Comparison of the enthalpy state of vesicles of different size by their interaction with α-lactalbumin

The characteristics of small unilamellar, large unilamellar and large multilamellar vesicles of dimyristoylphosphatidylcholine and their interaction with α-lactalbumin are compared at pH 4. (1) By differential scanning calorimetry and from steady-state fluorescence anisotropy data of the lipophilic probe 1,6-diphenyl-1,3,5-hexatriene it is shown that the transition characteristics of the phospholipids in the large unilamellar vesicles resemble more those of the multilamellar vesicles than of the small unilamellar vesicles. (2) The size and composition of the lipid-protein complex formed with α-lactalbumin around the transition temperature of the lipid are independent of the vesicle type used. Fluorescence anisotropy data indicate that in this complex the motions of the lipid molecules are strongly restricted in the presence of α-lactalbumin. (3) The previous data and a comparison of the enthalpy changes, $\text{ΔH}$, of the interaction of the three vesicle types with α-lactalbumin allow us to derive that the enthalpy state of the small unilamellar vesicles just below 24°C is about 24 kJ/mol lipid higher than the enthalpy state of both large vesicle types at the same temperature. The abrupt transition from endothermic to exothermic $\text{ΔH}$ values around 24°C for large vesicles approximates the transition enthalpy of the pure phospholipid     

The effect of anesthetic charge on anesthetic-phospholipid interactions

Cationic and uncharged forms of a tertiary amine local anesthetic are reported to have different properties and potencies as nerve blocking agents. However, the relative capacities of each form of the local anesthetic to perturb the properties of different model membrane systems is unknown. For this reason we have studied the effects of uncharged lidocaine (high pH) and its quaternary amine analogue (W49091) on the phase transition properties of DMPS, DPPE and DPPC liposomes using high-sensitivity differential scanning calorimetry. We report that neutral lidocaine interacts similarly with all three phospholipids. This interaction results in a decrease in the temperature of the gel å liquid crystalline phase transition ($\text{T}{}_{\mathrm{<mtext>m</mtext>}}$), an increase in the enthalpy of the transition ($\text{ΔH}$), and a slight decrease in the cooperativity of melting. Quaternary lidocaine (W49091), on the other hand, interacts significantly with only DMPS; the result being again a decrease in the temperature of DMPS melting, an increase in $\text{ΔH}$, and a slight decrease in the cooperativity of the phase transition. These results are interpreted to indicate that uncharged lidocaine enters the membrane during the DPPE and DPPC phase transitions. In the case of DMPS, an influx of both charged forms of lidocaine must occur at $\text{T}{}_{\mathrm{<mtext>m</mtext>}}$. These anesthetic fluxes at the lipid's phase transition are suggested to be responsible for the observed elevated enthalpies of the respective transitions. The observation that the cationic form of lidocaine does not significantly modify the behavior of DPPC and DPPE liposomes suggests that these lipids are not important components of the anesthetic's site in nerve membranes. However, the dramatic perturbation of the properties of DMPS by W49091 suggests that phosphatidylserine may comprise part of this inhibitory site.     

Raman spectroscopic study of an interdigitated lipid bilayer. Dipalmitoylphosphatidylcholine dispersed in glycerol

Dipalmitoylphosphatidylcholine (DPPC) dispersed in perdeuterated glycerol was investigated in order to determine the effects on the Raman spectra of hydrocarbon chain interdigitation in gel-phase lipid bilayers. Interdigitated DPPC bilayers formed from glycerol dispersions in the gel phase showed a decrease in the peak height intensity $\text{I}{}_{2850}\text{/I}{}_{2880}$ ratio, for the symmetric and asymmetric methylene CH stretching modes, respectively, as compared to non-interdigitated DPPC/water gel-phase dispersions. The decrease in this spectral ratio is interpreted as an increase in chain-chain lateral interactions. Spectra recorded in the 700–740 cm^?1 CN stretching mode region, the 1000–1200 cm^?1 CC stretching mode region and the 1700–1800 cm^? CO stretching mode region were identical for both the interdigitated and non-interdigitated hydrocarbon chain systems. At low temperatures the Raman peak height intensity ratios $\text{I}{}_{2935}\text{/I}{}_{2880}$ were identical for the DPPC/glycerol and DPPC/water dispersions, indicating that this specific index for monitoring bilayer behavior is insensitive to acyl chain interdigitation. The increase, however, in the change of this index at the gel-liquid crystalline phase transition temperature for the DPPC/glycerol dispersions implies a larger entropy of transition in comparison to the non-interdigitated DPPC/water bilayer system.     

The interaction of spectrin · actin and synthetic phospholipids

Using differential scanning calorimetry and freeze fracture electron microscopy interactions were studied between lipids and a spectrin · action complex isolated from human erythrocyte membranes. With dispersions of $\text{1,2-}\text{dimyristoyl}\text{-sn-}\text{glycero}\text{-3-}\text{phosphocholine}$, $\text{1,2-}\text{dimyristoyl}\text{-sn-}\text{glycero}\text{-3-}\text{phosphoglycerol}$ and mixtures of these two compounds, which for experimental reasons were chosen as the lipid counterpart, such an interaction could clearly be deduced from changes in the temperature and the enthalpy of the phase transition. Furthermore it was demonstrated that the interaction with this membrane protein protects the bilayer against the action of Ca²⁺ and Mg²⁺ and prevents fusion of lipid vesicles which easily occurs in some of the systems when divalent ions were added to the pure lipid vesicles.     

Phase transition properties of aqueous dispersions of homologues of all-trans 2,3-dipalmitoylcyclopentano-1-phosphocholine

In a previous publication, (Singer, M.A., Jain, M.K., Sable, H.Z., Pownall, H.H., Mantulin, W.W., Lister, M.D. and Hancock, A.J. (1983) Biochim. Biophys. Acta, 731, 373–377), we reported the properties of aqueous dispersions of the six diastereo-isomers of cyclopentanoid analogues of dipalmitoylphosphatidylcholine. Two of these isomers displayed unusually high enthalpies of transition, about double that of dipalmitoylphosphatidylcholine. One of the high enthalpy isomers whose configuration is all-trans has now been modified by the insertion of extra methylene residues ($\text{n = 3}$ through 9) between the nitrogen and phosphorus atoms of the headgroup. Vesicles were formed from these lipids and studied by ²²Na permeability measurements, differential scanning calorimetry, fluorescence polarization, ³¹P-NMR, and freeze-fracture electron microscopy. Vesicles composed of lipids with $\text{n = 2}$ or 3 exhibit a sharp transition at 46°C or 49°C, respectively, and a high enthalpy with no detectable sub- or pretransitions. Lipids with $\text{n > 3}$ exhibit a main transition between 38 and 43°C with enthalpies $\text{< 10}\text{kcal/mol}$ and after prolonged cooling (more than 3 days at 4°C) a broad endotherm at about $\text{20 ± 3°}\text{C}$ with enthalpies $\text{> 4}\text{kcal/mol}$. These same dispersions display a permeability peak at 20–25°C and a second increase in ²²Na efflux in the temperature range 30–40°C. The results of ³¹P-NMR measurements suggest that the acyl chains in 2,3-dipalmitoylcyclopentanol-phosphocholine ($\text{n = 2}$) bilayers have restricted rotation below the main phase transition temperature.     

Thermotropic behavior of dipalmitoylphosphatidylcholine vesicles reconstituted with the glycoprotein of vesicular stomatitis virus

The vesicular stomatitis virus glycoprotein reconstituted into dipalmitoylphosphatidylcholine (DPPC) vesicles exerts a profound effect upon the DPPC gel to liquid-crystalline phase transition. The glycoprotein was reconstituted into DPPC vesicles by octyl glucoside dialysis. The gel to liquid-crystalline phase transition of these vesicles was monitored by differential scanning calorimetry. Vesicles formed in the absence of glycoprotein (600--2100-A diameter) underwent the phase transition at 41.0 degrees C and had an associated enthalpy change of 8.0 +/- 1.6 kcal/mol. Increasing the mole ratio of glycoprotein to DPPC in the vesicles to 0.15 mol % reduced both the transition temperature and the transition enthalpy change. The enthalpy change as a function of the mole percent glycoprotein could be fit to a straight line by a least-squares procedure. Extrapolation of the results to the glycoprotein concentration where the enthalpy change was zero indicated one glycoprotein molecule bound 270 +/- 150 molecules of DPPC.     

Binding of insulin to the external surface of liposomes: Effect of surface curvature,temperature, and lipid composition

The binding of insulin to the external surface of phosphatidylcholine liposomes as a function of the temperature, the surface curvature, and the composition of lipids was studied. The amount of the saturated binding of insulin to liposomes was assessed by gel-filtration chromatography. The binding of insulin to small unilamellar vesicles was highly dependent upon the temperature, favoring low temperatures. As the temperature increased, there was a distinct temperature range where the binding of insulin to small unilamellar vesicles decreased. The temperature ranges for dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) small unilamellar vesicles were found to be 10–20°C and 21–37°C, respectively. These temperature ranges were quite different from the reported ranges of the gel → liquid crystalline phase transition temperatures ($\text{T}{}_{\mathrm{<mtext>c</mtext>}}$) for DMPC or DPPC small unilamellar vesicles. In contrast to other proteins, the amount of insulin bound to DMPC and DPPC small unilamellar vesicles was negligible at or above the upper limit of the above temperature ranges, and increased steadily to 6–7 μmol of insulin per mmol of phospholipid as the temperature decreased to or below the lower limit of these temperature ranges. On the other hand, the binding of insulin to the large multilamellar liposomes cannot be detected at all temperatures tested. The affinity of insulin to neutral phosphatidylcholine small unilamellar vesicles appeared to be related to the surface curvature of the liposomes, favoring the liposomes with a high surface curvature. Furthermore, the amount of insulin bound to small unilamellar vesicles decreased as the content of the cholesterol increased. The presence of 10% molar fraction of phosphatidic acid did not appear to affect the binding of insulin to small unilamellar vesicles. However, the presence of 5% molar fraction of stearylamine in DPPC small unilamellar vesicles increased the amount of bound insulin as well as the extent of aggregation of liposomes. The results of the present study suggest that the interstitial regions of the acyl chains of phospholipids between the faceted planes of small unilamellar vesicles below $\text{T}{}_{\mathrm{<mtext>c</mtext>}}$ may be responsible for the hydrophobic interaction of insulin and small unilamellar vesicles. The tight binding of insulin to certain small unilamellar liposomes could lead to an overestimation of the true amount of insulin encapsulated in liposomes, if care is not taken to eliminate the bound insulin during the procedure of encapsulating insulin in liposomes.     

Phase transition characteristics of diphosphatidylglycerol (cardiolipin) and stereoisomeric phosphatidyldiacylglycerol bilayers: Mono- and divalent metal ion effects

Synthesis and phase transition characteristics of aqueous dispersions of the homologous (12 : 0, 14 : 0, 16 : 0) diphosphatidylglycerols (cardiolipins) and phosphatidyldiacylglycerols are reported. Electron microscopy of the negatively stained aqueous dispersions reveals a characteristic lamellar structure suggesting that these phospholipid molecules are organized as bilayers in the aqueous dispersions. The phase transition temperature ($\text{T}{}_{\mathrm{<mtext>m</mtext>}}$) and the enthalpy of transition ($\text{ΔH}$) increase monotonically with chain length in the cardiolipin and phosphatidyldiacylglycerol series; $\text{T}{}_{\mathrm{<mtext>m</mtext>}}$ for phosphatidyldiacylglycerol is higher than that for cardiolipin of the same chain-length. The transition temperatures for the enantiomeric $\text{sn-3,3-}\text{and}\text{sn-1,1-}\text{phosphatidyldiacylglycerol}$ and for the diastereomeric, $\text{meso}\text{-sn-1,3-}\text{phosphatidyldiacylglycerol}$ are approximately the same. The molar enthalpy for the transition of cardiolipin-NH₄⁺ bilayers is approximately twice the value for the phosphatidylcholines of the same chain length, i.e., the molar enthalpy per acyl chain is approximately the same in the two systems. The transition temperatures for metal ion salts of C_{1 6}-cardiolipin exhibit a biphasic dependence upon the unhydrated ionic radii, i.e. the highest $\text{T}{}_{\mathrm{<mtext>m</mtext>}}$ is observed for Ca²⁺- cardiolipin and decreases for the salts of ions with smaller and larger ionic radii than that of Ca²⁺. The lowest $\text{T}{}_{\mathrm{<mtext>m</mtext>}}$ is observed for Rb⁺-cardiolipin. Monovalent metal salts of cardiolipin exhibit two phase transitions. This effect may result from different conformational packing of the four acyl chains due to differences in metal-phosphate binding.     

A study on the interaction between 1-decyloxymethyl-3-carbamoylpyridinium salts and model membranes--the effect of counterions

The interaction between 1-decyloxymethyl-3-carbamoylpyridinium salts (PS-X) and two types of vesicles (multilamellar vesicle and sonicated vesicle) was investigated. Vesicles were formed from two classes of phospholipids: 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphatidylethanolamine (DPPE). The PS-X salts used had nitrate, perchlorate, tetrafluoroborate and halides as counterions. Measurements were carried out using differential scanning calorimetry and 1H NMR. All studied compounds decreased the main phase transition temperatures of both DPPC and DPPE bilayers. All of them also decreased the transition enthalpy of DPPC bilayers, however they had a dual effect on the transition enthalpy of DPPE. Namely, at low concentrations the PS-X salts studied significantly increased the main transition enthalpy of DPPE (perchlorate and tetrafluoroborate the least among them) and decreased it at higher concentrations. We have suggested that surfactant rich and pure domains form on the DPPE bilayer in the presence of PS-ClO4, PS-BF4 and PS-NO3, whereas they form on DPPC bilayer only in the presence of PS-ClO4. Results are discussed in terms of counterion molecular geometry and the ability of amide group to form hydrogen bonds with lipids.     

Phospholipid interactions with cytochrome P-450 in reconstituted vesicles Preference for negatively-charged phosphatidic acid

Cytochrome $\text{P-450}$ LM2 was reconstituted by the cholate-dialysis method into vesicles containing a mixture of either phosphatidylcholine or phosphatidylethanolamine with up to 50 mol% of phosphatidic acid. Phase transition curves in the presence or absence of cytochrome $\text{P-450}$ were obtained from electron paramagnetic resonance experiments by measuring the partitioning of 2,2,6,6-tetramethylpiperidine-1-oxyl. Protein-free phospholipid vesicles exhibit a phase separation into domains of gel phase enriched in phosphatidic acid in a surrounding fluid matrix containing mainly phosphatidylcholine. The phase transition of the phosphatidic acid domains disappeared following incorporation of cytochrome $\text{P-450}$ into the bilayers. In contrast, in vesicles containing mixtures of egg-phosphatidic acid and dimyristoyl phosphatidylcholine, the phase transition of the domains enriched in dimyristoyl phosphatidylcholine was less sharp than in the corresponding vesicles containing cytochrome $\text{P-450}$. The results of both of these experiments could be explained by a redistribution of the mol fraction of the two phospholipids in the gel phase due to preferential binding of the egg-phosphatidic acid to the cytochrome $\text{P-450}$. For comparison, incorporation of cytochrome $\text{P-450}$ into uncharged vesicles of dimyristoyl phosphatidylcholine and egg-phosphatidylethanolamine did not alter the     

The kinetics of ionophore X-537A-mediated transport of manganese through dipalmitoylphosphatidylcholine vesicles A 1H-NMR study

We have studied the kinetics of ionophore X-537A-mediated transport of manganese ions into small unilamellar vesicles formed from dipalmitoylphosphatidylcholine. To follow the transport we used the paramagnetic effect of manganese on the ¹H-NMR signal from choline trimethylammonium groups on the inner phospholipid monolayer. The transport of only one manganese ion produces an intravesicular concentration which is high enough (approx. 1 mM) to substantially broaden this signal. The observed signal thus arises predominantly from those vesicles which contain no manganese. Therefore, as manganese is transported into the vesicles the observed signal decreases in intensity, but does not broaden. The initial time-dependence of the intensity of the signal, $\text{S(t)}$, can be approximated by the simple first-order rate law: $\text{S(t) = S}\text{(O)}\text{exp}\text{(?K′t)}$, where $\text{K′}$ is the probability per unit time for the transport of a manganese ion from the external medium to the intravesicular space. From the dependence of $\text{K′}$ on the ionophore X-537A concentration we conclude that manganese is transported into the vesicles via both 1 : 1 and 2 : 1 complexes with ionophore X-537A. At low ratios of ionophore X-537A to vesicles transport via the 1 : 1 complex predominates; at high ratios transport via the 2 : 1 complex predominates. From the dependence of $\text{K′}$ on manganese concentration we determined that under our conditions the equilibration of ionophore X-537A between vesicles is much faster than the transport of manganese through the vesicles. Lastly, from the dependence of $\text{K′}$ on temperature, we conclude that the ionophore X-537A-mediated transport of manganese into the dipalmitoylphosphatidylcholine vesicles is very sensitive to the gel-liquid crystalline phase transition.     

The effect of anaesthetic-like molecules on the phase transition in smectic mesophases of dipalmitoyllecithin I. The normal alcohol up to C = 9 and three inhalation anaesthetics

The effect of the normal alcohols (up to $\text{C = 9}$) and three clinically used anaesthetics, on the crystalline-liquid crystalline phase transition in $\text{1,2-}\text{dihexadecyl}\text{-sn-}\text{glycero}\text{-3-}\text{phosphorylcholine}$ have been studied. A one-degree depression was produced by a 4.4% concentration in the membrane of $\text{n-}\text{octanol}$ and $\text{n-}\text{nonanol}$ agreeing well with the value calculated from the temperature and enthalpy of the transition. It is also shown that the relationship between the partition coefficient $\text{P}$ and the water solubility $\text{S (P · S = 2)}$, holds for the solutes investigated here. The experimental method described offers a simple way of assessing the anaesthetic potency of a wide range of compounds.     

The influence of cholesterol on head group mobility in phospholipid membranes

The dielectric dispersion in the MHz range of the zwitterionic dipolar phosphocholine head groups has been measured from 0–70°C for various mixtures of $\text{1,2-}\text{dipalmitoyl}\text{-sn-}\text{glycero-3-phosphocholine}$ (DPPC) and cholesterol. The abrupt change in the derived relaxation frequency $\text{f}{}_2$ observed for pure DPPC at the gel-to-liquid crystalline phase transition at 42°C reduces to a more gradual increase of frequency with temperature as the cholesterol content is increased. In general the presence of cholesterol increases the DPPC head group mobility due to its spacing effect. Below 42°C no sudden changes in $\text{f}{}_2$ are found at 20 or 33 mol% cholesterol, where phase boundaries have been suggested from other methods. Above 42°C, however, a decrease in $\text{f}{}_2$ at cholesterol contents up to 20–30 mol% is found. This is thought to be partly due to an additional restricting effect of the cholesterol on the number of hydrocarbon chain conformations and consequently on the area occupied by the DPPC molecules.     

Kinetics of monensin complexation with sodium ions by 23Na NMR spectroscopy

The kinetics of the sodium binding to the ionophore monensin (Mon) in methanol has been studied by ²³Na NMR spectroscopy. Fast quadrupole relaxation of the bound sodium affected the relaxation rate of the free sodium through an exchange process between these two species. The exchange was found to be dominated by the reaction: Na⁺ + Mon^? ? MonNa. The dissociation rate constant at 25°C is 63 s^?1, with an activation enthalpy of 10.3 $\text{kcal}\text{mol}$ and activation entropy of ?15.8 $\text{cal}\text{mol}$ deg. These results indicate that the specificity of the binding of sodium ions to monensin is reflected in the relatively slow dissociation process. The entropy changes indicate that the activated monensin-sodium complex undergoes a conformational change, but the existence of a conformational change in monensin anion prior to complexation is excluded.     

Nonelectrolyte substitution for water in phosphatidylcholine bilayers

Glycerol substitutes for water in multilamellar phosphatidylcholine liposomes in that the fluid spaces between bilayers, as well as their main transition temperatures, heat capacities, and ethalpies are very similar in water and in pure glycerol. One major difference is that the gel state phase of dipalmitoylphosphatidylcholine (DPPC) in glycerol consists of bilayers with fully interdigitated hydrocarbon chains. Interdigitated DPPC phases are also formed in ethylene glycol or in methanol (at low methanol content). In solutions of glycerol and water, the fluid spacing between bilayers is a function of mole fraction of glycerol X_g, reaching maximum values at $\text{X}{}_{\mathrm{<mtext>g</mtext>}}\text{≌ 0.1}$ for lipid in the liquid crystalline phase and at $\text{X}{}_{\mathrm{<mtext>g</mtext>}}\text{≌ 0.3}$ for the gel phase. These changes are explained in terms of a modification of the long-range Van der Waals attractive forces by glycerol.     

Phospholipid exchange between bilayer membranes

The mode of interaction of aqueous dispersions of phospholipid vesicles is investigated. The vesicles (average diameter 950 Å) are prepared from total lipid extracts of Escherichia coli composed of phosphatidylethanolamine, phosphatidylglycerol and cardiolipin. One type of vesicle contains $\text{trans-}\text{Δ}{}^9\text{-octadecenoate}$, the other type $\text{trans-}\text{Δ}{}^9\text{-hexadecenoate}$ as predominant acyl chain component. The vesicles show order?disorder transitions at transition temperatures, $\text{T}{}_{\mathrm{<mtext>t</mtext>}}\text{= 42°}\text{C}$ and $\text{T}{}_{\mathrm{<mtext>t</mtext>}}\text{= 29°}\text{C}$, respectively. A mixture of these vesicles is incubated at 45° C and lipid transfer is studied as a function of time using the phase transition as an indicator. The system reveals the following properties: Lipids are transferred between the two vesicle types giving rise to a vesicle population where both lipid components are homogeneously mixed. Lipid transfer is asymmetric, i.e. $\text{trans-}\text{Δ}{}^9\text{-hexadecenoate}$-containing lipid molecules appear more rapidly in the $\text{trans-}\text{Δ}{}^9\text{-octadecenoate}$-containing vesicles than vice versa. At a given molar ratio of the two types of vesicles the rate of lipid transfer is independent of the total vesicle concentration. It is concluded that lipid exchange through the water phase by way of single molecules or micelles is the mode of communication of these negatively charged lipid vesicles.     

Orientational order in the choline headgroup of sphingomyelin: A 14N-NMR study

An aqueous dispersion of fully hydrated bovine sphingomyelin was studied using ¹⁴N-NMR spectroscopy. Spectra were obtained as a function of temperature over the range 15–80°C, in both the liquid crystal and gel phases. In the liquid crystal phase, powder pattern lineshapes were obtained, whose quadrupolar splitting slowly decreases with increasing temperature. The spectra are increasingly broadened as the temperature is lowered through the phase transition into the gel phase. The linewidths and the second moments of these spectra indicate that the onset of a broad phase transition occurs at approx. 35°C, in agreement with previous calorimetric and ³¹P-NMR measurements. There is no evidence from the lineshapes for an hexagonal phase in this system, and this conclusion is supported by X-ray diffraction measurements carried out on aqueous dispersions of sphingomyelin in both phases. Assuming that the static nitrogen quadrupole coupling constant is the same for both sphingomyelin and dipalmitoyl-l-α-phosphatidylcholine (DPPC), the decrease observed in the quadrupolar splitting of sphingomyelin compared to that of DPPC indicates that the orientational order of the choline headgroup in liquid crystalline sphingomyelin is not the same as that of its counterpart in DPPC. Preliminary relaxation time measurements of $\text{T}{}_1$ and $\text{T}{}_2$ are presented which suggest that there are also dynamic differences between sphingomyelin and DPPC in the choline headgroup.     

Action of insulin analogues on cultured human fibroblasts reflects biological potency.

The biological sensitivity of cultured human skin-derived fibroblasts was examined in terms of the ability of insulin and insulin analogues to stimulate the uptake of alpha-aminoisobutyrate in these cells. The relative biological activity measured: insulin=desalanine insulin > proinsulin > desoctapeptide insulin parallels exactly the relative activity of these compounds on fat and muscle preparations both $\text{in}$$\text{vitro}$ and $\text{in}$$\text{vivo}$. Inactive insulin analogues do not stimulate alpha-aminoisobutyrate uptake. It is concluded that the chemical specificity of human fibroblast insulin receptors is retained in culture and that the biological responsiveness (alpha-aminoisobutyrate uptake) of such cells to insulin should prove useful for comparative studies of receptors obtained from different individuals.     

Cooperative unit size in the gel-liquid crytalline phase transition of dipalmitoyl phosphatidylcholine-water multilayers: An estimate from raman spectroscopy

The temperature dependence of the Raman spectral transitions assigned to the acyl chain C-C stretching modes of dipalmitoyl phosphatidylcholine was determined for the gel, phase transition and liquid crystalline states of the lipid multilayers. The van't Hoff enthalpy differences $\text{ΔH}{}_{\mathrm{<mtext>VH</mtext>}}$ between trans and gauche rotational isomers were obtained from the Raman spectral data for the temperature region characteristics of each bilayer state. An average size for the cooperative unit undergoing the chain melting process during the phase transition was estimated from the ratio of the appropriate van't Hoff enthalpy to an adjusted calorimetric enthalpy.     

Increase in temperature induces the Z to B transition of poly[d(G-C)] in water-ethanol solution

An increase in temperature from 20 to 50° C results in the complete transition from the Z to B form of poly(d(G-C)], dissolved in a 55% ethanol-water solution. The transition is fully reversible and displays a slow kinetics. The transition profiles for the free polynucleotide and for that in the presence of ethidium bromide, which is known to stabilize the B form, are obtained by circular dichroism. Based on these data the enthalpy value for the B-Z transition in our conditions is estimated to be ΔH_BZ = ?0.7 $\text{kcal}\text{mol}$.