Effects of cis and trans unsaturation on the structure of phospholipid bilayers: a high-pressure infrared spectroscopic study

Abstract

In order to compare the effects of cis and trans unsaturation on the structure and packing of phospholipid bilayers, infrared spectra of aqueous dispersions of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dielaidoyl-sn-glycero-3-phosphocholine (DEPC) were measured in a diamond anvil cell at 28 degrees C as a function of pressure up to 36 kbar. The infrared spectra indicate that DEPC and DOPC undergo pressure-induced liquid-crystalline to gel phase transitions at critical pressures of 0.7 and 5.2 kbar, respectively. Below their respective critical pressures, the infrared spectra of DOPC and DEPC are essentially indistinguishable, whereas above these pressures, there are very pronounced differences in the barotropic behavior of these two lipids. Specifically, at the 5.2-kbar transition in DOPC, there are significant changes in the frequencies, intensities, and widths of bands associated with the interfacial C = O groups, the olefinic CH = CH groups, and the terminal CH3 groups, whereas the corresponding bands of DEPC are, by contrast, relatively insensitive to the pressure-induced phase transition. The unusual band shape changes in DOPC are attributed to a unique packing arrangement of the oleoyl acyl chains required to accommodate the bent geometries of adjacent cis double bonds. Moreover, above 5 kbar in DEPC, well-defined correlation field splittings of the CH2 scissoring and rocking modes are observed, with magnitudes very similar to those observed at comparable pressures in saturated lipid systems. The absence of correlation field splittings of the corresponding bands of DOPC up to 36 kbar suggests that the bent oleoyl acyl chains are closely packed with all chains oriented parallel to each other.

Conformational transmission in condensed lipid model compounds

“…Such a change in the glyceryl backbone orientation from a layer-perpendicular to a more layer-parallel arrangement was also assumed in the high-pressure-induced transition in dioleoylglycerophosphocholine, thereby allowing an optimal packing of the cis double bonds [38]. As a result of the rearrangement of the glyceryl backbone, a condensing effect on the chain-end methyl groups was found with infrared spectroscopy, analogous to our observations for the silatranes.…”

The Temperature‐Dependent Hydrogen‐Bonding Signature of Lipids Monitored in the Far‐Infrared Domain

“…[9,[12][13][14][15] We used these well-known temperature-dependent phase transitions in the mid-infrared (MIR) range to analyze the FIR contributions and differentiate between water and internal hydrogen-bonding contributions. Interestingly, intermolecular hydrogen bonding was found to strongly vary in function of the lipid structure and the formed layers.…”

Vibrational spectroscopic studies of newly developed synthetic biopolymers

“…From the literature it is known that the saturated lipid molecules have higher melting points than unsaturated lipids. 54 In addition, such long acyl chains in saturated lipid molecules can be packed quite closely together thus maximizing the van der Waals forces. On the other hand, the presence of unsaturated cis bonds introduces a ''kink'' into the acyl chain since there is restricted rotation available around these bonds.…”

“…Consequently, the intermolecular forces are weaker and unsaturated lipids have lower melting point than saturated lipids. 54 It is also worth pointing out that lipid molecules made from trans unsaturated bonds are characterized by higher melting points than their cis equivalents. Furthermore, when cis carbon-carbon double bonds are incorporated into saturated lipid molecules, the resulting monounsaturated molecules will, in excess water, exhibit reduced phase transitions temperature and transition enthalpy as they undergo the gel to liquid-crystalline phase transitions.…”