based on the tendency to form bioactive conformations. Therefore the investigation of their structure yields an understanding of their functionalities. Mass-selective combined IR/UV spectroscopy in molecular beam experiments represents an ideal tool for structural analyses on isolated molecules in the gas phase, such as the investigated cyclo[L-Tyr(Me)-d-Pro](2) peptide and its complexes with water. Using the chosen spectroscopic method in combination with DFT calculations, an assignment of a structure with two intramolecular hydrogen bonds for the naked cyclopeptide is possible. For the monohydrated cluster two isomers have to be discussed: in one of them the water molecule is simply attached to the assigned monomer structure as hydrogen donor, whereas the second isomer can be characterized by a water molecule that is inserted into one of the intramolecular hydrogen bonds.