Acyl Chain Structure Research Articles

Phase transition of triglycerides during semi-hard cheese ripening

Sixteen semi-hard cheeses varying by their protein (20.2–24.1%), fat (23.7–31.1%) and dry matter (50.2–57.9%) contents were manufactured and ripened in controlled conditions. Fluorescence (vitamin A) and mid-infrared (2780–3000 cm−1 region) spectra were collected on the 16 different cheeses at 1, 21, 51 and 81 days of ripening. The data tables containing the mid-infrared and fluorescence spectra of the different cheeses were analyzed by principal component analysis and canonical correlation analysis methods. The obtained similarity maps showed that it was possible to discriminate the cheeses at the four different ripening times from the infrared and fluorescence spectra. In addition, the interest to combine the two spectroscopies was demonstrated since a better discrimination of the four stages was obtained using canonical correlation analysis. The examination of the spectral patterns associated with the principal components and the canonical variates showed that the structure of triglycerides acyl chains in fat globules was modified during cheeses ripening. The shift of the methylene bands in the infrared region and the changes of the shape of vitamin A fluorescence spectra agree with partial crystallization of triglycerides from 21 to 81 days of storage.

Hydrocarbon chain packing and the effect of ethanol on the thermotropic phase behavior of mixed-chain phosphatidylglycerols

Previous studies in this laboratory have delineated the relationship between the acyl chain asymmetry of mixed-chain phosphatidylcholines and the effect of ethanol concentration ([EtOH]) on their melting behavior (Li et al., Biophys J., 70 (1996) 2784–2794). This present investigation extends these findings to another phospholipid family by using high-resolution differential scanning calorimetry (DSC) to characterize the effect of ethanol concentration on the main phase transition temperature ( T m) of five molecular species of mixed-chain phosphatidylglycerol (PG). For C(14):C(18)PG, C(15):C(17)PG, C(16):C(16)PG, and C(17):C(15)PG, a biphasic profile in the T m versus [EtOH] plot was observed, and the minimum in the plot for each PG occurred at 33, 15, 19, and 36 mg/ml, respectively. This biphasic behavior is typical of phospholipids whose acyl chain asymmetry is fairly small. For C(18):C(14)PG, only a linear decrease in the T m was observed as a function of ethanol concentration; this effect is characteristic of highly asymmetric phospholipids. Our DSC results obtained with mixed-chain PG in the presence of ethanol demonstrate that the acyl chain asymmetry of the five lipids studied can be ranked as follows: C(15):C(17)PG<C(16):C(16)PG<C(14):C(18)PG<C(17):C(15)PG<C(18):C(14)PG, confirming independent results from this laboratory that the overall acyl chain structure of PG in the hydrated gel phase is opposite to that detected in the single crystals. In fully hydrated bilayers, the acyl chain conformation of PG is most likely to be similar to that of phosphatidylcholine.

The Thioesterase of the Erythromycin-Producing Polyketide Synthase: Influence of Acyl Chain Structure on the Mode of Release of Substrate Analogues from the Acyl Enzyme Intermediates.

The production of genetically engineered polyketides depends critically on thioesterase activity for product release. In vitro studies with the thioesterase from the erythromycin polyketide synthase (PKS) have demonstrated that the ability of this enzyme to act as a universal decoupler is limited, but stereochemical variation is readily tolerated. Synthetic analogues with all four stereochemical configurations of the natural substrate's 2-methyl-3-hydroxy substitution pattern (1-4; X=p-nitrophenoxy) were substrates for the enzyme.

Evidence that nonbilayer phase propensity of the membrane is important for the side chain cleavage activity of cytochrome P450SCC.

To analyze whether specific protein-lipid interactions or physical features of the membrane contribute to cytochrome P450SCC (CYP11A1) activation by lipids, dimyristoylphosphatidylcholine/cardiolipin and dimyristoylphosphatidylcholine/branched phosphatidylcholine vesicles of defined acyl chain structure were studied for their ability to stimulate the side chain cleavage activity of the enzyme. Activation was found to increase with the mole percent of nonbilayer lipids in the system and the chain lengths of both the branched and main fatty acyl chains of the activator lipid. Unsaturation provided by dioleoylphosphatidylcholine as host lipid leads to a further increase in the potency of the branched phosphatidylcholines to activate the enzyme. The observed activation can be qualitatively interpreted in terms of the effect of these lipids on the hydrophobic volume of the membrane. Using differential scanning calorimetry, we showed that the branched phosphatidylcholines perturb the bilayer membrane structure of dimyristoylphosphatidylcholine and lower the bilayer to hexagonal phase transition temperature of dielaidoylphosphatidylethanolamine, i.e., promote hexagonal phase formation. We also examined the effect of eicosane on both the cytochrome P450SCC activity and the lipid polymorphism and found that eicosane increases both the activity and the hexagonal phase propensity of the vesicle membrane. Because of these correlations, we conclude that the nonbilayer phase propensity of the membrane rather than specific binding of activator lipids to the enzyme explains best the observed activation of enzymatic activity by the lipids.

Spontaneous transfer of monoacyl amphiphiles between lipid and protein surfaces

The kinetics of transfer of natural and fluorescent nonesterified fatty acids (NEFA) and lysolecithins (lysoPC) from phospholipid and protein surfaces were measured. The kinetics of transfer of 12-(1-pyrenyl)dodecanoic acid, from liquid crystalline and gel phase single unilamellar phospholipid vesicles, very low, low, and high density lipoproteins, human serum albumin, and rat liver fatty acid-binding protein, were first-order and characterized by similar rate constants. The halftimes (t1/2) of NEFA transfer from lipids and proteins were dependent on the acyl chain structure according to log t1/2 = -0.62n + 0.59m + 12.0, where n and m, respectively, are the numbers of carbon atoms and double bonds. The structure of the donor surface had a measurable but smaller effect on transfer rates. The kinetics of NEFA and lysoPC transfer are slow relative to the lipolytic processes that liberate them. Therefore, one would predict a transient accumulation of NEFA and lysoPC during lipolysis and an attendant modulation of many metabolic processes within living cells and within the plasma compartment of blood. These data will be useful in the refinement of current models of membrane and lipoprotein function and in the selection of fluorescent NEFA analogs for studying transport in living cells.

Possible involvement of nonbilayer lipids in the stimulation of the activity of cytochrome P450SCC (CYP11A1) and its propensity to induce vesicle aggregation

Phosphatidylcholines with saturated branched fatty acyl chains substituted in the 2-positions of the main chains have a significant stimulating effect on both the side chain cleavage activity of cytochrome P450SCC and the ability of P450SCC to induce a specific vesicle aggregation in lipid-vesicle reconstituted systems. Stimulation increases with increasing content of the branched phosphatidylcholines and is sensitive to its acyl chain structure and composition. Because branched phosphatidylcholines containing 2-alkyl substituted fatty acid chains belong to a new class of phosphatidylcholines forming inverted nonbilayer phases the results indicate that nonbilayer lipids might play an important role in the function of mitochondrial P450SCC.

α-Branched 1,2-Diacyl Phosphatidylcholines as Effectors of Activity of Cytochrome P450SCC (CYP11A1): MODELING THE STRUCTURE OF THE FATTY ACYL CHAIN REGION OF CARDIOLIPIN

Cardiolipin has been shown to be the most effective activator of cholesterol side chain cleavage activity of cytochrome P450SCC, and evidence has been provided for a lipid effector site on the enzyme. Results suggested the headgroup of cardiolipin as major determinant of lipid interaction with P450SCC (Lambeth, J. D. (1981) J. Biol. Chem. 256, 4757-4762). The role of unsaturation is contradictory and open to question (Igarashi, Y. and Kimura, T. (1986) Biochemistry 25, 6461-6466). We synthesized phosphatidylcholines with fully saturated branched fatty acyl chains substituted in the 2-positions of the main chains and studied the influence of these lipids on the activity and other properties of P450SCC in vesicle-reconstituted systems. These saturated branched lipids, with regard to the fatty acyl moiety in molecular shape similar to cardiolipin but with the headgroup of phosphatidylcholines retained, showed a stimulatory efficiency higher than any other phospholipid and at least comparable to cardiolipin. Activation is sensitive to the acyl chain structure and composition. Results suggest that the shape of the molecule at least partially plays an important role in the process of stimulation of the activity of P450SCC. Because binding of cholesterol was increased by the branched lipids monitored optically by the fraction of P450SCC in the high spin form, it was concluded that these lipids, like cardiolipin and other lipids, exert their effects by regulating the binding of cholesterol to P450SCC. These data suggest that polymorphic lipids such as branched phosphatidylcholines and cardiolipin might influence P450SCC function by maintenance of the membrane curvature at a value optimal for activity.

Wild-type Escherichia coli Cells Regulate the Membrane Lipid Composition in a “Window” between Gel and Non-lamellar Structures

Escherichia coli strain K12 was grown at 17, 27, and 37 degrees C. The acyl chain composition of the membrane lipids varied with the growth temperature; the fraction of cis-vaccenoyl chains decreased, and the fraction of palmitoyl chains increased, when the growth temperature was increased. However, the polar head group composition did not change significantly. The equilibria between lamellar and reversed non-lamellar phases of lipids extracted from the inner membrane (IM), and from both the membranes (IOM), were studied with NMR and x-ray diffraction. At temperatures above the growth temperature the lipid extracts formed a reversed hexagonal phase, or a bicontinuous cubic phase, depending on the degree of hydration of the lipids. It was observed that: 1) at equal elevations above the growth temperature, IM lipid extracts, as well as IOM lipid extracts, have a nearly equal ability to form non-lamellar phases; 2) IM extracts have a stronger tendency than IOM extracts to form non-lamellar phases; 3) non-lamellar phases are formed under conditions that are relatively close to the physiological ones; the membrane lipid monolayers are thus "frustrated"; and 4) as a consequence of the change of the acyl chain structures, the temperature for the lamellar gel to liquid crystalline phase transition is changed simultaneously, and in the same direction, as the temperature for the lamellar to non-lamellar phase transition. With a too large fraction of saturated acyl chains the membrane lipids enter a gel state, and with a too large fraction of unsaturated acyl chains the lipids transform to non-lamellar phases. It is thus concluded that the regulation of the acyl chain composition in wild-type cells of E. coli is necessary for the organism to be able to grow in a "window" between a lamellar gel phase and reversed non-lamellar phases.

A Novel Series of Amphiphilic Imidazolinium Compounds for in Vitro and in Vivo Gene Delivery

We have developed three catioinic amphiphiles based on the structure 1-[2-(acyloxy)ethyl]-2-alkyl(alkenyl)-3-(2-hydroxyethyl)imidazolinium chloride. Although these three compounds differ only in the structure of the hydrophobic acyl chains, they differ greatly in their ability to mediate in vivo and in vitro gene delivery. Moreover, in vitro efficiency is not predictive of in vivo efficiency. The myristoyl form is the most effective compound in vitro, and the oleoyl form is the most effective compound in vivo. The compounds readily form suspensions in aqueous media, both in the pure form and as mixtures with either cholesterol or dioleoylphosphatidylethanolamine. These suspensions can be sonicated to produce smaller particles. Particle size, electron microscopy, and the ability to capture glucose suggest that these lipids form liposomes on suspension in aqueous media. When mixed with plasmid DNA, the lipid particles appear to fuse and form larger particles. Fusion is maximal at the critical DNA:lipid ratio where extensive aggregation and precipitation are observed. Therefore, these compounds behave similarly to other cationic liposome-forming lipids upon interaction with DNA.

Bombesin stimulates distinct time-dependent changes in the sn-1,2-diradylglycerol molecular species profile from Swiss 3T3 fibroblasts as analysed by 3,5-dinitrobenzoyl derivatization and h.p.l.c. separation.

We have developed procedures for the analysis of endogenous diradylglycerol (DRG) molecular species using derivatization with 3,5-dinitrobenzoyl chloride. The introduction of this strong chromatophore enabled us to separate less than 1 nmol of DRG into its three classes (diacylglycerol, alkylacylglycerol and alkenylacylglycerol) using a combination of h.p.l.c. and t.l.c. followed by reversed-phase h.p.l.c. to resolve these classes into their component molecular species. When applied to Swiss 3T3 mouse fibroblasts stimulated with bombesin for 25 s, 5 min or 30 min, subtle time-dependent changes in the DRG patterns were observed, with only certain polyunsaturated 1,2-diacyglycerol species [18:0/20:3(n-9), 18:0/20:4(n-6), 18:0/20:4(n-3), 18:0/20:5(n-3), 18:1(n-9)/20:3(n-9), 18:1(n-9)/20:4(n-6), 16:0/22:6(n-3), 18:0/20:3(n-6) and 16:0/20:5(n-3)] showing significant agonist-stimulated increases. The amounts of the first six species were all raised at 25 s, whereas all except the latter two were elevated at 5 min. By 30 min these last species were also increased but 18:0/20:3(n-9) had returned to basal levels. Overall DRG levels, as measured by total molecular-species peak area, remained effectively constant. No changes in the amount or species profile of 1-alkyl-2-acylglycerol were observed. Comparison of these species with the acyl-chain structure of phospholipids supports the idea that inositol lipids could be the source of DRG at early stimulation times, but phosphatidylcholine appears to be a phospholipase substrate at all times. These results indicate sequential activation of several phospholipases with different substrate specificities and/or access to different phospholipid pools. They also suggest that only polyunsaturated DRGs act as second messengers and that changes in the relative amounts of these species may trigger activation of different proteins and/or isoforms (e.g. the different isoforms of protein kinase C).

Affinity of phosphatidylcholine molecular species for the bovine phosphatidylcholine and phosphatidylinositol transfer proteins. Properties of the sn-1 and sn-2 acyl binding sites

Both the phosphatidylcholine transfer protein (PC-TP) and the phosphatidylinositol transfer protein (PI-TP) act as carriers of phosphatidylcholine (PC) molecules between membranes. To study the structure of the acyl binding sites of these proteins, the affinity of 32 distinct natural and related PC molecular species was determined by using a previously developed fluorometric competition assay. Marked differences in affinity between species were observed with both proteins. Affinity vs lipid hydrophobicity (determined by reverse-phase HPLC) plots displayed a well-defined maximum indicating that the acyl chain hydrophobicity is an important determinant of binding of a phospholipid molecule by these transfer proteins. However, besides the overall lipid hydrophobicity, steric properties of the individual acyl chains contribute considerably to the affinity, and PC-TP and PI-TP respond differently to modifications of the acyl chain structure. The affinity of PC-TP increased steadily with increasing unsaturation of the sn-2 acyl moiety, resulting in high affinity for species containing four and six double bonds in the sn-2 chain, whereas the affinity of PI-TP first increased up to two to three double bonds and then declined. These data, as well as the distinct effects of sn-2 chain double bond position and bromination, indicate that the sn-2 acyl chain binding sites of the two proteins are structurally quite different. The sn-1 acyl binding sites are dissimilar as well, since variation of the length of saturated sn-1 chain affected the affinity differently. The data are discussed in terms of the structural organization of the sn-1 and sn-2 acyl binding sites of PC-TP and PI-TP.(ABSTRACT TRUNCATED AT 250 WORDS)

The chemical synthesis and physical characterization of [formula omitted], an important class of membrane glycolipids

Improved procedures for the preparation of 1,2- di-O- acyl-3-O-( α- d-glucopyranosyl )-sn- glycerols have been developed. The procedures are based on a halide ion condensation of a benzylated 1-bromo sugar with 1,2-di- O-butenyl- sn-glycerol and have enabled a relatively straightforward and inexpensive synthesis of 1,2- di-O- acyl-3-O-( α- d-Glucopyranosyl- and 1,2- di-O- acyl-3-O- d- galactopyranosyl)-sn- glcerols with a wide variety of acyl chain lengths and structure and in quantities suitable for detailed structural studies. The analytical data on these compounds are consistent with the expected structure and high sample purity. A physical characterization of the α-D-glycosyl diacylglycerols also indicates that the solid phase and solution properties of these glucolipids differ markedly from those previously reported for the β-anomers. Such differences should also be manifest in the polymorphic phase behavior of lipid bilayers composed of these lipids.

The pH dependence of headgroup and acyl chain structure and dynamics of phosphatidylserine, studied by 2H-NMR

By varying the pH, the influence of the ionization degree on the structure and dynamics of aqueous dispersions of 1,2-dioleoyl-sn- glycero-3-phospho- l-serine (DOPS) was studied, using 2H-NMR methods. For this purpose DOPS was synthesized with deuterium labels incorporated either stereospecifically at the β-position of the serine headgroup ([2- 2H]DOPS) or at the 11-position of both acyl chains ([11,11- 2H 2]DOPS), allowing the effects of pH on headgroup and acyl chains to be measured in parallel. A large scale synthesis procedure of stereospecific 1,2-dioleoyl-sn- glycero-3-phospho-[2- 2H]- l-serine is described. The quadrupolar splitting ( Δν q ) of [2- 2H]DOPS is shown to be a sensitive sensor for the degree of protonation of the molecule. Whereas the Δν q of [2- 2H]DOPS decreases upon lowering the pH, that of [11,11- 2H 2]DOPS gradually increases, indicating an increase in acyl chain ordering. In the pH range below the p K α value, DOPS exhibits a temperature-dependent bilayer to hexagonal H II phase transition, apparent from the 31P-NMR spectra and the occurrence of a second component in the [11,11- 2H 2]DOPS 2H-NMR spectrum, with a much smaller Δν q . The H II phase component in spectra from [2- 2H]DOPS coincides with the isotropic position and has no defined Δν q . In the bilayer organization Δν q and spin-lattice relaxation time (T1) values for the acyl chain deuterated DOPS are similar to those obtained for other lipid systems. In contrast the PS headgroup region displays a relatively rigid structure as evidenced by a large Δν q and very small T1 values. Upon adopting the H II phase the T1 values of the acyl chain deuterons are hardly affected. The uniqueness of the PS headgroup with respect to structure and motional properties is reinforced by the occurrence of a T1 minimum at 45°C in the measurement of the temperature dependence of T1 for [2- 2H]DOPS in the hexagonal H II configuration. Quantitative analysis yields a correlation time ( τ c ) for the motions determining T1 under these conditions, of 3.45 ns.

Secondary structure of substance P bound to liposomes in organic solvents and in solution from Raman and CD spectroscopy.

The membrane lipid phase may be an important mediator of the peptide-receptor interaction. In order to understand the mechanism of this interaction, it is important to know the peptide structure, not only in the hydrophobic lipid bilayer environment, but also at the bilayer surface and in solution. To investigate this problem we have measured the secondary structure of the 11-residue neuropeptide substance P (SP) and its fragments in aqueous solutions, in membrane mimetic solvents, and associated with lipid bilayers using Raman and CD spectroscopy. Raman and CD spectra of SP bound to liposomes indicate a less than 20% helix content. We interpret these results to indicate that SP contains virtually no helix when bound to negatively charged liposomes. These spectra are similar to spectra of peptides in type I and III beta-turns. SP forms between 10 and 30% (1-3 residues) helical structure in sodium dodecyl sulfate micelles and less than 10% helix in methanol and trifluoroethanol. The binding of SP to negatively charged liposomes significantly changes the structure of the lipid acyl chains, decreasing order in some cases and increasing it in others. Raman spectra of SP in water indicates that SP near 30 mM forms an ensemble of structures in water that is distinct from completely unfolded peptide and from the aggregated beta-sheet form observed in saline solutions. We conclude from our CD results that methods used to quantitate secondary structure from CD spectra of short peptides cannot be used to distinguish between very short helical segments and beta-turns.

Effect of fatty acyl chain length and structure on the lamellar gel to liquid-crystalline and lamellar to reversed hexagonal phase transitions of aqueous phosphatidylethanolamine dispersions.

The lamellar gel/liquid-crystalline and the lamellar liquid-crystalline/reversed hexagonal phase transitions of aqueous dispersions of a number of synthetic phosphatidylethanolamines containing linear saturated, branched chain, and alicyclic fatty acyl chains of varying length were studied by differential scanning calorimetry, 31P nuclear magnetic resonance spectroscopy, and X-ray diffraction. For any given homologous series of phosphatidylethanolamines containing a single chemical class of fatty acids, the lamellar gel/liquid-crystalline phase transition temperature increases and the lamellar liquid-crystalline/reversed hexagonal phase transition temperature decreases with increases in hydrocarbon chain length. For a series of phosphatidylethanolamines of the same hydrocarbon chain length but with different chemical structures, both the lamellar gel/liquid-crystalline and the lamellar liquid-crystalline/reversed hexagonal phase transition temperatures vary markedly and in the same direction. In particular, at comparable effective hydrocarbon chain lengths, both the lamellar gel/liquid-crystalline and the lamellar liquid-crystalline/reversed hexagonal phase transition temperatures vary in parallel, such that the temperature difference between these two phase transitions is nearly constant. Moreover, at comparable effective acyl chain lengths, the d spacings of the lamellar liquid-crystalline phases and of the inverted hexagonal phases are all similar, implying that the thickness of the phosphatidylethanolamine bilayers at the onset of the lamellar liquid-crystalline/reversed hexagonal phase transition and the diameter of the water-filled cylinders formed at the completion of this phase transition are comparable and independent of the chemical structure of the acyl chain.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative analysis of hemolytic action of lysophosphatidylcholines in vitro: effect of acyl chain structure.

Hemolysis of rabbit erythrocytes by several lysophosphatidylcholines (lysoPCs) containing C12-C18 saturated and cis unsaturated acyl chains was investigated in vitro. The degree of hemolysis was dependent on the erythrocyte concentration used.Based on this dependency, two determinants for the drugs'hemolytic activity, namely, their affinity for the cell membrane and their intrinsic hemolytic activity, were analyzed separately to clarify the relationship between these factors and the acyl chain structure. Elongation of the acyl chain in saturated lysoPCs was found to greatly increase the affinity and slightly increase the intrinsic activity. In contrast, the introduction of a cis double bond into the acyl chain was found to slightly decrease the affinity and to significantly decrease the intrinsic activity. The affinity and the intrinsic hemolytic activity are discussed in relation to critical micellar concentration and bilayer solubilization ability, respectively.

Heterogeneity effects on permeability-partition coefficient relationships in human stratum corneum.

The relationship between the permeability of solutes undergoing transport via the lipid pathway of the stratum corneum and the degree to which the same solutes partition into the stratum corneum has been explored by measuring the permeability coefficients and stratum corneum/water partition coefficients of a series of hydrocortisone esters varying in lipophilicity. Isolated human stratum corneum, used in both the permeability and the uptake experiments, was shown to resemble full-thickness skin in its overall resistance and selectivity to solute structure. As with full-thickness skin, delipidization destroys the barrier properties of isolated stratum corneum. Although a linear relationship is frequently assumed to exist between permeability coefficients and membrane/water partition coefficients, a log-log plot of permeability coefficients versus the intrinsic stratum corneum/water partition coefficients for the series of hydrocortisone esters studied is distinctly nonlinear. This nonlinearity arises from the fact that the transport of these solutes is rate limited by a lipid pathway in the stratum corneum, while uptake reflects both lipid and protein domains. From the relative permeability coefficients of 21-esters of hydrocortisone varying in acyl-chain structure, group contributions to the free energy of transfer of solute into the rate-limiting barrier microenvironment of the stratum corneum lipid pathway are calculate for a variety of functional groups including the -CH2-, -CONH2, -CON(CH3)2, -COOCH3, -COOH, and -OH groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Electron diffraction structure analysis of phospholipids.

The use of lamellar electron diffraction data from epitaxially oriented phospholipid crystals for quantitative structure analysis is described in this review of the technique. It is seen that an appropriate correction for crystal texture, which is justified by the analysis of low-dose lattice images, enables these intensity data to be used much as they would be used in X-ray crystallography. Analyses of two classes of phospholipids are reviewed, revealing that the recently determined lamellar structures of ether-linked phosphatidylethanolamine and phosphatidylcholine and quite similar to the acyl-chain structures. Preliminary analysis of 1,2-dihexadecyl-sn-glycerophospho-N-methyl ethanolamine indicates that the headgroup conformation may be similar to that of the ether-linked lecithin.

Defects in the Ripple Structure of Phospholipids

Abstract The authors propose a model for the arrangement of the phospholipid molecules in a lipid bilayer membrane, based upon both the structure of the fatty acyl chains packed in highly ordered hexagonal arrays below the main transition temperature and the ripple structures in which the ripple lines have a hexagonal symmetry. This assumption results in three kinds of the configurations, whose boundaries make the angle of 120° among them. According to this domain structure, the texture appearing on the freeze-fractured surface in the ripple phase can be explained consistently, and furthermore, the kinetics of the domain boundary motion can be understood satisfactorily.

A Raman spectroscopic study on the interaction of an ion‐channel protein with a phospholipid in a model membrane system (gramicidin A/l‐α‐lysophosphatidylcholine)

The interaction of the ion channel polypeptide gramicidin A with the L-alpha-lysophosphatidylcholine micelles in a membrane state association (approximative molar ratio 1:9) was investigated by Raman spectroscopy. Studies were carried out over the spectral ranges of 700-1700 cm-1 and 2800-3100 cm-1 at 10 degrees C. The Raman spectrum of L-alpha-lysophosphatidylcholine micelles indicated a disordered structure of the lipid acyl chains by the high intensities of the gauche conformation vibrations. Changing from the micellar phase to the membrane state of association with gramicidin A, the intensities of all-trans stretching modes increased whereas the intensities of gauche conformation vibrations decreased, reflecting the emergence of ordered lipid chains. Hydrophobic interactions between the acyl chains and the polypeptide side chain residues were demonstrated. The absence of modifications in intensities of the very strong tryptophan vibrations in the complex spectrum indicated that, if the tryptophan-stacking interactions suggested by some authors exist, they are very weak ones.