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Experimental and theoretical study of the THz absorption spectra of selected tripeptides

We investigate small peptides using standard terahertz (THz) time-domain spectroscopy. As a test set we examine the tripeptides glutathione, gly-gly-gly and enalapril maleate at room temperature. While earlier investigations of short-chain polypeptides with a conventional FTIR spectrometer were performed at higher THz frequencies, we present first measurements between 150 GHz and 2 THz and compare our measurements to density functional theory (DFT) calculations in order to assign the measured resonances to distinct molecular motions. DFT calculations obtained for a single molecule (glutathione), dimers (gly-gly-gly) and ion pairs (enalapril maleate) coupled via H-bonds, reproduce correctly the number of resonances observed in the experiment.