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

Abstract

Previous vibrational spectroscopic studies of solid acyl-alkyl and diacyl phosphatidylcholines suggested that the sn1- and sn2-carbonyl stretching modes of 1,2-diacylglycerolipids have different absorption maxima. To address the assignment of sn1- and sn2-carbonyl stretching modes of hydrated 1,2-diacylglycerolipids, aqueous dispersions of 1-palmitoyl-2-hexadecyl phosphatidylcholine (PHPC), 1-hexadecyl-2-palmitoyl phosphatidylcholine (HPPC), 1,2-dipalmitoylphosphatidylcholine (DPPC), as well as hydrated samples of unlabeled, sn1-13C=O-labeled, sn2-13C=O-labeled, and doubly 13C=O-labeled dimyristoylphosphatidylcholine (DMPC) were examined by Fourier transform infrared spectroscopy. The ester carbonyl stretching (nu C=O) bands of HPPC and PHPC each exhibit maxima near 1726 cm-1 and appear to be a summation of three subcomponents with maxima near 1740 cm-1, 1725 and 1705-1711 cm-1. In contrast, the nu C=O band of DPPC exhibits its maximum near 1733 cm-1 and appears to be a summation of two components centered near 1742 and 1727 cm-1. Thus the ester carbonyl group of the acyl-alkyl PCs appears to reside in a more polar environment than the ester carbonyl groups of their diacyl analogue. This observation implies that the polar/apolar interfaces of hydrated bilayers formed by PHPC and by HPPC are significantly different from that of DPPC and raises the question of whether the acyl-alkyl PCs are suitable models of their diacyl analogue. The absorption maximum of the nu C=O band of the doubly 13C=O-labeled DMPC occurs near 1691 cm-1 and those of its subcomponents occur near 1699 and 1685 cm-1. These frequencies are consistent with a 12C=0/13C0 'isotopic shift' of 42-43cm-1. snl - and snY2-13C0O-labeled DMPC each exhibit well resolved 12C and 13C vc-0 bands with absorption maxima near 1734 and 1692 cm-1, respectively. With both specifically 13C=O-labeled lipids, the 12C and 13C vo bands each seem to be a summation of subcomponents with absorption maxima near 1742 and 1727 cm-1 (12C vc=o) and 1699 and 1685 cm-1 (13C VC_o),regardless of whether the 13C=O-labeled fatty acyl chain is esterified at the snl - or sn2- positions of the glycerol backbone.We conclude that in hydrated 1,2-diacyl PC bilayers, the patterns of infrared absorption exhibited by ester carbonyl groups located at the primary and secondary positions of the glycerol backbone are similar. Also, the resolvable subcomponents of their v0 bands are each a summation of comparable contributions from both ester carbonyl groups and therefore cannot be attributed to the inequivalent locations of the two ester carbonyl groups. This result differs from that of the vibrational spectroscopic studies alluded to above and raises the question of whether data obtained in studies of dry (or poorly hydrated) lipids are applicable to fully hydrated lipid bilayers. To address questions of why the results of the two studies differ, we have also examined the vc=o bands of solid samples of DPPC, HPPC, and PHPC. We find that the vc-0 bands of all solid lipids studied differ from those...

A mechanistic view of lipid membrane disrupting effect of PAMAM dendrimers

“…Formerly, these components were assigned solely to sn-1 and sn-2 ester bonds of the glycerol backbone. Now it is widely accepted that these bands are assigned to differently hydrated subpopulations of both carbonyls [58]. Figure 7.…”

Interaction of skullcap (Scutellaria baicalensis Georgi) and buckwheat (Fagopyrum esculentum Moench) extracts with lipid bilayers

“…The frequencies of this group of alkyl chains depend on mobility (fluidity) of the chains and increase, e.g., with increasing temperature or during transition from the gel to the liquid crystalline state. The increase in the wavenumber of these bands testifies to an increase in liquidity of the hydrophobic part of the membrane [58][59][60].…”

“…Examining the C=O stretching band provides information about the strength of hydrogen bonding. The changes observed in carbonyl (1760-1700 cm -1 ) bands indicate changes in the degree of hydration of carbonyl groups [58]. In DMPC, there are two bands because of two ester carbonyl groups in the molecule: one around 1742 cm -1 with a trans-conformation in the C-C bond and the other one around 1728 cm -1 [58].…”

“…The changes observed in carbonyl (1760-1700 cm -1 ) bands indicate changes in the degree of hydration of carbonyl groups [58]. In DMPC, there are two bands because of two ester carbonyl groups in the molecule: one around 1742 cm -1 with a trans-conformation in the C-C bond and the other one around 1728 cm -1 [58]. However, for hydrated samples, we observed only one broad band contour with a mid-point around 1735 cm -1 and divided into two subcomponents.…”

“…However, for hydrated samples, we observed only one broad band contour with a mid-point around 1735 cm -1 and divided into two subcomponents. The low-wavenumber subcomponent reflects the contribution of a subpopulation of hydrogen-bonded carbonyl groups, while the high-wavenumber one arises from free (non-bonded) groups [58]. The relationship of the maximum position of the vibration of the C=O band at different temperatures in pure and extract-doped DMPC liposomes is shown in Fig.…”