Molecular details of melittin-induced lysis of phospholipid membranes as revealed by deuterium and phosphorus NMR

Solid-state deuterium and phosphorus-31 nuclear magnetic resonance (2H and 31P NMR) studies of deuterium-enriched phosphatidylcholine [( 3',3'-2H2]DPPC, [sn-2-2H31]DPPC) and ditetradecylphosphatidylglycerol (DMPG-diether), as water dispersions, were undertaken to investigate the action of melittin on zwitterionic and negatively charged membrane phospholipids. When the lipid-to-protein ratio (Ri) is greater than or equal to 20, the 2H and 31P NMR spectral features indicate that the system is constituted by large bilayer structures of several thousand angstrom curvature radius, at T greater than Tc (Tc, temperature of "gel-to-liquid crystal" phase transition of pure lipid dispersions). At T approximately Tc, a detailed analysis of the lipid chain ordering shows that melittin induces a slight disordering of the "plateau" positions concomitantly with a substantial ordering of positions near the bilayer center. At T much greater than Tc, an apparent general chain disordering is observed. These findings suggest that melittin is in contact with the acyl chain segments and that its position within the bilayer may depend on the temperature. On a cooling down below Tc, for Ri greater than 20, two-phase spectra are observed, i.e., narrow single resonances superimposed on gel-type phosphorus and deuterium powder patterns. These narrow resonances are characteristic of small structures (vesicles, micelles, ... of a few hundred angstrom curvature radius) undergoing fast isotropic reorientation, which averages to zero both the quadrupolar and chemical shift anisotropy interactions. On an increase of the temperature above Tc, the NMR spectra indicate that the system returns reversibly to large bilayer structures.(ABSTRACT TRUNCATED AT 250 WORDS)

A detailed analysis of the motions of cholesterol in biological membranes by 2H-NMR relaxation

Spin-lattice relaxation, T1z, measurements of [2,2,3,4,4,6-2H6]cholesterol in model membranes of DMPC were performed as a function of temperature, Larmor frequency and position of labelling in the fused ring system. The results are interpreted according to a hierarchy of motions, such that motion i of correlation time tau i reduces the residual ordering set, characterizing motions i-1, i-2, etc..., by the amount Si = d(2)00(beta i), where beta i is the angle between the axes of motional averaging of motions i and i-1, respectively and d(2)00 is the Wigner rotation matrix element. The appearance of minima in the temperature dependence of T1z for cholesterol, at 46.1 MHz and 30.7 MHz, and the scaling of these T1z (min) according to the orientation of each individual C-2H bond with respect to the axis of motional averaging of cholesterol, allows assignment of the sterol axial rotation to the second fastest motion, characterized by a correlation time of 3.2 X 10(-9) s at 25 degrees C and an activation energy of 32 +/- 5 kJ X mole-1. The fastest motion of cholesterol in DMPC could be a very rapid libration, 'wobbling', which does not contribute significantly to the T1z relaxation of cholesterol at physiological temperatures and Larmor frequencies smaller than 50 MHz, but does reduce the ordering of the cholesterol molecule in DMPC from S0 = 1 to S1 = 0.8, at 25 degrees C.

Large-scale selection synchrony of Tetrahymena thermophila

A method is described, based on the phagocytosis of colloidal ferrite particles, which gives highly synchronous populations of Tetrahymena thermophila. To ensure a successful synchrony, the cell culture doubling time, the limits of the phagocytic period and the distribution of cell stages must first be determined. Once these parameters are known, synchrony can be achieved under a variety of growth conditions and with cultures ranging in volume from a few millilitres to 12 litres or more. The main advantages of the method are that the apparatus required is simple, large volumes of cells can be handled easily, and the synchronous populations can be prepared within a few hours. In principle, the method should be applicable to any cell population in which phagocytosis occurs discontinuously over the cell cycle.

Changes in miniature end-plate potentials due to moderate hypertonicity at the frog neuromuscular junction

The effect of an increase in tonicity on the amplitude and frequency of miniature end-plate potentials (m.e.p.p.s) following superfusion of 50 mM- or 100 mM-sucrose-Ringer solution was determined from intracellular recording at end-plates of the frog cutaneous pectoris muscle, in the absence of drugs and using on-line statistical analysis. An immediate decrease in mean amplitude of the order of 20% control was associated with a marked increase in mean frequency. A delayed increase in mean amplitude, independent of frequency, followed the initial response in the majority of end-plates exposed to 100 mM-sucrose-Ringer solution. The net gain was of the order of 34% and was attained over 20-40 min. This was not reversible over at least 20 min. There was an increase in the variability of m.e.p.p. amplitudes associated with the increase of mean amplitude but this was reversible. A moderate increase in tonicity is considered to induce two kinds of presynaptic changes. First, an immediate increase in the probability of release of smaller quantal packets and secondly, an increased loading of transmitter into vesicles within the readily releasable store.

Oxygen consumption in Plasmodium berghei-infected murine red cells: a direct spectrophotometric assay in intact erythrocytes

A spectrophotometric assay has been devised to measure oxygen consumption non-invasively in intact murine red cells parasitized by Plasmodium berghei. The method uses oxyhemoglobin in the erythrocytes both as a source of oxygen and as an indicator of oxygen consumption. Spectra of intact cells show broad peaks and sloping baselines due to light-scattering. In order to ascertain the number of varying components in the 370-450 nm range, the resolution of the spectra was enhanced using Fourier transforms of the frequency domain spectra. Calculation of oxygen consumption was carried out for two-component systems (oxyhemoglobin, deoxyhemoglobin) using absorbances at 415 and 431 nm. Samples prepared from highly parasitized mice (greater than 80% parasitemia, 5% hematocrit) showed oxygen consumption rates of (4-8) X 10(-8) microliter/cell per h. This rate was not attributable to the presence of white cells or reticulocytes. The rate of oxygen consumption in the erythrocytes is shown to be modulated by various agents: the respiratory inhibitors NaN3 and KCN (1 mM) reduced oxygen consumption 2-3-fold; salicylhydroxamic acid (2.5 mM) caused a 20% reduction in rate and 10 mM NaN3, completely blocked deoxygenation. Antimalarial drugs and metal-chelating agents were also tested. Chloroquine, EDTA and desferal (desferoxamine mesylate) did not decrease the deoxygenation rate of hemoglobin in parasitized cells. Quinacrine, quinine and primaquine reduced the rate of formation of deoxyhemoglobin but also produced substantial quantities of methemoglobin. The lipophilic chelator, 5-hydroxyquinoline, decreased the rate of deoxygenation one-third. The spectrophotometric assay provides a convenient means to monitor oxygen consumption in parasitized red cells, to test the effects of various agents thereon, and potentially to explore possible mechanisms for oxygen utilization.

Mevalonic acid is partially synthesized from amino acids in Halobacterium cutirubrum: a 13C nuclear magnetic resonance study

13C nuclear magnetic resonance revealed an unusual pathway for the biosynthesis of lipids in Halobacterium cutirubrum and H. halobium. Mevalonic acid was not synthesized from three acetyl-coenzyme A molecules, as has been suggested previously, and the branch-methyl and methine carbons in phytanyl chains were derived from neither acetate nor glycerol. Instead, they were supplied by the degradation of amino acids, in particular of lysine. Presumably, two different types of two-carbon fragments were used simultaneously by halobacteria for the biosynthesis of mevalonate. The labeling pattern of squalene supported the above conclusions. Based on these data, a general scheme is proposed to account for the contribution of lysine-to-lipid biosynthesis.

Differential effects of calcium channel blockers on the responses of the rat vas deferens to intramural nerve stimulation and exogenous drugs

The calcium channel blockers, nifedipine and verapamil, have separate effects on the phases of nerve-induced twitches which are not reflected by their actions on the responses to exogenous NA, ATP and the stable ATP analogue, alpha, beta-mATP. This implies that different calcium channels are used according to the manner of stimulation.

Effects of antimalarial drugs on oxygen consumption by erythrocytes infected with Plasmodium berghei: an ESR study

Oxygen consumption in mouse erythrocytes infected with Plasmodium berghei has been followed by electron spin resonance (ESR) spectroscopy of nitroxide radical spin probes. The parasitized red cell suspension is mixed with the spin probe CTPO (3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy) in a closed chamber. Oxygen consumption is monitored by the increasing resolution of the superhyperfine splittings of the spin label. The antimalarial drugs quinacrine, primaquine, and quinine are shown to decrease the rate of oxygen consumption of the parasitized erythrocyte suspensions. The spin-label method offers advantages over conventional polarographic and spectrophotometric assays for highly parasitized cell populations where cells are fragile and contain oxidized hemoglobin as well as hemoglobin-derived pigments.

A 2H-NMR study on the interactions of the local anesthetic tetracaine with membranes containing phosphatidylserine

The interaction of the local anesthetic tetracaine with phosphatidylserine-containing model membranes has been studied by 2H-NMR. Charged tetracaine exhibited an unusually large partition coefficient into multilamellar dispersions of phosphatidylserine. The 2H-NMR spectra consisted of a Pake doublet and a narrow line, with the former corresponding to tetracaine in the bilayer and the latter to tetracaine free in solution. A strong pH dependence of the quadrupole splittings indicated different membrane locations for charged and uncharged tetracaine. In equimolar mixtures of phosphatidylserine and phosphatidylcholine the partition coefficients and 2H-NMR spectra were much more like those observed in neat phosphatidylcholine than in neat phosphatidylserine. Dilution studies at pH 5.5 indicated that in phosphatidylserine/phosphatidylcholine mixtures tetracaine experiences a three-site exchange similar to that found earlier for tetracaine in phosphatidylcholine. Tetracaine is in fast exchange between sites weakly bound to membrane and free in solution, and in slow exchange with a strongly bound site in the membrane.

Classification of human tumours by high-resolution magnetic resonance spectroscopy

Malignant tumours yield a high-resolution proton magnetic resonance (MR) spectrum. Fifty-one tumour biopsy specimens from patients with cancer of the ovary or colon were examined by magnetic resonance spectroscopy (MRS) to determine whether it was possible to identify tumour subtypes with metastatic potential. Relaxation parameters (T2) for visible lipid methylene protons were within the range of those measured for three animal metastasis models. Primary carcinomas with metastases gave T2 values greater than 350 ms, whereas carcinomas not associated with known metastases at the time of tumour excision gave a range of values from 150 to 1500 ms. All but two carcinomas gave a long T2 (greater than 350 ms) indicating metastatic potential. The MRS method designated five of six histologically borderline epithelial ovarian tumours as malignant with metastatic potential. MRS may be sensitive enough to detect malignant cells in a tumour which is of intermediate or borderline malignancy by light microscopy. Malignancy without a potential for metastasis is uncommon.

Assignment of methylene proton resonances in NMR spectra of embryonic and transformed cells to plasma membrane triglyceride

Some biological characteristics of cancer cells and solid tumors are identifiable by the high resolution NMR relaxation behavior of their nonaqueous components. Chemical analysis and two-dimensional scalar correlated (COSY) NMR spectroscopy show these resonances arise from neutral lipid in the plasma membrane. Triglyceride is shown to be the main plasma membrane component giving rise to the NMR spectrum, while soluble nonmembrane components account for 90% of the remaining resonances in the spectrum of intact cells. The presence of triglyceride has been detected by chemical analysis in highly purified plasma membranes from two different cell lines. The COSY spectra of cancer cells are comparable with that obtained for the triglyceride-rich very low density human lipoprotein.

Microcomputers and biologists

Microcomputers have now become a working tool of the biologist. This article explores some of the consequences of this revolution both in terms of the skills and working habits needed and of the range of uses which have been found. The primary consequence is that biologists must learn how to become computer managers. This does not mean learning BASIC and computer architecture, though this will usually prove valuable, but that we must be able to grasp from the flood of sales literature and computer specialists what tasks can or should be undertaken with the aid of microcomputers. Secondly it means being able to choose and maintain the right machinery for the tasks needed. An overview of applications shows four main areas of use. 1) The standard packages where one can expect the computer manufacturer to provide a complete working system. These are word-processing, database reference systems, spreadsheet calculations, graphics production and communications. 2) Specialist scientific programs such as statistics, data processing and model making. 3) The use of the microcomputer as a laboratory instrument for control or measurement. 4) Educational applications. Each of these areas has separate, often conflicting, requirements which can only be balanced by the user.

Biological antioxidants

The mechanism of lipid peroxidation and the ways in which the rate of this reaction can be reduced by small quantities of certain specific chemicals, called antioxidants, are described. The types and roles of the different antioxidants found in living systems are considered. Vitamin E (alpha-tocopherol) has long been recognized as an important lipid-soluble, chain-breaking antioxidant. It has an unexpectedly high reactivity towards peroxyl radicals, which can be understood only after detailed consideration of its structure. It is the major antioxidant of its class in human blood and its effectiveness in plasma is greatly improved by a synergistic interaction with water-soluble reducing agents such as ascorbic acid. Experiments designed to locate vitamin E within phospholipid bilayers and to discover the origin of the different biopotencies of stereoisomers of alpha-tocopherol are also described.

Estimation of the location of natural alpha-tocopherol in lipid bilayers by 13C-NMR spectroscopy

Natural, 2R,4R',8R'-alpha-tocopherol (vitamin E), labelled selectively with 13C in the methyl group at position 5, was incorporated into unilamellar vesicles of egg phosphatidylcholine. The vesicles are impermeable to the shift reagent Pr3+ and, in the presence of this reagent, separate 13C resonances due to labelled alpha-tocopherol in the outer and inner monolayers could be observed with relative intensities, 2:1. Subsequent addition of the relaxation reagent Gd2+ causes broadening and greatly shortened spin-lattice relaxation times for the resonance due to alpha-tocopherol in the outer monolayer only. These data confirm that alpha-tocopherol is located in both halves of the bilayers with its more hydrophilic chroman moiety very near the lipid-water interface, and indicate that the methyl group at position 5 of the alpha-tocopherol in the inner monolayer must be at least 40 A from the aqueous interface of the outer monolayer.

Acyl chain dynamics of phosphatidylethanolamines containing oleic acid and dihydrosterculic acid: 2H NMR relaxation studies

The dynamical behavior of the acyl chains of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine, and 1-palmitoyl-2-dihydrosterculoyl-sn-glycero-3-phosphoethanolamine has been investigated by using 2H T1 and T2 relaxation times. Lipids were labeled at the 5-,9-,10-, and 16-positions of the sn-2 acyl chain. The profile of deuterium spin-lattice relaxation rate (T1(-1) vs. chain position is characterized in all systems by a marked discontinuity at the positions of the carbon-carbon double bond and the cyclopropane ring; the deuterons at these positions have relaxation rates which are greater than at any other labeled position of the sn-2 chain. For both types of sn-2 acyl chain, assuming a single-exponential correlation time and that the motion is within the rapid regime, the phosphatidylcholine lipid systems are less mobile than their phosphatidylethanolamine analogues. Systems containing an oleoyl chain are more dynamic than their analogues containing a dihydrosterculoyl chain. The rates of motion of the sn-2 acyl chains of phosphatidylethanolamine in a bilayer structure are slower than those of the lipid in an inverted hexagonal structure. In the hexagonal phase, the motional rates of a dihydrosterculoyl chain are slower than those of the corresponding positions of an oleoyl chain.

Plasmodium berghei: gluconeogenesis in the infected mouse liver studied by 13C nuclear magnetic resonance

Using 13C nuclear magnetic resonance, we have compared the gluconeogenic activity of perfused livers isolated from normal starved mice and mice highly parasitized with Plasmodium berghei, using [2-13C]pyruvate as substrate. In both types of livers, 13C labeling of glucose carbons occurred in positions 1, 2, 5, and 6. The equal proportions of [1,6-13C]- and [2,5-13C]glucose in livers from malarial and normal mice suggests that pyruvate enters the gluconeogenic pathway directly and, to an equal extent, via the tricarboxylic acid cycle. The normalized signal heights indicated that at a given time after the addition of [2-13C]pyruvate the degree of 13C labeling in glucose carbons was reduced in livers from malarial animals, when compared to livers from normal animals. During the course of the perfusion experiment, the [2-13C]lactate resonance signal was always more intense from livers of malarial animals than from normal animals. A reduced activity of hepatic gluconeogenesis in malarial animals was further confirmed by a separate set of perfusion experiments which showed a 56% reduction of the measured rate of glucose production in livers from malarial animals, with respect to that of normal animals. A lowered NAD/NADH ratio in livers from malarial animals would explain the increased proportion of lactate observed in the spectra and be related to a decreased gluconeogenic rate. A more reduced oxidoreduction level in the hepatocytes of a malarial animal would result from a defect in the oxidative phosphorylation activity of mitochondria.

31P nuclear magnetic resonance studies of crayfish (Orconectes virilis). The use of inversion spin transfer to monitor enzyme kinetics in vivo

31P nuclear-magnetic resonance (NMR) has been used to observed in vivo the steady-state levels of phosphorus-containing metabolites in the crayfish Orconectes virilis and the intracellular pH of the abdominal muscle was determined. Measurement of spin-lattice relaxation times and spin transfer experiments have enabled calculation of unidirectional rate constants and activation energies for the arginine kinase reaction in vivo.

2H NMR evidence for antibiotic-induced cholesterol immobilization in biological model membranes

The interaction of the polyene antibiotic filipin with membrane sterols has been studied by deuterium nuclear magnetic resonance of the molecular probes [2,2,3,4,4,6-2H6]cholesterol and 1-myristoyl-2-[4',4',14',14',14'-2H5]myristoyl-sn-glycero-3-phospho- choline. At physiological temperatures, there is evidence of filipin-induced cholesterol immobilization in the membrane. The 2H NMR spectra of cholesterol show two domains in which ordering and dynamics are very different. In one of these, cholesterol is static on the 2H NMR time scale, whereas in the other it undergoes rapid axially symmetric motions similar to those it exhibits in the drug-free membrane; this indicates that the jumping frequency of cholesterol between the labile and immobilized domains is less than 10(5) s-1. The distribution of cholesterol between these two sites is temperature dependent; at 0 degrees C all sterol molecules are immobilized, whereas at 60 degrees C they are almost totally in the labile site. In contrast to cholesterol, the phospholipids sense only one type of environment, at both the top and center of the bilayer, indicating that cholesterol acts as a screen, preventing the lipids from direct interaction with the antibiotic. At low temperature, the ordering of the lipid in the presence of cholesterol does not change upon filipin addition, whereas at elevated temperatures the local ordering of both the lipid and the labile cholesterol is significantly lower than that in the absence of the drug. Moreover, there is a very important difference between the degree of local ordering as measured by the lipids and by cholesterol at high temperatures.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of replacement of a double bond by a cyclopropane ring in phosphatidylethanolamines: a 2H NMR study of phase transitions and molecular organization

The thermotropic behavior and molecular properties of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and 1-palmitoyl-2-dihydrosterculoyl-sn-glycero-3-phosphoethanolamine (PDSPE) have been investigated by 2H NMR spectroscopy using samples selectively labeled at the 5'-, 9'-, 10'-, and 16'-positions of the sn-2 chains. Comparison with the corresponding phosphocholine analogues (POPC and PDSPC), obtained as intermediate synthetic products, was used to monitor the role of the polar head group. Replacement of the choline moiety by ethanolamine increased the gel to liquid-crystal transition temperature by 10-32 degrees C and led to a significantly higher ordering of the fatty acyl chains in the liquid-crystalline bilayer state. The lateral compression effect, due to the smaller area per polar head group in PE, results in a bilayer to hexagonal phase transition at elevated temperatures. The effects on both PC and PE due to replacement of the olefinic group by a cyclopropane unit are similar. A decrease in the temperature of the gel to liquid-crystal phase transition, Tc, is observed upon introduction of a cyclopropane ring; it goes from 26 degrees C in POPE to approximately 10 degrees C in PDSPE. In addition, a very significant broadening of the transition profile is observed. These observations are consistent with the poor packing ability of mixed saturated and cyclopropane-containing chains due to the bulky substituent effect. The temperature of the bilayer-hexagonal phase transition of PE samples was decreased by 15-20 degrees C on replacement of oleoyl chains by dihydrosterculoyl chains at the sn-2 position.(ABSTRACT TRUNCATED AT 250 WORDS)

Magnetic resonance studies of the pathophysiology of murine malaria

The non-invasive and non-destructive aspects of NMR and ESR spectroscopy have prompted a variety of research on the pathophysiological impact of murine malaria. NMR is unique in its ability to monitor intracellular pH non-invasively in a heterogeneous sample, a compartmentalized cell and in a whole organism. It has also been shown to be sensitive to unusual structures and metabolic products in free-living protozoa such asAcanthamoeba(Deslaurierset al.1982a) andTetrahymena(Deslaurierset al.1982b; Jarrellet al.1981). Using the appropriate spin probe, ESR can give valuable information on membrane structure (Schreier, Polnaszek & Smith; 1978). It is particularly useful when quantities of material are limited.

A versatile perfusion technique for metabolic studies by NMR

A perfusion technique applicable to metabolic studies by NMR is described in detail. The utility and versatility of the approach are demonstrated by following gluconeogenesis from [2-13C]pyruvate in the perfused, isolated mouse liver, and lipogenesis from [2-13C]acetate in perfused Acanthamoeba castellanii cells embedded in agarose filaments.

Interaction of amphotericin B with membrane lipids as viewed by 2H-NMR

The effects of amphotericin B upon the organization and dynamics of multibilayer membranes of dimyristoylphosphatidylcholine (DMPC) were investigated by means of 2H-NMR. At high amphotericin B concentrations (30 mol% with respect to the lipid) and at temperatures above 25 degrees C, DMPC experiences two different environments which are in slow exchange on the 2H-NMR time scale. In one of these, the lipid is immobilized by the antibiotic, in a molar ratio of approximately 1:1, whereas the lipid unsequestered by amphotericin B is more ordered than in its pure state. This ordering effect is perceived at relatively low antibiotic doses (4%). The local lipid order, and the relative percentage, of sequestered DMPC, are temperature-independent (up to 65 degrees C), whereas the ordering of the unsequestered lipid domain is not. The perturbation induced by amphotericin B is manifest similarly at the edges as well as in the center of the bilayer. Antibiotic addition leads to large decreases in the transverse relaxation time, T2, of the labelled lipid, but not in the spin-lattice relaxation time, T1. This indicates an increased density of slow motional modes and little change in rapid motions.

Use of 13C nuclear magnetic resonance spectroscopy for the evaluation of hepatic drug-metabolizing enzyme activity in perfused mouse livers

Experiments were performed to determine whether the activity of hepatic drug-metabolizing enzymes could be estimated in intact livers by 13C NMR spectroscopy. 13C-labelled aminopyrine was administered to isolated perfused mouse livers and the rate of decline in the NMR signal arising from the drug was used to estimate the half-life of aminopyrine. The technique was sufficiently sensitive to discriminate between the enzyme activities in normal livers and in livers whose enzyme activity was enhanced by treatment with sodium phenobarbitone. It was also demonstrated that demethylation of aminopyrine was not impaired in perfused livers from mice infected with Plasmodium berghei.

Changes in miniature end-plate potentials after brief nervous stimulation at the frog neuromuscular junction

The amplitude of miniature end-plate potentials (m.e.p.p.s), recorded at the frog neuromuscular junction in a normal ionic environment and in absence of drugs, was examined following 10-450 nerve impulses using conventional electrophysiological techniques and on-line computational analysis. In both contracting preparations and non-contracting preparations pre-treated with glycerol, 100 or more nerve impulses resulted in a maximal fall in mean amplitude of about 20% with recovery apparent over the next 10-20 min. In an altered ionic environment with a lowered Ca and raised Mg concentration, 450 nerve impulses did not produce a decrease in mean amplitude but a similar reduction was seen following a larger number of impulses. The reduction in amplitude was estimated to follow the release of the order of 5000-10000 quanta at end-plates in a normal ionic environment and on average 17000 quanta in the presence of a lowered Ca and raised Mg concentration. Changes in the mean size of the spontaneous quantal response is considered to be a presynaptic event and to reflect the loss and slow recovery of larger packets of transmitter from a vesicular store that is readily released by nerve impulses.

Amphotericin and model membranes. The effect of amphotericin B on cholesterol-containing systems as viewed by 2H-NMR

The interactions between amphotericin B and sterol-containing model membranes were monitored by 2H-NMR of deuterium-labelled dimyristoylphosphatidylcholine (DMPC), cholesterol and epicholesterol. The addition of amphotericin B to a cholesterol/DMPC (3:7) system was perceived differently by the lipid, depending upon the depth in the bilayer: no structural change was manifest in the acyl chain region associated with the plateau in molecular ordering (C4'), whereas the lipid clearly senses two environments near the center of the bilayer (C13', C14'). The amount as well as the ordering properties of the more ordered antibiotic-induced component, sensed at C14', increased with decreasing temperature. The structural parameters of deuterium-labelled cholesterol in cholesterol/DMPC mixtures were unchanged upon addition of amphotericin B, regardless of the bilayer depth. Upon addition of amphotericin B, the lipid T1 values are unchanged, whereas the T2 values are reduced by a factor of 2. The minimum in T1 observed for cholesterol in DMPC at 32-35 degrees C was shifted towards 38-40 degrees C in the presence of amphotericin B. Epicholesterol-containing dispersions of DMPC had properties similar to those of their cholesterol-containing analogs; a noticeable difference between the systems was an approx. 10% increase in the segmental order parameters on the addition of amphotericin B to the system containing the alpha-isomer of cholesterol. The concept of a dynamic complexation between amphotericin B and sterol is discussed.