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PubMed2004

Supersymmetry theory of microphase separation in homopolymer-oligomer mixtures.

The mesoscopic structure of periodically alternating layers of stretched homopolymer chains surrounded by perpendicularly oriented oligomeric tails is studied for systems with both strong (ionic) and weak (hydrogen) interactions. We focus on the consideration of the distribution of oligomers along the homopolymer chains that is described by the effective equation of motion with the segment number playing the role of imaginary time. The supersymmetry technique is developed to consider associative hydrogen bonding, self-action effects, inhomogeneity, and temperature fluctuations in the oligomer distribution. Making use of the self-consistent approach allows one to explain experimentally observed temperature dependence of the structure period and the order-disorder transition temperature and period as functions of the oligomeric fraction for systems with different bonding strengths. A whole set of parameters of the model used is found for strong, intermediate, and weak coupled systems being Poly (4-vinyl pyridine)-dodecyl benzene sulfonic acid [P4VP-(DBSA)(x)], P4VP-[Zn(DBS)(2)](x), and P4VP- 3-pentadecyl Phenol(x), respectively. A passage from the former two to the latter is shown to cause a crucial decrease in the magnitude of both parameters of hydrogen bonding and self-action, as well as the order-disorder transition temperature.

Journal Article↗
PubMed2005

Kinetic study of the Maillard reaction between metoclopramide hydrochloride and lactose.

The purpose of this work is to study the apparent solid-state kinetics of the Maillard reaction of the colyophilized metoclopramide hydrochloride (MCP) and lactose system and to elucidate some of the effects of molecular mobility on the kinetic behavior of the amorphous mixture. Colyophilized MCP-lactose mixtures (1:9 molar ratio) were stored at temperatures ranging from 100 to 115 degrees C (above the glass transition temperature, Tg=99.7 degrees C). This temperature range, which corresponds to the change between the glass and liquid states of the lactose-MCP mixtures, is also the temperature region where molecular mobility represents a kinetic impediment of enough significance as to affect the observed order of the reaction. A pseudo second-order kinetic model was developed to fit the MCP loss data. The proposed model gives better fit to the degradation data than many of the commonly used kinetic models available in the literature. The second-order rate constant of the model follows Arrhenius kinetics and the activation energy was found to be 53.8 kcal mol(-1). The molecular mobility (relaxation time) of the mixture was calculated from the heating rate dependence of the DSC-determined glass transition temperature of the mixtures. From molecular mobility considerations alone, it is possible to accurately predict the temperature dependence of the reaction rate constant. These results support the hypothesis that the solid-state reaction is mobility controlled.

Freeze Drying↗
PubMed2005

Phase transitions as a function of osmotic pressure in Saccharomyces cerevisiae whole cells, membrane extracts and phospholipid mixtures.

Fourier Transform Infrared spectroscopy (FTIR) was used to determine the phase transition temperature of whole Saccharomyces cerevisiae W303-1 A cells as a function of Aw in binary water-glycerol media. A phase transition occurred at 12 degrees C in water, at 16.5 degrees C at Aw=0.75, and at 19.5 degrees C at Aw=0.65. The temperature ranges over which transition occurred increased with decreasing Aw. A total lipid extract of the plasma membranes isolated from S. cerevisiae cells was also studied, with a phase transition temperature determined at 20 degrees C in pure water and at 27 degrees C in binary water-glycerol solutions for both Aw levels tested. The pure phospholipids dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE) and three binary mixtures of these phospholipids (percentage molar mixtures of DMPC/DMPE of 90.5/9.5, 74.8/25.2, and 39.7/60.3) were studied. For DMPC, there was no influence of Aw on the phase transition temperature (always 23 degrees C). On the other hand, the phase transition temperature of DMPE increased with decreasing Aw for the three aqueous solutions tested (glycerol, sorbitol and sucrose), from 48 degrees C in water, to 64 degrees C for a solution at Aw=0.67. For the DMPC/DMPE mixtures, transitions were found intermediate between those of the two phospholipids, and a cooperative state was observed between species at the gel and at the fluid phases.

Cell Membrane↗
PubMed1993

Co-existing structures of an mRNA stability determinant. The 5' region of the Escherichia coli and Serratia marcescens ompA mRNA.

The structure of untranslated regions of mRNA is thought to play a key role in the degradation of mRNAs by specific RNases. As a model system, in vitro transcripts of the stability determining 5' non-coding region of bacterial ompA mRNA were investigated by calculation of secondary structure models and by experiments applying the temperature-gradient gel electrophoresis (TGGE). For the theoretical prediction of secondary structures an algorithm was used, which yields the structure of lowest free energy as well as a large set of suboptimal structures. Three structures were predicted to co-exist in similar concentrations under native conditions. They denature in a low temperature transition leading to a unique structure which denatures in a high temperature transition. The prediction of three structures and two transitions could be confirmed experimentally by TGGE. Due to the use of transcripts of different length the conformational transitions could be attributed to distinct parts of the molecules. A pseudoknot structural motif was predicted theoretically, but could not be confirmed experimentally. Comparing ompA transcripts of E. coli and S. marcescens, a conservation of structural features could be shown in spite of a sequence homology of only 63%. Regarding the sequential folding of the transcript after synthesis, a metastable structure is formed first and is converted slowly into structures of lower free energy. The biological implications for in vivo degradation are discussed.

Bacterial Outer Membrane Proteins↗
PubMed1982

Lack of transbilayer coupling in phase transitions of phosphatidylcholine vesicles.

Praseodymium and ytterbium chlorides were used as nuclear magnetic resonance shift reagents to resolve the inner and outer choline methyl resonances of single-walled dimyristoylphosphatidylcholine bilayer vesicles. The gel to liquid-crystalline phase transition of these vesicles was monitored by observing the proton and carbon-13 nuclear magnetic resonance line widths of the choline methyl group nuclei. In the absence of ions the transition occurred at 21.5 degrees C in both halves of the bilayer. With Pr3+ or Yb3+ added to the outside of the vesicles, the phase transition temperature of the outer half of the bilayer was raised several degrees, while the transition temperature of the inner half was unchanged. In vesicles containing 20 mol % cholesterol the phase transition of the outer monolayer was considerably broadened, while the inner half still melted sharply at 21.6 degrees C. By use of dipalmitoylphosphatidylcholine vesicles with UO2(2)+ added to the outside, phase transitions at 41.5 and 44 degrees C were detected by electron spin resonance with the spin-label 2,2,6,6-tetramethylpiperidinyl-l-oxy. These results imply that the two halves of the bilayer in phospholipid vesicles are so weakly coupled that they can undergo the gel to liquid-crystalline phase transition independently.

Dimyristoylphosphatidylcholine↗
PubMed2001

Solvent isotope effects on the phase-transition properties of lipid bilayers.

Highly sensitive differential scanning microcalorimetry (DSC) has been used to investigate the phase transition properties of lipid vesicles prepared from 1,2-distearoyl-L-3-glyceryl-phosphatidylcholine (DSPC) in H(2)O and D(2)O. The data show that the response of pre-transition properties to D(2)O-->H(2)O substitution is stronger than the main transition properties. We find that there is a small increase in the phase transition temperature (DeltaT approximately 0.5 K) and in the co-operative unit in the main transition. The increase in enthalpy (DeltaH congruent with1 kJ(.)mol(-1)) and in transition temperature (DeltaT congruent with2 K) observed in the pre-transition is comparable with that observed in quite different processes and systems, i.e. melting of nucleic acids and proteins and gel formation. It is suggested that D(2)O-->H(2)O substitution affects the thermal transition in these systems in such a way that the contributions of enthalpy and entropy to structural reorganization of water in these processes is modified.

Journal Article↗
PubMed1997

Effect of sphingomyelin composition on the phase structure of phosphatidylcholine-sphingomyelin bilayers.

In this study, we examine the phase behavior as well as lateral diffusion and percolation in the region of coexisting gel and fluid phases in binary mixtures composed of dimyristoylphosphatidylcholine and one of two totally synthetic D-erythro-sphingomyelins (having either C16 or C24 acyl chains, both having similar gel to liquid-crystalline phase transition temperatures). This study stems from the uniqueness of sphingomyelins having gel to liquid-crystalline transition temperatures in the range of physiological interest, and the fact that more than 50% of the naturally occurring sphingomyelin species have a chain mismatch. The presence of sphingomyelin in biological membranes can thus be expected to give rise to a complex phase structure. Fluorescence recovery after photobleaching, differential scanning calorimetry, and electron microscopy are used to show that, despite similarity in the temperature range of the gel to liquid-cystalline phase transition of the two sphingomyelins, the two differ in their phase structure. Also they differ to a large extent in their mixing with dimyristoylphosphatidylcholine. Dimyristoylphosphatidylcholine and C16 sphingomyelin mix nearly ideally, with the percolation threshold locus lying close to the liquidus on the phase diagram. In contrast, the C24 sphingomyelin and dimyristoylphosphatidylcholine mix nonideally, with the percolation threshold locus lying close to the solidus. In addition, mixtures containing C24 sphingomyelin have a complex thermotropic behavior which may be related to the observation that these dispersions contain several types of particles, some of which are not multilamellar vesicles. These studies suggest that the degree of sphingomyelin chain mismatch is an important factor in determining lateral organization in the membrane.

Calorimetry, Differential Scanning↗
PubMed1984

Interaction of calcium and neomycin with anionic phospholipid-lecithin liposomes. A differential scanning calorimetry study.

The interactions of calcium and neomycin with liposomes of various anionic phospholipids plus lecithin were studied by differential scanning calorimetry. Phosphatidylinositol bisphosphate differed from other acidic phospholipids in its interactions with both calcium and neomycin. Calcium, at concentrations as low as 1 mM, induced the appearance of a second transition peak in phosphatidylinositol bisphosphate-enriched liposomes only. Neomycin acted antagonistically and precluded this phase separation. In addition, neomycin lowered the phase transition temperature of phosphatidylinositol bisphosphate-lecithin liposomes while it raised the transition temperature of all other anionic phospholipid-lecithin liposomes tested. This fluidizing effect of neomycin and the antagonism to calcium may induce critical alterations of properties of biological membranes. The study supports and extends our previous findings and conclusions that phosphatidylinositol bisphosphate may play a crucial role in the expression of aminoglycoside toxicity.

Calcium↗
PubMed1997

Lipogastrins as potent inhibitors of viral fusion.

The rate and extent of membrane fusion is markedly sensitive to membrane interfacial properties. Lipopeptides with hydrophilic peptide moieties will insert into membranes, leaving the peptide portion at the membrane-water interface. In this work, we have used a lipopeptide composed of the peptide [Nle15]-gastrin-(2-17)-amide covalently linked to 1,2-diacyl-3-mercaptoglycerol-N(alpha)-maleoyl-beta-alanine to give DM-gastrin or DP-gastrin having 14 or 16 carbon atom acyl chains, respectively. The fluorescence emission from the two Trp residues of these lipopeptides exhibited little or no blue shift upon addition of liposomes of egg-phosphatidylethanolamine containing 5 mol% G(D1a). Iodide quenching of DP-gastrin fluorescence was also independent of lipid. These results indicate that the peptide moiety is exposed to the aqueous environment even though the lipopeptide is firmly anchored to the membrane. Both DM and DP-gastrin markedly raise the bilayer to hexagonal phase transition temperature of dipalmitoleoyl phosphatidylethanolamine. However, DM-E5 lowers this phase transition temperature. These lipopeptides have effects on the overall fusion of Sendai virus to liposomes in accord with their opposite effects on lipid curvature. The lipogastrins are potent inhibitors of viral fusion, while DM-E5 slightly promotes this process. Truncated forms of DM-gastrin are also inhibitory to viral fusion, but are less inhibitory than the full lipopeptide. Analysis of the fusion kinetics shows that DP-gastrin causes a reduction in the final extent of fusion and a marked lowering of the fusion rate constant. Binding of Sendai virus to the ganglioside receptor-containing liposomes was not affected. Consideration of the various contributions to the mechanism of inhibition of viral fusion suggests that effects of lipogastrin on membrane intrinsic monolayer curvature is of primary importance.

Amino Acid Sequence↗
PubMed1984

Role of peptide structure in lipid-peptide interactions: a fluorescence study of the binding of pentagastrin-related pentapeptides to phospholipid vesicles.

The binding of pentagastrin and three other structurally related pentapeptides to phospholipid vesicles has been studied by fluorescence spectroscopy. The fluorescence of the tryptophan residues of these peptides exhibits an increased quantum yield upon binding to phospholipid vesicles. This is accompanied by a blue shift of the maximum emission, indicative of the incorporation of the tryptophan residue into a more hydrophobic environment. The affinity of the peptides for a zwitterionic phospholipid, dimyristoylphosphatidylcholine (DMPC), increases in the following order: N-t-Boc-beta-Ala-Trp-Met-Gly-Phe-NH2 greater than N-t-Boc-beta-Ala-Trp-Met-Arg-Phe-NH2 greater than N-t-Boc-beta-Ala-Trp-Met-Asp-Phe-NH2 greater than N-t-Boc-beta-Ala-Trp-Met-Phe-Asp-NH2. Comparison of the interaction of these various peptides with this phospholipid indicates that although the interaction is largely of hydrophobic nature, the structure of the polar amino acids and their electrostatic charge have significant influence on the nature of the bindings. In addition, the sequence of polar and apolar amino acids appears to be of importance. The higher affinity for DMPC of N-t-Boc-beta-Ala-Trp-Met-Asp-Phe-NH2 as compared to its "reversed" analogue N-t-Boc-beta-Ala-Trp-Met-Phe-Asp-NH2 suggests that the ability of the peptides to fold into amphiphatic structures can enhance their lipid binding affinity. For all peptides the interaction with DMPC is greater at 8 degrees C, i.e., below the lipid phase transition temperature, than at 40 degrees C, i.e., above the lipid phase transition temperature.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1987

Kinetics of ganglioside transfer between liposomal and synaptosomal membranes.

The transfer of ganglioside GM1 from micelles to membranes and between different membrane populations has been examined by using a pyrene fatty acid derivative of the ganglioside. The transfer of gangliosides from micelles to membranes depends on the physical state as well as the molecular composition of the acceptor vesicles. At 30 degrees C, the transfer of micellar gangliosides to dipalmitoylphosphatidylcholine (DPPC) large unilameller vesicles (Tm = 41.3 degrees C) is characterized by a rate constant of 0.01 min-1; at 48 degrees C, however, the rate constant is 0.11 min-1. Below the phase transition temperature, the activation energy is 25 kcal/mol whereas above the phase transition it is 17 kcal/mol. Similar experiments performed with synaptic plasma membranes yielded a rate constant of 0.05 min-1 at 37 degrees C. The rate of transfer of ganglioside molecules, asymmetrically located on the outer layer of donor vesicles, to acceptor vesicles lacking ganglioside depends on the physical state of both the donor and acceptor vesicles. For the transfer of ganglioside from DPPC (donor) vesicles to dimyristoylphosphatidylcholine (DMPC) (acceptor) vesicles, the rates were essentially zero at 15 degrees C in which both vesicle populations were in the gel phase, 0.008 min-1 at 30 degrees C in which DPPC is in the gel phase and DMPC is in the fluid phase, and 0.031 min-1 at 48 degrees C in which both vesicle populations are in the fluid phase. The transfer of ganglioside from DPPC vesicles to synaptic plasma membranes was also dependent on the physical state of the donor vesicles and showed an inflection point at the phase transition temperature of DPPC.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Binding of cytochrome c to liposomes as revealed by the quenching of fluorescence from pyrene-labeled phospholipids.

Resonance energy transfer from pyrene-fatty acid containing phospholipid derivatives to the heme of cytochrome c (cyt c) was used to observe the binding of this protein to liposomal membranes. Liposomes were formed of egg yolk phosphatidic acid (PA) and either egg yolk phosphatidylcholine or dipalmitoylphosphatidylcholine with 1 mol % of the fluorescent lipid. Binding of cyt c to liposomes was monitored by measuring the decrease either in the fluorescence intensity or in the lifetime of pyrene emission. The requirement for the presence of the acidic phospholipid in the membrane for the binding of cyt c could be reconfirmed. Below 5 mol % of phosphatidic acid in the membrane, no significant attachment of cyt c to liquid-crystalline bilayers was evident whereas upon increasing the concentration of PA further the association of cyt c progressively increased until a saturation was reached at about 30 mol % of phosphatidic acid. Addition of NaCl caused the fluorescence intensity and lifetimes to return to values observed in the absence of cyt c, thus revealing the dissociation of the protein from the membrane. The pyrene-labeled phosphatidic acid derivatives PPHPA and PPDPA were quenched more effectively than the corresponding phosphatidylcholines, apparently due to the direct involvement of the acidic head group in binding cyt c. When dipalmitoylphosphatidylcholine (DPPC) with 5 mol % of phosphatidic acid was used, no binding of cyt c to the liposomes above the phase transition temperature of the former lipid could be demonstrated whereas below the transition temperature (Tm) binding did take place.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytochrome c Group↗
PubMed2002

N(4)C-alkyl-N(4)C cross-linked DNA: bending deformations in duplexes that contain a -CNG- interstrand cross-link.

Short DNA duplexes containing a 1,3-N(4)C-alkyl-N(4)C interstrand cross-link that joins the two C residues of a -CNG- sequence were prepared using either a phosphoramidite or convertible nucleoside approach. The alkyl cross-link consists of 2, 4, or 7 methylene groups. The duplexes, which contain a seven-base-pair core and A(3)/T(3) complementary 3'-overhanging ends, were characterized by enzymatic digestion and MALDI-TOF mass spectrometry. Ultraviolet thermal denaturation studies showed that the duplexes denature in a cooperative manner and that the length of the cross-link affects the thermal stability. Thus, the transition temperature of the ethyl cross-linked duplex, 42 degrees C, is 16 degrees C higher than the melting temperature of the corresponding non-cross-linked control, whereas the transition temperatures of the butyl and heptyl cross-linked duplexes, 73 and 72 degrees C, respectively, are 46-47 degrees C higher. The reduced molecularity of denaturation of the cross-linked duplexes versus melting of the non-cross-linked duplex most likely accounts for these differences. Examination of molecular models suggests that the ethyl cross-link is too short to span the distance between the two C residues at the site of the cross-link in B-form DNA without causing distortion of the helix, whereas less and no distortion would be expected for the butyl and heptyl cross-links, respectively. The circular dichroism spectra, which show greatest deviation in the ethyl cross-linked duplex from B-form DNA, are consistent with this expectation. Anomalous mobilities on native polyacrylamide gels of multimers produced by self-ligation of each of the cross-linked duplexes suggest that the ethyl and butyl cross-linked duplexes undergo bending deformations, whereas multimers derived from the heptyl cross-linked duplex migrated normally. The bending angle was estimated to be 20 degrees, 13 degrees, and 0 degrees for the ethyl, butyl, and heptyl cross-linked duplexes, respectively. Thus, it appears that the degree of bending in these N(4)C-alkyl-N(4)C cross-linked duplexes is controlled by the length of the cross-link.

Alkylation↗
PubMed2000

Freeze-concentration separates proteins and polymer excipients into different amorphous phases.

PURPOSE: To study the miscibility of proteins and polymer excipients in frozen solutions and freeze-dried solids as protein formulation models. METHODS: Thermal profiles of frozen solutions and freeze-dried solids containing various proteins (lysozyme, ovalbumin, BSA), nonionic polymers (Ficoll, polyvinylpyrrolidone [PVP]), and salts were analyzed by differential scanning calorimetry (DSC). The polymer miscibility was determined from the glass transition temperature of maximally freeze-concentrated solute (Tg') and the glass transition temperature of freeze-dried solid (Tg). RESULTS: Frozen Ficoll or PVP 40k solutions showed Tg' at -22 degrees C, while protein solutions did not show an apparent Tg'. All the protein and nonionic polymer combinations (5% w/w, each) were miscible in frozen solutions and presented single Tg's that rose with increases in the protein ratio. Various salts concentration-dependently lowered the single Tg's of the proteins and Ficoll combinations maintaining the mixed amorphous phase. In contrast, some salts induced the separation of the proteins and PVP combinations into protein-rich and PVP-rich phases among ice crystals. The Tg's of these polymer combinations were jump-shifted to PVP's intrinsic Tg' at certain salt concentrations. Freeze-dried solids showed varied polymer miscibilities identical to those in frozen solutions. CONCLUSIONS: Freeze-concentration separates some combinations of proteins and nonionic polymers into different amorphous phases in a frozen solution. Controlling the polymer miscibility is important in designing protein formulations.

Calorimetry, Differential Scanning↗
PubMed1976

The effects from combining urea and an alcohol on the heat-induced reversible denaturation of ribonuclease.

The effects of a combination of an alcohol and urea on the transition temperature of bovine ribonuclease were investigated. The combined effects on the transition temperature of ribonuclease of a polyvalent alcohol and urea are about equal to the algebraic sum of the effects of each individual additive. The effects of a monovalent alcohol and urea are not cummulative, especially not at low temperatures (30 degrees C). The presence of urea decreases the hydrophobic effect of a monovalent alcohol, strongly at low temperatures, to a lesser degree at high temperatures (60 degrees C). Consequently, urea hinders the interhydrophobic interactions by affecting the water molecules.

1-Propanol↗
PubMed1980

A comparison of vesicles derived from terminal cisternae and longitudinal tubules of sarcoplasmic reticulum isolated from rabbit skeletal muscle.

Sarcoplasmic reticulum vesicles were separated into heavy (derived from terminal cisternae) and light (derived from longitudinal tubules) fractions, according to Meissner [Biochim. Biophys. Acta, 389, 51-68 (1975)]. The similar Ca2+ sensitivities of phosphoprotein formation, ATPase activity and calcium uptake, and the similar phosphoprotein turnover rates (ATPase/phosphoprotein formation) of both fractions indicate that the same ATPase enzyme is present in the terminal cisternae and longitudinal sarcoplaxmic reticulum. The higher V for Ca2+-activated ATPase activity and calcium uptake in the light fraction correlated with the higher concentration of ATPase enzyme per mg of membrane protein in this fraction. In both the presence and absence of calcium-precipitating anions, the light fraction stored more calcium than the heavy. The Ca2+ dependence of calcium release after addition of EGTA appeared similar in both fractions, but the rate of calcium release was more rapid in the light fraction. These findings suggest that calcium release may occur more rapidly from longitudinal than terminal cisternae portions of the sarcoplasmic reticulum and that calcium release, like calcium uptake, may be mediated by the ATPase enzyme in the sarcoplasmic reticulum membrane. Although the activation energies for Ca2+-activated ATPase activity above and below the transition temperature were significantly different for the heavy and light fractions, their transition temperatures were similar. Partial purification of the ATpase enzyme by deoxycholate treatment modified the activation energies of the light but not the heavy fraction and caused the activation energies to become similar. The phosphoprotein levels of heavy and light vesicles did not become similar after deoxycholate treatment, although gel electrophoretograms indicated both samples contained > 90% ATPase protein. These results indicate the protein-lipid associations in these two fractions may be different.

Adenosine Triphosphatases↗
PubMed2000

Probing local thermal stabilities of bovine, horse, and tuna ferricytochromes c at pH 7.

Correlation between the flexibility of the Met80 loop (residues 75-86) and the local stabilities of native ferricytochromes c from horse, bovine, and tuna was examined. By monitoring the heme bands versus temperature, absorption changes associated with altered ligation in the alkaline isomers were observed. In addition, the intensity of the 695-nm absorption band, which is associated with the heme-crevice stability, decreased with increasing temperature and exhibited biphasic temperature dependence, with transition temperatures (Tc) at 35 degrees C in tuna c, 55 degrees C in horse c, and 58 C in bovine c. Since the heme crevice plays a key role in the thermal stabilities of cytochromes c, their susceptibility to proteolytic attack was examined as a function of temperature. Proteolytic digestion, which requires local conformational instability, revealed that the local stabilities of the cytochromes follow the order: bovine > horse >> tuna, and increased digestion occurred at temperatures close to the 695-nm Tc for each protein. This is consistent with the actual substitution of the Met80 ligand above the 695-nm Tc, which is reflected in the thermodynamic parameters for the two phases. Also, tuna c, unlike horse and bovine c, exhibits different 695-nm (35 degrees C) and Soret (approximately 46 degrees C) Tc values, but its local stability is controlled by the transition detected at 695 nm. The combined spectroscopic and proteolysis results clearly indicate that the flexibility of the Met80 loop determines the local stability of cytochromes c.

Amino Acid Motifs↗
PubMed1977

Thermodynamic values of the helix-coil transition of DNA in the presence of quaternary ammonium salt.

The knowledge of the enthalpy and entropy of the helix-coil transition in DNA is necessary for the understanding of the stabilization of its native conformation in solution. Reported here is the transition temperature Tm, the transition enthalpy deltaH, determined with the help of an adiabatic scanning calorimeter, the transition entropy deltaS and the breadth of the helix-coil transition as a function of tetramethyl and tetraethyl ammonium chloride concentration.

Animals↗