Observations by nuclear magnetic resonance of the interactions of water with lecithin micelles in carbon tetrachloride solution

Experimental and theoretical NMR studies of interaction between phenylalanine derivative and egg yolk lecithin

The interaction of phenylalanine diamide (Ac-Phe-NHMe) with egg yolk lecithin (EYL) in chloroform was studied by (1)H and (13)C NMR. Six complexes EYL-Ac-Phe-NHMe, stabilized by N-H···O or/and C-H···O hydrogen bonds, were optimized at M06-2X/6-31G(d,p) level. The assignment of EYL and Ac-Phe-NHMe NMR signals was supported using GIAO (gauge including atomic orbital) NMR calculations at VSXC and B3LYP level of theory combined with STO-3Gmag basis set. Results of our study indicate that the interaction of peptides with lecithin occurs mainly in the polar 'head' of the lecithin. Additionally, the most probable lecithin site of H-bond interaction with Ac-Phe-NHMe is the negatively charged oxygen in phosphate group that acts as proton acceptor.

Phospholipid-based reverse micelles

Physicochemical investigations on the aggregation of phospholipids (mainly phosphatidylcholines) in organic solvents are reviewed and compared with the aggregation behaviour of phospholipids in aqueous medium. In particular we review the data showing that phosphatidylcholines (lecithins) form reverse micellar structures in certain apolar solvents. In these systems not only low molecular weight compounds but also catalytically active enzymes and entire cells can be solubilized. In addition, highly viscous phosphatidylcholine gels can be obtained in organic solvents upon solubilizing a critical amount of water. Generally, phospholipid-based reverse micelles can be regarded as thermodynamically stable models for inverted micellar lipid structures possibly occurring in biological membranes.

Micelle formation of diacylglycerophosphocholines in organic solvents I. Effects of the solvents on Krafft points

Krafft points of diacylglycerophosphocholines (PC) were measured in alkanes-cyclohexane solutions by differential scanning calorimetry, and it was found that they were regularly increased following the increase in alkane content in the solutions and the chain length of the alkanes. From these results it was deduced that the mixing of PC with alkanes occurred in the gel state of the PC, but not in micelles at higher temperatures above the Krafft points, where micellar solutions are provided. The penetration of alkanes into gel state PC was found to be dominated by Langmuir type interaction, and the affinity of alkanes increases with increasing in chain lengths. Above the Krafft points, the micelle formation was confirmed by using the fluorescence probe technique.

Conformations of model peptides in membrane-mimetic environments

The influence of a membrane environment on the conformational energetics of a polypeptide chain has been investigated through studies of model peptides in a variety of membrane-mimetic media. Nuclear magnetic resonance (NMR) and circular dichroism (CD) data have been obtained for the peptides in bulk hydrophobic solvents, normal micelles, and reversed micelles. Several hydrophobic peptides which are sparingly soluble in water have been solubilized in aqueous sodium dodecyl sulfate (SDS) solution. NMR and CD data indicate that the micelle-solubilized peptides experience an environment with the conformational impact of bulk methanol, and have decreased conformational freedom. The site of residence of the peptides interacting with the micelles appears to be near the surfactant head groups, in a region permeated by water, and not in the micelle core. Strongly hydrophilic peptides have been solubilized in nonpolar solvents by reversed micelles. These peptides are located in small water pools in close association with the head groups of the surfactant. NMR and CD data show that there is a conformational impact of this interfacial water region on peptide solubilizates distinct from that of bulk water.

Importance of glycerol and fatty acid residues on the ionic properties of phosphatidylglycerols at the air-water interface

Ionic properties of didodecanoylphosphatidylglycerol (C12PG), didodecanolyphosphatidyl-l'-propanol (C12PP), di-(12-methyl, 13-methyl)-pentadecanoylphosphatidylglycerols (C15PG) and dihexadecanoylphosphatidylglycerol (C16PG) have been studied at the air-water interface using titration experiments at constant ionic strength and film expansion experiments at constant pH, with Li+, Na+, K+ and Cs+ in the subphase. For each lipid, the apparent pK in the surface is strongly dependent on the subphase salt concentration and differs from expected intrinsic pK in the bulk. Discrimination between alkaline cations is observed. These results can be accounted for by strong surface potentials, which are satisfactorily calculated by using the Gouy and Chapman theory of the diffuse double layer. The comparison of C12PP and PG expansion data shows the importance of the glycerol residue of PG ionic properties, favouring penetration of cations in the films. Lipids in the liquid-crystalline state, such as C12-and C15PG, do not interact with alkaline cations as does C16PG in the gel phase. In particular, film condensations bring about a clear-cut discrimination between Na+ and K+. Results are discussed with regard to cation penetration and the structure of water at the interface. The importance on membrane functions of these strong surface potentials generated by PG monolayers is suggested.

Theoretical study of the hydration of B-DNA

A theoretical study of the hydration of the B-form of DNA has been carried out using empirical potential energy functions. In the first stage the hydration scheme of a model compound representing the B-DNA has been determined and the results have been shown to agree to a large extent with those of refined ab initio SCF computations. In the second stage, the stabilization energy due to the presence of water in the first hydration shell was computed by considering the hydrated helix as a supermolecule. The computations indicate appreciable stabilization. The different components contributing to the overall stabilization are determined and analysed.

The interaction between water and the polar head in inverted phosphatidylcholine micelles. A 2H and 31P relaxation study

2H and 31P spin-lattice relaxation times (T1) were studied for inverted egg phosphatidylcholine micelles in CCl4 as functions of 2H2O concentration. When the 2h2O/phosphatidylcholine mole ratio changed from 1.0 to 18.0, T1 of 31P increased by about 2.6 fold, whereas T1 of 2H increased by about 50 fold. A quantitative analysis of the deuterium T1 data showed that there is only one water molecule tightly bound to the polar head, and it is in rapid exchange with the rest of the water molecules. The activation energy for the deuterium T1 was 7.1 +/- 0.8 kcal/mol(30 +/- 3 kJ/mol), and was independent of the 2H2O concentration.

Solubility of phosphatidylcholine in chloroform. Formation of hydrogen bonding between phosphatidylcholine and chloroform

The solubility of phosphatidylcholine (PC) was studied by the spectroscopic analysis and the measurement of the solubility. The qualitative analysis of infrared absorption spectra confirmed the existence of two types of hydrogen bondings between chloroform and PC, one between chloroform and the C=O group of PC and the other between chloroform and the phosphorylcholine group of PC. The quantitative analysis of the C-D stretching vibration bands of the chloroform-d solution of PC showed that the latter hydrogen bonding mainly contributes to the solubility and that PC dissolves in chloroform to form a complex consisting of a few or more molecules of chloroform and one molecule of PC. We discussed in this report about the molecular organization of PC in chloroform solution.

Interaction of water with egg lecithin in benzene solution

Benzene solutions of purified egg lecithin, with small amounts of water added, have been examined by 60 MGz and 100 MHz NMR spectroscopy, infrared spectrophotometry and phase contrast microscopy. The transverse relaxation times of the water, N-methyl and O--H protons are dependent on water concentration. This dependence changes sharply for the water proton at a level of one water molecule per lecithin monohydrate molecule. These results do not fully agree with those reported by other workers. Four mathematical models are examined which could account for the behaviour of the water protons. Models which assume a constant transverse relaxation time for water protons above a level of one water molecule per lecithin molecule cannot predict the behaviour observed. It is sufficient to assume that water protons above this concentration have a single relaxation time which is a linear function of water concentration. The added water associates primarily with the phosphate in the lecithin head group. Above nine water molecules per lecithin monohydrate molecule free water is present in the system.