Attenuated total reflection IR spectroscopy as a tool to investigate the structure, orientation and tertiary structure changes in peptides and membrane proteins.

During the last few years, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) has become one of the most powerful methods to determine the structure of biological materials and in particular of components of biological membranes, like proteins that cannot be studied by x-ray crystallography and NMR. ATR-FTIR requires a little amount of material (1-100 microg) and spectra are recorded in a matter of minutes. The environment of the molecules can be modulated so that their conformation can be studied as a function of temperature, pressure, pH, as well as in the presence of specific ligands. For instance, replacement of amide hydrogen by deuterium is extremely sensitive to environmental changes and the kinetics of exchange can be used to detect tertiary conformational changes in the protein structure. Moreover, in addition to the conformational parameters that can be deduced from the shape of the infrared spectra, the orientation of various parts of the molecule can be estimated with polarized IR. This allows more precise analysis of the general architecture of the membrane molecules within the biological membranes. The present review focuses on ATR-IR as an experimental approach of special interest for the study of the structure, orientation, and tertiary structure changes in peptides and membrane proteins.