Infrared spectroscopy is a useful technique for the determination of conformation and orientation of membrane-associated proteins and lipids. The technique is especially powerful for detecting conformational changes by recording spectral differences before and after perturbations in physiological solution. Polarized infrared measurements on oriented membrane samples have revealed valuable information on the orientation of chemical groupings and substructures within membrane molecules which is difficult to obtain by other methods. The application of infrared spectroscopy to the static and dynamic structure of proteins and peptides in lipid bilayers is reviewed with some emphasis on the importance of sample preparation. Limitations of the technique with regard to the absolute determination of secondary structure and orientation and new strategies for structural assignments are also discussed.

http://shaker.umh.es/docencia/doctorado_IR/FTIR_review3.pdf

Infrared spectroscopy of proteins and peptides in lipid bilayers

The effect of metal cations on the phase behavior and hydration characteristics of phospholipid membranes.

Molecular mechanism of membrane permeabilization by the peptide antibiotic surfactin.

Components of the carbonyl stretching band in the infrared spectra of hydrated 1,2-diacylglycerolipid bilayers: a reevaluation.

Interactions at the lipid–water interface

Fourier transform infrared spectroscopy has been used to characterize the carbonyl stretching vibration of DMPC, DMPE, DMPG, and DMPA, all labeled with 13C at the carbonyl group of the sn-2 chain. Due to the vibrational isotope effect, the 13C = O and the 12C = O vibrational bands are separated by ca. 40-43 cm-1. This frequency difference does not change when the labeling is reversed with the 13C = O group at the sn-1 chain. For lipids in organic solvents possible conformational differences between the sn-1 and sn-2 ester groups have no effect on the vibrational frequency of the C = O groups. In aqueous dispersion unlabeled phospholipids always show a superposition of two bands for the C = O vibration located at ca. 1740 and 1727 cm-1. These two bands have previously been assigned to the sn-1 and sn-2 C = O groups. FT-IR spectra of 13C-labeled phospholipids show that the vibrational bands of both, the sn-1 as well as the sn-2 C = O group, are clearly superpositions of at least two underlying components of different frequency and intensity. Band frequencies were determined by Fourier self-deconvolution and second-derivative spectroscopy. The difference between the component bands is ca. 11-17 cm-1. Again, the conformational effect as shown by reversed labeling is negligible with only 1-2 cm-1. The splitting of the C = O vibrational bands in H2O and D2O is caused by hydrogen bonding of water molecules to both C = O groups as shown by a comparison with spectra of model ester compounds in different solvents. To extract quantitative information about changes in hydration, band profiles were stimulated with Gaussian-Lorentzian functions. The chemical nature of the head group and its electronic charge have distinctive effects on the extent of hydration of the carbonyl groups. In the gel and liquid-crystalline phase of DMPC the sn-2 C = O group is more hydrated than the sn-1 C = O. This is accord with the conformation determined by X-ray analysis. In DMPG the sn-1 C = O group seems to be more accessible to water, indicating a different conformation of the glycerol backbone.

G Messner

Papers

Fourier transform infrared spectroscopy of 13C = O-labeled phospholipids hydrogen bonding to carbonyl groups.