Fourier transform infrared spectroscopy of 13C:O labeled phospholipids hydrogen bonding to carbonyl groups

Abstract

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.

Electrostatic interactions in phospholipid membranes revealed by coherent 2D IR spectroscopy

“…2 A). The elongation of these resonances along the diagonal direction of the 2D IR spectrum confirms the inhomogeneous character of the absorption bands of the carbonyl moieties (10,11,16). Each resonance has a negative (blue) contribution due to ground-state bleaching and to stimulated emission from the first excited state and a Author contributions: V…”

“…The carbonyl stretching line-shapes in membranes could yield direct information about molecular architecture and fluctuations in the membrane interface (10, 11), provided that the origin of the spectral inhomogeneity of the carbonyl IR response in phospholipid membranes is understood. The inhomogeneity was attributed to differences in the local environment of the sn-1 and sn-2 carbonyl moieties stemming from the packing arrangements (6, 8), the local chain conformations (8,9,(12)(13)(14), the relative positions of the two CAO groups with respect to the interface (9, 15), and the degree of hydration (16,17). In an elegant work, Blume et al (16) ruled out all of the scenarios involving local structural differences except hydrogen bonding.…”

“…The inhomogeneity was attributed to differences in the local environment of the sn-1 and sn-2 carbonyl moieties stemming from the packing arrangements (6,8), the local chain conformations (8,9,(12)(13)(14), the relative positions of the two CAO groups with respect to the interface (9,15), and the degree of hydration (16,17). In an elegant work, Blume et al (16) ruled out all of the scenarios involving local structural differences except hydrogen bonding. In a recent study, we eliminated the variance in hydration as a possible source of inhomogeneity (18).…”

Infrared studies of protein-induced perturbation of lipids in lipoproteins and membranes

“…However, with the use of fully hydrated, specifically 13 C O labelled phospholipid bilayers, this assignment was questioned, since, should the two bands correspond to the sn-1 and sn-2 carbonyls, their maxima should not be more than 4 cm − 1 apart. In fact, 13 C labelling of only one the positions still gave rise to two bands around 1743 and 1728 cm − 1 (Blume et al, 1988;Lewis and McElhaney, 1992). These studies concluded that the underlying components normally resolved in the stretching vibrations of C O absorption bands are the summation of comparable contributions from both of the ester carbonyl groups and assigned to subpopulations of free and hydrogenbonded ester carbonyl groups.…”

The Role of Liposome -Encapsulated Ascorbic Acid on Hemoglobin Damage by Gamma Irradiation.

“…The decrease in the frequency indicates a strengthening of the hydrogen bonds or even a formation of new hydrogen bonds between the components Korkmaz and Severcan, (2005). Also, the degree of hydrogen-bond formation was monitored in the glycerol backbone region of the DPPC molecule by changes in the contours of the ester C=O stretching bands Blume et al (1988). …”