Phase Transition Of Dimyristoylphosphatidylcholine Research Articles

Kinetics of lipid mixing between bicelles and nanolipoprotein particles

Nanolipoprotein particles (NLPs), also known as nanodiscs, are lipid bilayers bounded by apolipoprotein. Lipids and membrane proteins cannot exchange between NLPs. However, the addition of bicelles opens NLPs and transfers their contents to bicelles, which freely exchange lipids and proteins. NLP–bicelle interactions may provide a new method for studying membrane protein oligomerization. The interaction mechanism was investigated by stopped flow fluorometry. NLPs with lipids having fluorescence resonance energy transfer (FRET) donors and acceptors were mixed with a 200-fold molar excess of dihexanoyl phosphatidylcholine (DHPC)/dimyristoyl phosphatidylcholine (DMPC) bicelles, and the rate of lipid transfer was monitored by the disappearance of FRET. Near or below the DMPC phase transition temperature, the kinetics were sigmoidal. Free DHPC and apolipoprotein were ruled out as participants in autocatalytic mechanisms. The NLP–bicelle mixing rate showed a strong temperature dependence (activation energy=28kcal/mol). Models are proposed for the NLP–bicelle mixing, including one involving fusion pores.

Structural features and functional properties of water in model DMPC membranes: thermally stimulated depolarization currents (TSDCs) and Fourier transform infrared (FTIR) studies

Thermally stimulated depolarization currents (TSDCs) and Fourier transform infrared (FTIR) spectroscopies were employed to investigate the state of water incorporated in a model DMPC (dimyristoyl-phosphatidylcholine) membrane. The lipid multilayers, highly inhomogeneous from the dielectric point of view, originate complex TSDC spectra critically dependent on the sample water content and thermal history. Different temperature ranges were chosen to polarize the sample, i.e. 100–300 K (type I) and 100–285 K (type II). The purpose of the latter choice was to avoid any sample heating above the DMPC phase transition temperature (295 K) along the sample polarization. According to the results, water in a fully hydrated system (aw = 0.92) (1) is ordered around the hydrophilic head molecular groups, (2) is layered in the interbilayer space and (3) penetrates among the hydrocarbon chains. It can assume different local structural configurations depending on the lipid packing. Irreversible conformational transitions in the lipid array system were monitored as a consequence of different dehydration treatments.FTIR absorption measurements were performed to study the water sorption kinetics into a DMPC thin film. The water related OH band was decomposed into three components, describing three water states, with different propensity to the H-bond formation. The changes of the lipid characteristic groups (CH2/CH3, and C=O) absorption bands as a function of increasing hydration level were monitored and discussed.

Abstract 105: Free Cholesterol Determines the Phospholipid Domain Size and Stability of rHDL

Background: Microsolubilization of dimyristoyl phosphatidylcholine (DMPC) by human apolipoprotein A-I is a frequently used model system for the identification of determinants of macrophage cholesterol efflux, the first step in reverse cholesterol transport (RCT) to hepatic disposal. Although many studies have focused on microsolubilization of DMPC, most have been conducted under stoichiometric conditions resulting in ∼9 nm rHDL, and few have included free cholesterol (FC), a key lipid in RCT. Methods: Various rHDL species (excess DMPC + FC), were prepared and analyzed by size exclusion chromatography (SEC), differential scanning calorimetry (DSC) and circular dichroism (CD) spectroscopy as a function of temperature. Results: At each mol% of FC/DMPC, multiple rHDL species differing in size from ∼10 to ∼25 nm were formed. In SEC, rHDL size increased with the mol% FC in the microsolubilization reaction and in a given reaction mixture the large rHDL were always more FC-rich than the small rHDL. In DSC, the DMPC transition range of rHDL was broader than that of pure DMPC and although the transition temperature moderately increases with increasing mol% FC, the apparent enthalpy of the transition is diminished. The thermal dependence of the CD spectra showed that increasing mol% FC and rHDL size moderately increases the stability of apo A-I. Conclusion: Although large FC-rich and small FC-poor domains coexist on DMPC surfaces; the size of these domains increases with mol% FC. This effect of FC on rHDL size decreases the overall cooperativity of the DMPC phase transition and reduces the number of acyl chain interactions. Physiologically, these data suggest that increased FC-loading of macrophages should increase the size of nascent HDL produced by the interaction of apo A-I with cellular ABCA1. The lower stability of FC-poor HDL would likely make it release more lipid-free apo A-I in response to important plasma proteins including LCAT, PLTP, and CETP and in response to serum opacity factor.

Effects of Small Neutral Molecules on Phospholipid Bicelle Ordering

The effects of small neutral molecules on the liquid-crystalline ordering of dimyristoyl-phosphatidylcholine (DMPC)/dihexanoyl-phosphatidylcholine (DHPC) bicelles (q = 3.0 and 3.5) were studied via 2H, 31P, and 13C variable-temperature NMR. The addition of chloroform (up to approximately 90 mM, with a lipid concentration of approximately 120 mM) was observed to reduce the temperature onset of bicelle ordering by up to approximately 10 degrees C, likely resulting from the depression of the DMPC phase transition temperature. The temperature for the collapse of the bicelle phase was also significantly reduced; the observed effects amount to a downward shift in temperature (and reduction in range) of the liquid-crystalline portion of the bicelle phase diagram with increasing dopant concentration. Other model dopants (e.g., tetrahydrofuran and benzene) yielded smaller effects. Additionally, the variable bicelle alignment permitted the characterization of the ordering of chloroform molecules within the lipid phase.

The DMPC lipid phase transition influences differently the first and the second electron transfer reactions in bacterial reaction centers

Photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides were incorporated in dimyristoylphosphatidylcholine (DMPC) liposomes. The first and second electron transfer rates ( k AB(1) and k AB(2), respectively) between the first and the second quinone electron acceptors have been measured as a function of temperature, across the phase transition of DMPC (23°C). The Eyring plots of k AB(1) display straight lines. In contrast, the Eyring plots for k AB(2) in proteoliposomes show a break at about 23.5°C. This physical discrimination between the two electron transfer reactions demonstrates that the stiffness of the lipid environment of the RCs and/or the protein–protein interactions influence the parameters governing k AB(2), but not the gating process limiting k AB(1).

Effects of cholesterol and temperature on the permeability of dimyristoylphosphatidylcholine bilayers near the chain melting phase transition

The passive leakage of glucose across bilayers of dimyristoylphosphatidylcholine (DMPC), cholesterol (variable), and dicetyl phosphate (constant 5.9 mol%) has been measured as efflux over 30 min from multilamellar vesicles. Bilayer cholesterol was varied from 20 mol% to 40 mol%. Glucose permeation rates were measured from 10°C to 36°C, and showed a maximum in permeability at 24°C, the DMPC phase transition temperature. Increasing the bilayer cholesterol content above 20 mol% reduced that permeability peak. These results are quite consistent with a large number of similar bilayer permeability studies over the past 25 years. However, they are not consistent with a previous study of these same systems, which reported increased glucose permeability with temperature, without any maximum at or near the lipid chain melting temperature (K. Inoue, Biochim. Biophys. Acta 339 (1974) 390–402).

Solvent interconnectedness permits measurement of proximal as well as distant phase transitions in polymer mixtures by fluorescence

We monitored the fluorescence intensity and anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) incorporated in bovine serum albumin (BSA) and dimyristoylphosphatidylcholine (DMPC) vesicle membranes, which in turn were embedded in optically clear gelatin solutions, as a function of temperature. DPH in BSA gave unanticipated large changes in fluorescence intensity and anisotropy at the instant of gelatin gel melting. Both steady state anisotropy and fluorescence intensity reported the gel–sol transition point in gelatin unambiguously, which was independently confirmed as physical-pour point of the gel. In the case of DMPC vesicles, fluorescence intensity indicated the gelatin transition, while the anisotropy indicated DMPC phase transition. This fluorescence methodology uniquely offered a common probe for two distinct transitions in two distinct domains interconnected by the solvent, water.

Reconstitution of the glycosylphosphatidylinositol-anchored protein Thy-1: interaction with membrane phospholipids and galactosylceramide

Glycosylphosphatidylinositol (GPI)-anchored membrane proteins are proposed to interact preferentially with glycosphingolipids and cholesterol to form microdomains, which may play an important role in apical targeting and signal transduction. The objective of the present study was to investigate the interaction of the GPI-anchored protein Thy-1 with phospholipids and a glycosphingolipid. Purified Thy-1 was reconstituted into lipid bilayer vesicles of dimyristoyl-phosphatidylcholine (DMPC) alone or in combination with galactosylceramide (GC). The ability of Thy-1 to perturb the gel to a liquid-crystalline phase transition of DMPC was examined by differential scanning calorimetry. As the mole fraction of Thy-1 increased, the phase transition enthalpy, deltaH, declined. Analysis indicated that each molecule of Thy-1 perturbed over 50 phospholipids, suggesting that, in addition to the anchor insertion into the bilayer, the protein itself may interact with the membrane surface. Inclusion of 5% w/w GC in the bilayer resulted in a striking change in the interaction of Thy-1 with phospholipids. At low Thy-1 content, there was a reduction in the phase transition temperature and an increase in phospholipid cooperativity, suggesting the formation of Thy-1/GC-enriched domains. DeltaH initially decreased with increasing Thy-1 content of the bilayer; however, at higher Thy-1 mole ratios, deltaH rose again. These results are interpreted in terms of a model whereby, at low protein:lipid mole ratios, Thy-1 preferentially sequesters GC to form enriched microdomains. At high protein:lipid mole ratios, Thy-1 may alter its conformation in response to steric crowding within these domains such that its interaction with the bilayer surface is reduced.

A Calorimetric Investigation of the Interaction of Short Chain Alcohols with Unilamellar DMPC Liposomes

The molecular mechanisms by which ethanol and other short-chain alcohols perturb biological systems have been suggested to involve specific interactions with proteins and/or nonspecific interactions with the lipid bilayer part of biological membranes. To gain insight into the influence of alcohols on well-defined lipid bilayers, isothermal titration calorimetry (ITC) is used to determine the excess partial molar enthalpy of alcohol in liposome suspensions as well as the excess partial molar enthalpy of liposomes in dilute alcohol−water mixtures at temperatures above the main phase transition of dimyristoyl phosphatidylcholine (DMPC). The alcohols investigated were ethanol, 1-propanol, and 1-butanol. The calorimetric data were used to evaluate molecular interactions and based on a simple partitioning model the standard molar enthalpy of transferring alcohol from water into unilamellar DMPC liposomes was determined. The results reveal that the interaction of short-chain alcohols with the DMPC lipid bilayer ...

Temperature dependent intermediate structures during the main phase transition of dimyristoyl phosphatidylcholine vesicles a combined iodine laser-temperature jump and time resolved cryo-electron microscopy study

The kinetics of the main phase transition of dimyristoylphosphatidyl choline (DMPC) unilamellar vesicles were investigated in the time range from microseconds to seconds. Iodine laser-temperature jump (ILTJ) experiments showed three discrete relaxation phenomena. Time resolved cryo-electron microscopy (CEM) was applied to produce images of intermediate states typical for the relaxation times of lipid vesicles in the micro- to millisecond time window. A careful measurement of the rate of temperature decrease observed during the production of vitrified lamellae of aqueous samples on a copper grid was performed. The best conditions resulted in average rates of cooling of 3 × 10 4 K/s. By comparing the images from CEM of DMPC vesicle samples vitrified above, at, and below the phase transition temperature a structural model was designed, which explains the temperature jump relaxation times in the micro- to millisecond time range by the formation and disappearance of coexisting clusters of crystalline, intermediate, and fluid lipid areas inside the DMPC bilayers.

Antioxidant activity of probucol and its effects on phase transitions in phosphatidylcholine liposomes

The effect of probucol on the phase behavior of dimyristoylphosphatidylcholine (DMPC) was examined by fluorescence polarization and differential scanning calorimetry (DSC). Probucol broadens and shifts the temperature of the main phase transition of DMPC liposomes as measured by fluorescence polarization with diphenylhexatriene and trimethylammonium-diphenylhexatrine at concentrations as low as 5 mole%. As measured by DSC, probucol reduces the transition temperature of the gel å liquid-crystalline phase transition of DMPC by approx. 2 C° at all concentrations above about 5 mole% probucol and eliminates the pretransition at < 1 mole% . In addition, the phase transition of DMPC is broadened and the enthalpy of the transition reduced by approx. 50%. Even at high concentrations of probucol, the gel å liquid-crystalline phase transition of DMPC is not eliminated. Similar effects are observed with dipalmitoylphosphatidylcholine liposomes. Based on these DSC measurements, measurements of the melting of probucol in dry mixtures with DMPC and observations of probucol mixtures with DMPC under polarizing optics, the maximum solubility of probucol in DMPC is approx. 10 mole%. This concentration exceeds that required (approx. 0.5 mole%) to prevent peroxidation of 10 mole% arachidonic acid in DMPC liposomes for 30 min in the presence of 0.05 mM Fe(NH 4)(SO 4) 2 at 4°C. Thus, probucol has a limited solubility in saturated phosphatidylcholine bilayers, but is an effective antioxidant at concentrations lower than its maximum solubility.

The effects of lipid composition on the binding of lasalocid A to small unilamellar vesicles

The binding of the carboxylic ionophore lasalocid A (X537A) to small unilamellar phospholipid vesicles of varying composition was examined in an effort to determine what structural features of the phospholipid membrane influence the ionophore-membrane interaction. Apparent dissociation constants ( K app) were calculated for both the acidic and anionic forms of the ionophore using the change in fluorescence intensity observed for lasalocid A upon addition of phospholipid vesicles. The K app for binding to fluid phase dimyristoylphosphatidylcholine (DMPC) vesicles is 46 μM for the anion and 14 μM for the acid. While the phase transition of DMPC had no effect on the K app of the anion, an increase was observed in the K app of the acid below the phase transition temperature. The K app of the anion was not affected by the incorporation of 10% dimyristoylphosphatidylethanolamine (DMPE), but increased slightly upon incorporation of cholesterol. The p K a values of the ionophore were the same in DMPC and DMPC/DMPE membranes. Incorporation of the negative lipids phosphatidylglycerol, phosphatidic acid, or phosphatidylethanolamine (at pH 9.4 where PE carries a negative charge) decreases binding of the anion in accord with the increase in surface potential estimated from Gouy-Chapman theory. The CD spectrum of membrane-bound lasalocid A anion indicated the ionophore to be in an extended acyclic conformation on the membrane surface with the C-1 carboxylate rotated out of the plane of the salicylate ring. The out-of-plane rotation of the carboxylate may be the result of facial binding by the amphiphilic ionophore on the membrane surface or of weak ion pairing to the polar lipid head groups. These results suggest that the primary determinants of binding of the anionic ionophore on the membrane surface are packing density of the polar head groups and membrane surface potential. There is no evidence of strong hydrogen bond formation between the lipid polar head groups and the ionophore as has previously been suggested.

The effects of lipid composition on the rate and extent of heme binding to membranes.

The effects of membrane composition on heme binding to large unilamellar vesicles were examined using 30 separate phospholipid mixtures. Although there was some variation, most lecithins with Tm values less than or equal to 20 degrees C showed overall equilibrium partition constants equal to approximately 5 x 10(5) and association and dissociation partition rate constants equal to approximately 3 x 10(6) s-1 and 7 s-1, respectively, for CO-heme binding at 30 degrees C. A sharp decrease in the association rate for CO-heme uptake was observed as the lipid vesicles changed from liquid-crystalline to the gel phase. The addition of dicetyl phosphate or dimyristoylphosphatidylglycerol, which are negatively charged at neutral pH, decreased the affinity of the vesicles for CO-heme. The association rate and equilibrium partition constants for CO-heme uptake in unsaturated lecithins were unaffected by cholesterol content at levels up to 40%/mol. The affinity of saturated dimyristoylphosphatidylcholine (DMPC) vesicles for CO-heme decreased with increasing cholesterol content at 30 degrees C. This effect appears to be related to the influence of cholesterol on the DMPC phase transition temperature (Tm) since at low temperatures (less than or equal to 20 degrees C) little CO-heme binds to vesicles composed of DMPC even in the absence of cholesterol.

NMR, calorimetric, spin-label, and optical studies on a trifluoromethyl-substituted styryl molecular probe in dimyristoylphosphatidylcholine vesicles and multilamellar suspensions: a model for location of optical probes

NMR, calorimetric, and optical spectroscopic studies have been performed on a trifluoromethyl-substituted styryl molecular probe bound to vesicles and multilamellar suspensions formed from dimyristoylphosphatidylcholine (DMPC). In the fluorine NMR spectrum at 35° C there are two partially resolved resonances, but these collapse to an apparently single resonance at temperatures above 60° C. However, a line-shape analysis is not consistent with exchange between two sites on an NMR time scale, and the two resonances are assumed to be due to probe sites in the inner and outer leaflets of the vesicles. Two fluorescence lifetimes, each associated with one of these sites, characterize the decay curves for the molecular probe bound to DMPC vesicles. The shift reagent Eu(FOD) 3 and several nitroxide spin labels covalently bound to lipophilic structures strongly attenuate the lower frequency component of the fluorine NMR spectrum and also shift the other resonance to higher frequencies. The effect of two spin labels on the probe fluorine T2 relaxation time has been used to estimate the distance between the spin label unpaired electron and the trifluoromethyl group. The location of the spin label site in the membrane was determined from the effect of the unpaired electron on the lipid 13C linewidths. A model for the location of the probe in the bilayer was developed from the above information and refined using molecular mechanics calculations on a probe-DMPC lipid complex. The long axis of the probe parallels the bilayer normal; the styryl-group portion of the optical chromophore is located slightly below the glycerol backbone, and the remainder of the chromophore extends well into the hydrophobic region of the bilayer. Therefore, the optical properties of the probe should not be significantly influenced by alterations of the membrane surface charge density. Parameters derived from DSC studies in the gel-to-lipid crystal phase transition of DMPC are extremely sensitive to the probe. Even at 0.0001 mol fraction of probe, the transition is substantially broadened, and the ΔH for the transition has increased, just as one predicts for the formation of a tight complex described above.

Lateral diffusivity of lipid analogue excimeric probes in dimyristoylphosphatidylcholine bilayers

The lateral mobility of pyrenyl phospholipid probes in dimyristoylphosphatidylcholine (DMPC) vesicles was determined from the dependence of the pyrene monomeric and excimeric fluorescence yields on the molar probe ratio. The analysis of the experimental data makes use of the milling crowd model for two-dimensional diffusivity and the computer simulated random walks of probes in an array of lipids. The fluorescence yields for 1-palmitoyl-2-(1'-pyrenedecanoyl)phosphatidylcholine (py10PC) in DMPC bilayers are well fitted by the model both below and above the fluid-gel phase transition temperature (Tc) and permit the evaluation of the probe diffusion rate (f), which is the frequency with which probes take random steps of length L, the host membrane lipid-lipid spacing. The lateral diffusion coefficient is then obtained from the relationship D = fL2/4. In passing through the fluid-gel phase transition of DMPC (Tc = 24 degrees C), the lateral mobility of py10PC determined in this way decrease only moderately, while D measured by fluorescence photobleaching recovery (FPR) experiments is lowered by two or more orders of magnitude in gel phase. This difference in gel phase diffusivities is discussed and considered to be related either to (a) the diffusion length in FPR experiments being about a micrometer or over 100 times greater than that of excimeric probes (approximately 1 nm), or (b) to nonrandomicity in the distribution of the pyrenyl probes in gel phase DMPC. At 35 degrees C, in fluid DMPC vesicles, the diffusion rate is f = 1.8 x 10(8) s-1, corresponding to D = 29 microns2 s-1, which is about three times larger than the value obtained in FPR experiments. The activation energy for lateral diffusion in fluid DMPC was determined to be 8.0 kcal/mol.

Thermomechanical and transition properties of dimyristoylphosphatidylcholine/cholesterol bilayers.

Mixtures of dimyristoylphosphatidylcholine (DMPC) and cholesterol (Chol) have been used to examine the effects of cholesterol on the chain crystallization transitions and thermomechanical properties in phospholipid bilayer membranes. The mechanical properties--elastic moduli and level of tension at membrane rupture--were derived from micropipet pressurization of giant single-walled vesicles. Also, the micropipet method allowed temperature-dependent area transitions to be measured at constant membrane tension. X-ray diffraction measurements were made on selected lipid/cholesterol mixtures. Wide-angle patterns and electron density profiles were used to measure bilayer thickness as an indication of chain tilt and fluidity. Vesicle area versus temperature plots showed that the main acyl chain crystallization transition of DMPC broadened and shifted to higher temperatures. Both above and below the broad transition, the elastic area compressibility modulus, K, was greatly increased with cholesterol addition. The value for the 1:1 DMPC/Chol complex was found to be approximately 700 dyn/cm, comparable to that for DMPC in the L beta' phase. However, for all concentrations above 12.5 mol % (which was weakly solid), vesicle bilayers behaved as surface liquids with no surface shear rigidity even at temperatures well below the DMPC phase transition. Area changes over the broadened transitions were reduced by cholesterol and disappeared with the addition of 50 mol % to leave the thermal area expansivity at 1.3 X 10(-3)/degrees C. These area changes are consistent with separate formation of a 1:1 DMPC/Chol complex that does not condense plus residual free lipid and lipid loosely associated with the 1:1 complex that freezes normally.(ABSTRACT TRUNCATED AT 250 WORDS)

Phospholipase A2 assay using an intramolecularly quenched pyrene-labeled phospholipid analog as a substrate

A phospholipid analog 1-palmitoyl-2–6(pyren-1-yl)hexanoyl- sn-glycero-3-phospho- N-(trinitrophenyl)aminoethanol (PPHTE) in which pyrene fluorescence is intramolecularly quenched by the trinitrophenyl group was used as a substrate for pancreatic phospholipase A2. Upon phospholipase A2 catalyzed hydrolysis of this molecule pyrene monomer fluorescence emission intensity increased as a result of the transfer of the pyrene fatty acid to the aqueous phase. Optimal conditions for phospholipase A2 hydrolysis of PPHTE were similar to those observed earlier for other pyrenephospholipids (T. Thuren, J. A. Virtanen, R. Verger, and P. K. J. Kinnunen (1987) Biochim. Biophys. Acta 917, 411–417) . Although differential scanning calorimetry revealed no thermal phase transitions for PPHTE between +5 and +60°C the Arrhenius plot of the enzymatic hydrolysis of the lipid showed a discontinuity at 30°C. The molecular origin of this discontinuity remains at present unknown. To study the effects of dimyristoylphosphatidylcholine (DMPC) phase transition at 23.9°C on phospholipase A2 reaction PPHTE was mixed with DMPC in a molar ratio of 1:200 in small unilamellar vesicles. The hydrolysis of DMPC-PPHTE vesicles was measured by following the increase in pyrene monomer fluorescence emission due to phospholipase A2 action on PPHTE. Below the phase transition of DMPC the enzymatic reaction exhibited a hyperbolic behavior. At the transition as well as at slightly higher temperatures a lag period was observed. The longest lag period was approximately 20 min. Above 26°C no lag time could be observed. However, the reaction rates were slower than below the phase transition temperature. After the lag period maximal reaction rates were observed at the transition temperature. These results are consistent with results from studies using pure DMPC as substrate for phospholipase A2 and indicate that the trinitrophenyl-labeled pyrenephosphatidylethanolamine did not cause any major perturbation in phospholipase A2-DMPC interaction. Accordingly, these results suggest that PPHTE can be used as a probe to monitor the enzymatic degradation of nonlabeled phosphatidylcholines.

A comparison of the interaction of glucagon, human parathyroid hormone-(1-34)-peptide and calcitonin with dimyristoylphosphatidylglycerol and with dimyristoylphosphatidylcholine

The interaction of glucagon, human parathyroid hormone-(1–34)-peptide and salmon calcitonin with dimyristoylphosphatidylglycerol (DMPG) and with dimyristoylphosphatidylcholine (DMPC) was studied as a function of pH and temperature. The effect of lipid on the secondary structure of the peptide was assessed by circular dichroism and the effect of the peptide on the phase transition properties of the lipid was studied using differential scanning calorimetry. Some peptides interact more strongly with anionic than with zwitterionic phospholipids. This does not require an overall positive charge on the peptide. Increased thermal stability is observed in complexes formed between cationic peptides and anionic lipids. Particularly marked effects of glucagon and human parathyroid hormone-(1–34)-peptide on the phase transition properties of DMPG at pH 5 have been observed. The transition temperature is raised over 10°C at a lipid/peptide molar ratio of less than 30:1 and the transition enthalpy is increased over 2-fold. These effects do not occur with any basic peptide and were not observed with metorphinamide, molluscan cardioexcitatory neuropeptide or myelin basic protein. The results demonstrate that certain peptides can affect the phase transition properties of lipids in a manner similar to divalent cations. The overall hydrophobicities of these peptides can be evaluated by their partitioning between aqueous and organic solvents. None of the above three peptide hormones partition into the organic phase. However, a closely related peptide, human calcitonin, does exhibit substantial partitioning into the organic phase. Nevertheless, human calcitonin has a weaker interaction with both DMPC and DMPG than does salmon calcitonin. The effects of human calcitonin on the phase transition of DMPC are qualitatively different from those of salmon calcitonin in that the human form more readily eliminates the pretransition but causes less change in the main transition. Like overall charge, overall hydrophobicity is not an overwhelming factor in determining the ability of peptides to interact with phospholipids but rather more specific interactions are required for strong complexes to form.

Selectivity in rhodopsin-phospholipid interactions

This series of experiments systematically evaluated the effect of phospholipid headgroup structure on the interaction between rhodopsin and phospholipids. Two types of experiments were reported. First, ESR experiments involving spin-labeled phosphatidylserine, phosphatidic acid, and phosphatidylcholine demonstrated that, in the fluid-isotropic phase of dimyristoylphosphatidylcholine (DMPC)-rhodopsin membranes, the relative order of rhodopsin-induced immobilization was phosphatidic acid > phosphatidylcholine > phosphatidylserine. Second, the effect of rhodopsin incorporation on the dimyristoylphosphatidylserine (DMPS) gel to liquid-crystalline phase transition was analyzed with ESR techniques. A partial, binary phase diagram for the DMPS-rhodopsin system at pH 7.0 was constructed by studying the partitioning of Tempo between polar and hydrophobic domains as a function of temperature and system composition. A main result of this analysis was the finding that rhodopsin broadens and reduces the amplitude of the DMPS phase transition to a much smaller extent than it does the DMPC phase transition. When interpreted in terms of theoretical treatments of integral protein-lipid interactions, this indicates that rhodopsin has a lower affinity for DMPS than DMPC.

A scanning calorimetric study of small molecule-lipid bilayer mixtures.

The influence of small molecules on the main phase transition of dimyristoyl phosphatidylcholine was observed by high-sensitivity differential scanning calorimetry. The variation with temperature of the excess heat capacity is markedly affected by solid solution formation, which appears to be of frequent occurrence in solutions in lipid bilayers. The experimental results are in some cases in reasonable agreement with ideal solution theory.