The conformations of a protected tetrathiopeptide have been analysed by isotope labelling and two-dimensional infrared spectroscopy (2DIR). It has been found that Boc-Ala-Gly(=S)-Ala-Aib-OMe in acetonitrile, as well as its oxopeptide analogue, can adopt a hydrogen-bonded loop conformation in coexistence with less restricted structures. The two types of conformations interconvert too quickly to be resolved on the nuclear magnetic resonance
(NMR) timescale, but give rise to different cross peaks in two-dimensional infrared spectra. The hydrogen bond between the Boc terminal group and the amide proton of Aib can be broken by photoisomerisation of the thioamide bond.