Model peptide–water complexes in Ar matrix: Complexation induced conformation change and chirality transfer

The hydrogen bonded complexes between water and N-acetyl N′-methyl glycine amide (Ac-Gly-NHMe), as well as between water and N-acetyl N′-methyl l-alanine amide (Ac-L-Ala-NHMe) have been studied by matrix isolation infrared spectroscopy and quantum chemical (B3LYP) calculations. Analysis of the spectra shows that from the βDL/βL(D) and the γD=L/γL conformers of the monomers mainly βDL/βL(D) complex with water. In the complexes water binds to both amide groups making water-bridged complexes in which the monomer has a γD=L/γL-like structure. The Ac-L-Ala-NHMe–water complexes have also been studied by matrix isolation vibrational circular dichroism (MI-VCD) spectroscopy as well as by recording the VCD spectra of Ac-L-Ala-NHMe in pure and water-saturated dichloromethane. Transitions of the achiral water molecule, which gained rotational activity through chirality transfer from the model peptide, could be identified in both MI and solution VCD spectra. It is also shown that the signs of these transitions are sensitive to the alignment of the complexed water and can be used for structural identification of the complexes.