1-oleoyl-2-palmitoylphosphatidylcholine and 1-palmitoyl-2-oleoylphosphatidylcholine

Nearest-neighbor recognition measurements have been made using exchangeable mimics of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine in the liquid-ordered (lo) and liquid-disordered (ld) states. In the ld phase, the net interaction between these two lipids is repulsive. In the lo phase, their interactions are neither attractive nor repulsive. These results, together with previous nearest-neighbor measurements, imply that the overall driving force for lipid domain formation in bilayers composed of high-melting lipids, low-melting lipids, and cholesterol, corresponds to a strong pull (attraction) between the high-melting lipids and cholesterol, a significant push (repulsion) between the low-melting and high-melting lipids, and a significant push between the low-melting lipids and cholesterol. In a broader context, these results provide strong support for the notion that repulsive forces play a major role in the formation of lipid rafts.

Topics: Biomechanical Phenomena; Dimerization; Mechanical Phenomena; Membrane Microdomains; Phosphatidylcholines

Gas-phase transformation of synthetic phosphatidylcholine (PC) monocations to structurally informative anions is demonstrated via ion/ion reactions with doubly deprotonated 1,4-phenylenedipropionic acid (PDPA). Two synthetic PC isomers, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (PC(16:0/18:1)) and 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (PC(18:1/16:0)), were subjected to this ion/ion chemistry. The product of the ion/ion reaction is a negatively charged complex, [PC + PDPA - H](-). Collisional activation of the long-lived complex causes transfer of a proton and methyl cation to PDPA, generating [PC - CH3](-). Subsequent collisional activation of the demethylated PC anions produces abundant fatty acid carboxylate anions and low-abundance acyl neutral losses as free acids and ketenes. Product ion spectra of [PC - CH3](-) suggest favorable cleavage at the sn-2 position over the sn-1 due to distinct differences in the relative abundances. In contrast, collisional activation of PC cations is absent of abundant fatty acid chain-related product ions and typically indicates only the lipid class via formation of the phosphocholine cation. A solution phase method to produce the gas-phase adducted PC anion is also demonstrated. Product ion spectra derived from the solution phase method are similar to the results generated via ion/ion chemistry. This work demonstrates a gas-phase means to increase structural characterization of phosphatidylcholines via ion/ion chemistry.

Topics: Gases; Ions; Phosphatidylcholines; Tandem Mass Spectrometry

The bilayer phase transitions of six kinds of mixed-chain phosphatidylcholines (PCs) with an unsaturated acyl chain in the sn-1 or sn-2 position, 1-oleoyl-2-stearoyl- (OSPC), 1-stearoyl-2-oleoyl- (SOPC), 1-oleoyl-2-palmitoyl- (OPPC), 1-palmitoyl-2-oleoyl- (POPC), 1-oleoyl-2-myristoyl- (OMPC) and 1-myristoyl-2-oleoyl-sn-glycero-3-phosphocholine (MOPC), were observed by means of differential scanning calorimetry (DSC) and high-pressure light transmittance measurements. Bilayer membranes of SOPC, POPC and MOPC with an unsaturated acyl chain in the sn-2 position exhibited only one phase transition, which was identified as the main transition between the lamellar gel (L(beta)) and liquid crystalline (L(alpha)) phases. On the other hand, the bilayer membranes of OSPC, OPPC and OMPC with an unsaturated acyl chain in the sn-1 position exhibited not only the main transition but also a transition from the lamellar crystal (L(c)) to the L(beta) (or L(alpha)) phase. The stability of their gel phases was markedly affected by pressure and chain length of the saturated acyl chain in the sn-2 position. Considering the effective chain lengths of unsaturated mixed-chain PCs, the difference in the effective chain length between the sn-1 and sn-2 acyl chains was proven to be closely related to the temperature difference of the main transition. That is, a mismatch of the effective chain length promotes a temperature difference of the main transition between the positional isomers. Anomalously small volume changes of the L(c)/L(alpha) transition for the OPPC and OMPC bilayers were found despite their large enthalpy changes. This behavior is attributable to the existence of a cis double bond and to significant inequivalence between the sn-1 and sn-2 acyl chains, which brings about a small volume change for chain melting due to loose chain packing, corresponding to a large partial molar volume, even in the L(c) phase. Further, the bilayer behavior of unsaturated mixed-chain PCs containing an unsaturated acyl chain in the sn-1 or sn-2 position was well explained by the chemical-potential diagram of a lipid in each phase.

Topics: Biophysical Phenomena; Calorimetry, Differential Scanning; Ethylene Glycol; Isomerism; Lipid Bilayers; Phase Transition; Phosphatidylcholines; Pressure; Thermodynamics

An algorithm for the study of the gradual hydration of phospholipid assemblies by means of Fourier transform infrared (FTIR) spectroscopy is presented. A complete series of diacyl phosphatidylcholines (PCs) including all possible analogues with palmitoyl and oleoyl residues, namely DPPC, DOPC, POPC, and OPPC, was investigated at room temperature. The lipid samples were prepared as cast films probably consisting of aligned multilamellar bilayers. The range of water activities studied in these films was regulated by adsorption via the gas phase corresponding to relative humidities of between 0 and 100%. Analyses of the IR-spectroscopic data have concentrated mainly on determining the amounts of water incorporated by each lipid as well as the hydration-induced response observed for some absorption bands of the different lipids. The water uptake at high relative humidity (RH) increases with the portion of unsaturated acyl chains in the molecular structure of the PCs. Isothermal phase transitions triggered lyotropically have been detected in demonstrating the occurrence of the main transition in POPC and OPPC films at room temperature. Moreover, it appears that both lamellar phases, the gel as well as the liquid-crystalline phase, are not uniform. They seem to comprise an amazingly large span of order/disorder states of the lipid chains generally depending on the degree of hydration. As exemplified by the significant variation in the onset of wavenumber shifts for the PO2- and C=O stretching-vibration modes, obtained as a function of hydration, a sequence of attachment to polar lipid binding sites by water molecules was established for DPPC.

Topics: 1,2-Dipalmitoylphosphatidylcholine; Adsorption; Humidity; Kinetics; Models, Theoretical; Phosphatidylcholines; Spectroscopy, Fourier Transform Infrared; Water

The gradual hydration of phospholipid films can be effectively probed by Fourier transform infrared (FTIR) spectroscopy (cf. part I of this series). The hydration-induced changes observed for lipid IR-absorption bands are probably composed of contributions arising from the effects of both the direct binding of water molecules and the thereby caused conformational changes and phase transitions in the lipid molecules and assemblies, respectively. In this article, an attempt is made to attribute some of the more indicative spectroscopic results to these molecular and supermolecular processes with a view to separating their individual contributions to the relevant spectroscopic data. This is done by considering a series of suitable PLs consisting of the palmitoyl and oleoyl lecithins, DPPC, DOPC, POPC, and OPPC, and one cephalin, DOPE. This choice of PCs and DOPE means that at room temperature and different degrees of hydration, several phase states including lamellar gel and liquid crystalline as well as certain nonlamellar phases are covered. The separation of the water-binding and phase-transition contributions to the FTIR-spectroscopic data, we believe, is clearly demonstrated by interpreting the hydration-dependent wavenumber shifts of the nu C=O band of the PCs. Carbonyl groups are affected to a more significant degree for lipids arrayed in the L alpha phase than in the gel phase. A number of spectral features reveal the lyotropically triggered chain-melting transition as well as other structural rearrangements of PCs. This is discussed in detail and demonstrates the excellent sensitivity of the FTIR methodology for the study of such systems.

Topics: 1,2-Dipalmitoylphosphatidylcholine; Kinetics; Phosphatidylcholines; Phosphatidylethanolamines; Spectroscopy, Fourier Transform Infrared; Structure-Activity Relationship; Thermodynamics; Water

Mixed-acid monounsaturated phosphatidylcholines containing palmitate and oleate chains have been synthesized by phospholipase A2 digestion of the appropriate single-acid phosphatidylcholine, followed by reacylation of the lysophosphatidylcholine with the desired fatty acid anhydride. The positional isomers 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine and 1-oleyl-2-palmitoyl-sn-phosphocholine have been thus obtained. The thermotropic behavior of these lipids dispersed in excess water has been studied by differential scanning calorimetry. Positional isomers of mixed-acid monounsaturated phosphatidylcholines are found to have different gel to liquid-crystalline transition temperatures and enthalpies. It was found that mixtures of 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine or 1,2-distearoyl-sn-glycero-3-phosphocholine exhibited inmiscibility in the phosphatidylcholine gel state. The dynamic structure of 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine and 1-oleyl-2-palmitoyl-sn-glycero-3-phosphocholine bilayers has been investigated by measuring the 13C nuclear spin-lattice relaxation times of sonicated aqueous dispersions. No difference was found between the two systems, suggesting that above the thermal transition the presence of the unsaturated acyl group in the 1 or 2 position does not affect significantly the dynamic structure of the bilayer.

Topics: Calorimetry, Differential Scanning; Deuterium; Magnetic Resonance Spectroscopy; Phosphatidylcholines; Phospholipases A; Phospholipases A2; Structure-Activity Relationship; Thermodynamics