Characterization of Alkyl Chain Conformation in an Intercalated Cationic Lipid Bilayer by IR Spectroscopy

Abstract

A structural analysis of alkyl chain conformation of an intercalated cationic lipid bilayer is described. Dialkyl dimethylammonium ions (di-C n DA) were ion exchanged into the galleries of layered cadmium thiophosphate to give Cd 0.83 PS 3 (di-C n DA) 0.34 . The grafting density and interlayer expansions were identical to those for the intercalated single chain alkyl trimethylammonium (C n TA) bilayers. The increased methylene chain density in the galleries, however, forces the intercalated lipid to adopt a more trans ordered structure. Progression bands arising from the coupling of vibrational modes of trans methylene units are used to establish the extent of trans registry. Two types of ordered structures of the intercalated cationic lipid may be distinguished. One in which both alkyl chains adopt an all-trans geometry, and one in which the methylene bond adjacent to the headgroup on one of the alkyl chains is gauche. The latter structure is typically found in the crystalline state of these cationic lipids. The concentrations of the two structures were determined from the ratio of the intensities of the progression bands and were found to remain unchanged with temperature.

In-Contact IR Spectroscopy of Hydrocarbon Lubricants

“…The gauche conformer is more globular and has a smaller volume (higher energy) than the extended trans form [28]. The high-pressure spectra are characterised by shifts and broadening of the m asym CH 2 band, also suggesting an increase in the number of gauche bonds [23]. Band broadening is due to overlapping of various local (asymmetric) CH 2 vibrations, which exhibit a range of frequencies due to the different conformational environments as a result of the gauche defects [25].…”

“…In the thick film regime (>200 nm), a linear increase in peak frequency with increasing pressure was obtained, giving coefficients of 11 cm )1 /GPa for m asym CH 2 and 6 cm )1 / GPa for m sym CH 2 bands. The m asym CH 2 band shifts to higher frequencies have been related to an increase in gauche defects [23] in hydrocarbons. In the liquid state, both trans and gauche conformations occur and a large number of distinct conformational states are possible depending on the alkyl chain length.…”

“…Firstly due to surface roughness effects the film might not be providing the primary mechanism of load support and thus the pressure experienced by the oil was reduced. A second explanation is that the molecule adopts the extended (trans) configuration in the near-surface region and thus the vibration frequency is reduced [23].…”

“…However more detailed analysis of the peak position, intensity and band shape can also reveal structural and conformational information. For example IR spectroscopy has been used extensively to characterise alkyl chain conformation in lipid bilayers [23] and conformational defects in polymethylene chains [21]. The effects of pressure on conformer equilibrium have been studied for simple alkanes by a number of workers [24,25].…”

Microstructure of different chain length ionic liquids intercalated into montmorillonite: A molecular dynamics study

“…The 1450-1480 cm − 1 and 1550-1600 cm −1 are C_C stretching vibration of absorption valley. If conformational disorder is included in the chains, their frequencies may redshift or blueshift, depending upon the average content of gauche conformers (Venkataraman and Vasudevan, 2002 …”