Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “CALORIMETRY”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 289 records · Page 16Linked to original sources
PubMed2005

Study of interaction between ibuprofen and nicotinamide using differential scanning calorimetry, spectroscopy, and microscopy and formulation of a fast-acting and possibly better ibuprofen suspension for osteoarthritis patients.

Solid-state interaction between ibuprofen and nicotinamide was studied using thermal, spectroscopic, and microscopic techniques. Solubility enhancement was calculated by high-performance liquid chromatography and suspension was found to be the suitable choice of formulation. Ibuprofen-nicotinamide binary mixtures were prepared by solvent evaporation method. Differential scanning calorimetry was used to investigate the stoichiometry and thermal properties of the complex between ibuprofen and nicotinamide. A sharp, single endotherm was observed between the melting endotherms of the individual components at a composition of 60% ibuprofen and 40% nicotinamide (w/w). Several spectroscopic techniques such as ultraviolet-visible, Fourier transform infrared, nuclear magnetic resonance, and powder X-ray diffraction were used to investigate the type of interaction between the two components. Optical microscopy was performed to observe changes with regard to particle size and crystal habit. It was concluded that the interaction that occurred was Pi donor-Pi acceptor in nature and too weak to sustain the integrity of the complex in the liquid state. The solubility of ibuprofen was enhanced by 62 times in the suspension when the concentration of nicotinamide was 13.3 mg/mL. The suspension prepared in this study has potential of being a better medication for pain relief in patients with osteoarthritis.

Anti-Inflammatory Agents, Non-Steroidal↗
PubMed1987

A differential scanning calorimetry study of the thermal behavior of water-dipalmitoylphosphatidylcholine mixtures at subzero temperatures: effects of water content, surface charge, and cholesterol.

The thermal behavior of partly and fully hydrated dipalmitoylphosphatidylcholine (DPPC) alone and mixed with dipalmitoylphosphatidylglycerol (DPPG) or cholesterol (chol) was studied at subzero temperatures with differential scanning calorimetry (DSC). The melting characteristics of the frozen water provided information on the fraction of nonfreezing and freezing (subzero melting) hydration water and excess or bulk water (melting at 0 degrees C). Incorporation of DPPG or cholesterol into the DPPC structure increased the subzero temperature interval compared with pure DPPC. The maximum number of molecules of hydration water (nonfreezing and freezing) was similar for DPPC and DPPC/cholesterol systems: approximately 20 mol water/mol of phospholipid; under the selected conditions, no maximum for DPPC/DPPG could be assessed. Combinations of DPPC/cholesterol-water showed a complex thermogram with both endothermic and exothermic responses.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1988

Polymorphic differences in alpha- and beta-form crystals of 2R, 4S, 6-fluoro-2-methyl-spiro[chroman-4,4'-imidazoline]-2',5-dione (M79175) as determined by X-ray diffraction, infrared spectroscopy, and differential scanning calorimetry.

Polymorphic differences in alpha- and beta-form crystals of 2R, 4S, 6-fluoro-2-methyl-spiro[chroman-4,4'-imidazoline]-2',5-dione (M79175; 1) were studied by X-ray diffractometry, infrared spectroscopy, and differential scanning calorimetry. X-ray powder diffraction indicated the longest spacing of the unit cells to be 14.9 and 12.4 A for the alpha- and beta-form crystals, respectively. The infrared spectra showed the absorption band assigned to NH streching vibration for the alpha-form crystals to be centered at 3250 cm-1 and that for the beta-form crystals to split into two peaks, at 3150 and 3425 cm-1. The enthalpies of fusion were 26.3 kJ/mol at 517.5 K and 31.3 kJ/mol at 501.0 K, respectively. Transformation from the beta- to alpha-form was observed at various heating rates, which were enhanced by the presence of a small amount of alpha-form crystals previously added to the beta-form. The former appeared to serve as a source of nuclei for the growth of both forms. These results confirm that the alpha-form crystal is more stable than the beta-form.

Calorimetry, Differential Scanning↗
PubMed1990

Differential scanning calorimetry study of the interaction of antidepressant drugs, noradrenaline, and 5-hydroxytryptamine with a membrane model.

Differential scanning calorimetry was used to study the interaction of a membrane model with two neuromediators [noradrenaline and 5-hydroxytryptamine (Serotonin)] and four antidepressant drugs (imipramine, indalpine, citalopram, and milnacipran), known to be uptake inhibitors of these neuromediators. The study was carried out on dipalmitoyl phosphatidylcholine liposomes, a bilayer phospholipid system taken as a simplified model of biological membranes. Analysis of the thermograms led us to classify the molecules according to their lipophilic action, as in the conventional measurement of the water-octanol partition coefficient, and also enabled us to precisely determine their location along the phospholipid bilayer. A hypothesis based on this localization is put forward concerning the competitive, or otherwise, character of the blocking of uptake of the neuromediators. The extreme cases of interaction and localization of imipramine and milnacipran, a new antidepressant, relative to the bilayer are also analyzed in terms of side effects.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2005

Structure influence on biophenols solubility in model biomembranes detected by differential scanning calorimetry.

The protective effects of some foods, in particular fruits and vegetables, against cardiovascular disease and cancer are believed to be due to the presence of antioxidant substances such as hydroxyaromatic compounds. The aim of this work was to study (i) the interaction of three biophenols derived from benzoic acid (p-hydroxybenzoic acid, vanillic acid, syringic acid and benzoic acid) with model biomembranes and (ii) their transfer through an aqueous medium to be absorbed into a lipid bilayer, investigating the effect they exert on the thermotropic behaviour of model membranes represented by dimyristoylphosphatidylcholine multilamellar vesicles using differential scanning calorimetry. The compounds, when dispersed in liposomes during their preparation, at pH = 4, were found to modify the gel to liquid crystal phase transition of the lipid vesicles, causing a temperature shift towards lower values. The temperature shift was a function of the concentration of acids in the lipid aqueous dispersions and their lipophilic character. The kinetic experiments of compounds transfer through the aqueous medium and the absorption by the bilayer were performed contacting the antioxidant compounds (at a fixed concentration) and the model membrane at increasing incubation times. These experiments reveal that the transfer of the examined compounds through the aqueous medium and their uptake by bilayer are influenced by the presence of substituents located on the ring, which should consequently modify their lipophilicity.

Benzoic Acid↗
PubMed1993

Thermal unfolding of staphylococcal nuclease and several mutant forms thereof studied by differential scanning calorimetry.

The effects of eight mutations on the thermodynamics of the reversible thermal unfolding of staphylococcal nuclease have been determined over a range of pH and protein concentration by means of differential scanning calorimetry. Variation of the protein concentration was included in our study because we found a significant dependence of the thermodynamics of protein unfolding on concentration. Values for the change in the standard free energy of unfolding, delta delta G0d, produced by the mutations in the pH range 5.0-7.0 varied from 1.9 kcal mol-1 (apparent stabilization) for H124L to -2.8 kcal mol-1 (apparent destabilization) for L25A. As has been observed in numerous other cases, there is no correlation in magnitude or sign between delta delta G0d and the corresponding values for delta delta Hd and T delta delta S0d, the latter quantities being in most cases much larger in magnitude than delta delta G0d. This fact emphasizes the difficulty in attempting to correlate the thermodynamic changes with structural changes observed by X-ray crystallography.

Calorimetry, Differential Scanning↗
PubMed1994

Heparin binding domain peptides of antithrombin III: analysis by isothermal titration calorimetry and circular dichroism spectroscopy.

The serine proteinase inhibitor antithrombin III (ATIII) is a key regulatory protein of intrinsic blood coagulation. ATIII attains its full biological activity only upon binding polysulfated oligosaccharides, such as heparin. A series of synthetic peptides have been prepared based on the proposed heparin binding regions of ATIII and their ability to bind heparin has been assessed by CD spectrometry, by isothermal titration calorimetry, and by the ability of the peptides to compete with ATIII for binding heparin in a factor Xa procoagulant enzyme assay. Peptide F123-G148, which encompasses both the purported high-affinity pentasaccharide binding region and an adjacent, C-terminally directed segment of ATIII, was found to bind heparin with good affinity, but amino-terminal truncations of this sequence, including L130-G148 and K136-G148 displayed attenuated heparin binding activities. In fact, K136-G148 appears to encompass only a low-affinity heparin binding site. In contrast, peptides based solely on the high-affinity binding site (K121-A134) displayed much higher affinities for heparin. By CD spectrometry, these high-affinity peptides are chiefly random coil in nature, but low microM concentrations of heparin induce significant alpha-helix conformation. K121-A134 also effectively competes with ATIII for binding heparin. Thus, through the use of synthetic peptides that encompass part, if not all, of the heparin binding site(s) within ATIII, we have further elucidated the structure-function relations of heparin-ATIII interactions.

Amino Acid Sequence↗
PubMed1998

Scan-rate dependence in protein calorimetry: the reversible transitions of Bacillus circulans xylanase and a disulfide-bridge mutant.

The stabilities of Bacillus circulans xylanase and a disulfide-bridge-containing mutant (S100C/N148C) were investigated by differential scanning calorimetry (DSC) and thermal inactivation kinetics. The thermal denaturation of both proteins was found to be irreversible, and the apparent transition temperatures showed a considerable dependence upon scanning rate. In the presence of low (nondenaturing) concentrations of urea, calorimetric transitions were observed for both proteins in the second heating cycle, indicating reversible denaturation occurs under those conditions. However, even for these reversible processes, the DSC curves for the wild-type protein showed a scan-rate dependence that was similar to that in the absence of urea. Calorimetric thermograms for the disulfide mutant were significantly less scan-rate dependent in the presence of urea than in the urea-free buffer. The present data show that, just as for irreversible transitions, the apparent transition temperature for the reversible denaturation of proteins can be scan-rate dependent, confirming the prediction of Lepock et al. (Lepock JR, Rithcie KP, Kolios MC, Rodahl AM, Heinz KA, Kruuf J, 1992, Biochemistry 31:12706-12712). The kinetic factors responsible for scan-rate dependence may lead to significant distortions and asymmetry of endotherms, especially at higher scanning rates. This points to the need to check for scan-rate dependence, even in the case of reversible denaturation, before any attempt is made to analyze asymmetric DSC curves by standard thermodynamic procedures. Experiments with the disulfide-bridge-containing mutant indicate that the introduction of the disulfide bond provides additional stabilization of xylanase by changing the rate-limiting step on the thermal denaturation pathway.

Amino Acid Substitution↗
PubMed1998

Domain structure analysis of elongation factor-3 from Saccharomyces cerevisiae by limited proteolysis and differential scanning calorimetry.

Elongation-factor-3 (EF-3) is an essential factor of the fungal protein synthesis machinery. In this communication the structure of EF-3 from Saccharomyces cerevisiae is characterized by differential scanning calorimetry (DSC), ultracentrifugation, and limited tryptic digestion. DSC shows a major transition at a relatively low temperature of 39 degrees C, and a minor transition at 58 degrees C. Ultracentrifugation shows that EF-3 is a monomer; thus, these transitions could not reflect the unfolding or dissociation of a multimeric structure. EF-3 forms small aggregates, however, when incubated at room temperature for an extended period of time. Limited proteolysis of EF-3 with trypsin produced the first cleavage at the N-side of Gln775, generating a 90-kDa N-terminal fragment and a 33-kDa C-terminal fragment. The N-terminal fragment slowly undergoes further digestion generating two major bands, one at approximately 75 kDa and the other at approximately 55 kDa. The latter was unusually resistant to further tryptic digestion. The 33-kDa C-terminal fragment was highly sensitive to tryptic digestion. A 30-min tryptic digest showed that the N-terminal 60% of EF-3 was relatively inaccessible to trypsin, whereas the C-terminal 40% was readily digested. These results suggest a tight structure of the N-terminus, which may give rise to the 58 degrees C transition, and a loose structure of the C-terminus, giving rise to the 39 degrees C transition. Three potentially functional domains of the protein were relatively resistant to proteolysis: the supposed S5-homologous domain (Lys102-Ile368), the N-terminal ATP-binding cassette (Gly463-Lys622), and the aminoacyl-tRNA-synthase homologous domain (Glu820-Gly865). Both the basal and ribosome-stimulated ATPase activities were inactivated by trypsin, but the ribosome-stimulated activity was inactivated faster.

Amino Acid Sequence↗
PubMed2002

Mechanistic investigation of the interaction between bisquaternary antimicrobial agents and phospholipids by liquid secondary ion mass spectrometry and differential scanning calorimetry.

Mechanisms of interaction between the antimicrobial drugs decamethoxinum and aethonium, which are based on bisquaternary ammonium compounds, and a phospholipid component of biological membranes, dipalmitoylphosphatidylcholine, were studied by means of liquid secondary ion mass spectrometry (LSIMS) and differential scanning calorimetry (DSC). Supramolecular complexes of the drugs with this phospholipid were recorded under secondary ion mass spectrometric conditions. The dependence of the structures of these complexes on structural parameters of the dications of the bisquaternary ammonium compounds was demonstrated. Tandem mass spectrometric investigations of the metastable decay of doubly charged ions of decamethoxinum and aethonium complexes with dipalmitoylphosphatidylcholine allowed estimation of structural parameters of these complexes in the gas phase. Interactions of decamethoxinum and aethonium with model membrane assemblies built from hydrated dipalmitoylphosphatidylcholine were studied using DSC. It was shown that while both drugs can interact with model membranes, the mechanisms of such interactions for decamethoxinum and aethonium differ. The correlation between the nature of these interactions and structural and electronic parameters of the dications of the two bisquaternary agents is discussed. Interpretation of combined mass spectrometric and calorimetric experimental data led to proposals that the molecular mechanisms of antimicrobial action of bisquaternary ammonium compounds are related to their effect on the membrane phospholipid components of microbial cells.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1993

Heparin-von Willebrand factor binding as assessed by isothermal titration calorimetry and by affinity fractionation of heparins using synthetic peptides.

The ability of proteins to bind heparin, a heterogeneous sulfated glycosaminoglycan, likely depends on the conformational uniqueness of specific binding domains. Based on the motif of a consensus heparin-binding synthetic peptide, a 23-residue sequence (Y565-A587) of human von Willebrand factor (vWF) was previously identified which binds heparin with affinity comparable to that of the native protein (Sobel, M., Soler, D. F., Kermode, J. C., and Harris, R. B. 1992 J. Biol. Chem. 267, 8857-8862). This peptide undergoes a conformational change upon binding heparin. Isothermal titration calorimetry has now been used to further quantify this binding reaction. In experiments done at 25 degrees C, Y565-A587 bound heparin with about the same affinity (Kd = 9.0 x 10(-7) M) as a "core" sequence peptide encompassing residues K569-I580. Binding between these peptides and heparin is overwhelmingly enthalpically favored and is dependent on the formation of productive electrostatic bonds; hydrophobic interactions do not play a significant role in mediating binding. Furthermore, when immobilized on Sepharose in a manner which does not compromise essential cationic residues, the vWF domain peptides are effective affinity ligands. They bind a species of heparin which possesses significantly enhanced affinity for native vWF. The Kd for binding between the high-affinity heparin and Y565-A587 is about threefold lower than that determined with crude, unfractionated heparin. Thus, the vWF peptides are a useful model for studying the physiological role of heparin binding to the native protein.

Amino Acid Sequence↗
PubMed1996

Interaction of 4-tert-butyl-[3-(2-chloroethyl) ureido] benzene with phosphatidylcholine bilayers: a differential scanning calorimetry and infrared spectroscopy study.

We have investigated the interaction between a new antineoplastic drug, 4-tert-butyl-[3-(2-chloroethyl)ureido] benzene (tBCEU), and distearoylphosphatidylcholine bilayers using differential scanning calorimetry, Fourier transform infrared spectroscopy (FT-IR), and high-pressure infrared spectroscopy. The results obtained with the three different techniques indicate that the drug incorporates in the lipid bilayer. More specifically, the incorporation of the tBCEU results in a decrease in the phase transition temperature of the lipid and in an increase in the amount of gauche conformers in the liquid-crystalline phase. In the gel phase, high-pressure FT-IR results indicate that the incorporation of tBCEU decreases the acyl chain packing. In addition, the results suggest the presence of hydrogen bonding between the lipid carbonyl group and a hydrogen bond donor in the tBCEU molecule. A possible candidate for this donor is the NH group adjacent to the phenyl ring. A model is proposed for the incorporation of tBCEU in lipid bilayers, with the hydrophobic portion of the drug intercalated between the lipid bilayers and the hydrophilic region located close to the interfacial region of the bilayer.

Antineoplastic Agents↗
PubMed2002

Determination of the volumetric properties of proteins and other solutes using pressure perturbation calorimetry.

Pressure perturbation calorimetry is a new technique that measures the heat change in a solution that results when the pressure above the solution is changed. When used in a differential calorimeter containing a dilute solution of solute in the sample cell and the corresponding buffer in the reference cell, the measured differential heat can be used to calculate the thermal coefficient of expansion of the partial volume of the solute, alpha. For proteins in dilute aqueous solution, alpha is dominated by a temperature-dependent contribution arising from the interaction of protein groups with water at the protein-solvent interface. This arises due to the effect of the protein groups on the hydrogen-bonded structure of water, and thereby clearly differentiates between structure-making hydrophobic groups and structure-breaking hydrophilic groups. This solvation contribution to alpha can be accentuated in solvents having more structure (deuterium oxide) than water and attenuated in solvents having less structure (2.8 M guanidinium sulfate). Six different proteins (chymotrypsinogen, pepsinogen, lysozyme, bovine pancreatic trypsin inhibitor, ribonuclease A, and T4 lysozyme) were examined carefully by this technique, allowing estimates of various volumetric parameters including the volume change resulting from thermal unfolding of each protein. For ribonuclease A, results obtained in both water and deuterium oxide led to an estimate of the accessible surface area of the native protein of approximately 45% relative to the fully reduced unfolded protein. Also, it was also found that ligand binding to ribonuclease A led to changes in alpha, suggesting a burial of some surface area in the ligand-protein complex.

Animals↗
PubMed1993

Kinetic study of the thermal denaturation of G actin using differential scanning calorimetry and intrinsic fluorescence spectroscopy.

Previous studies of the thermal denaturation of G actin have yielded conflicting results which have led to contradictory interpretations of the denaturation process. In their interpretations, these authors postulated that the thermal denaturation of G actin can be described by thermodynamical equilibrium laws despite the fact that this transition is irreversible. Using differential scanning calorimetry and fluorescence spectroscopy, we now show that thermal denaturation of G actin does not obey to equilibrium thermodynamics, but rather, is a two-step irreversible phenomenon under kinetic control. It can therefore be analyzed using the recently developed Sanchez-Ruiz equations.

Actins↗
PubMed1997

Differential scanning calorimetry studies on an Antarctic nematode (Panagrolaimus davidi) which survives intracellular freezing.

Differential scanning calorimetry was used to characterize thermal events associated with freezing and melting of suspensions and extracts of Panagrolaimus davidi, an Antarctic nematode which can survive intracellular freezing. Nematode suspensions produced a single freezing exotherm with a shoulder on the peak representing the freezing of the nematodes. A shoulder on the peak of melting endotherms indicates the melting of the nematodes and of the water surrounding them. Exotherms were also detected from individual nematodes mounted in liquid paraffin. The freezing of nematodes was very rapid and in marked contrast to that of freezing-tolerant insects and vertebrates, which take hours or days to freeze. Eighty-two percent of the nematodes' body water froze. High levels of survival were obtained in nematodes exposed to temperatures down to -40 degrees C. No additional thermal events were observed after the freezing event and before the melting of samples cooled to -40 degrees C, indicating no changes in the proportion of body water frozen. Ice nucleating activity is present in nematode suspensions but not in supernatants from nematode extracts. No thermal hysteresis activity was detected in nematode extracts.

Animals↗
PubMed1995

The kinetics of the thermal denaturation of collagen in unrestrained rat tail tendon determined by differential scanning calorimetry.

This paper shows that the position and shape of the denaturation endothem of collagen fibrils are governed by the kinetics of an irreversible rate process. This was proved by measuring the rate of denaturation in rat tail tendons held isothermally at different temperatures, thereby determining rate constant characteristics such as the activation enthalpy and entropy and predicting endotherm position and shape therefrom. Comparison with actual scanning results showed good correspondence. Isothermal measurements of the rate of collagen denaturation, measured continuously using a calorimetric method, were used to determine rate constants for collagen denaturation in tendons immersed in water and 0.5 M acetic acid. The temperature dependence of the rate constants were fitted to the three rate process models, previously examined theoretically: the D and z formulation, the Arrhenius equation and the absolute rate theory. For example, in water the activation enthalpy was 0.518 (+/- 0.016) Mj mol-1 and the activation entropy 1.485 (+/- 0.049) kj mol-1 K-1, while in acetic acid the corresponding figures were 1.306 (+/- 0.099) Mj mol-1 and 4.142 (+/- 0.323) kj mol-1 K-1. These characteristics are discussed in terms of the thermal activation of a region of the molecule, the co-operative unit. The ratio of the activation enthalpy to the calorimetry enthalpy of denaturation indicated a co-operative unit that was 66 (+/- 5) residues long when fibrils were swollen in acetic and the collagen molecules acted essentially independently. On the other hand the intact fibrils in water gave a co-operative unit of 26 (+/- 1) residues long. The reason for the reduction in size of the co-operative unit is that it is surrounded, and therefore stabilized by other molecules in the fibre. It is interesting to note that the suggested co-operative unit lies almost entirely within the "gap" zone of the collagen fibril in its quarter-staggered arrangement of molecules. We believe that the co-operative unit would be represented by a domain that is free of stabilising hydroxyproline residues. Indeed such a domain exists near the C terminus of the triple helix from Gly877 to Pro941, i.e. 65 residues. In acetic acid, activation is similar to that of collagen molecules in solution. All the inter alpha-chain hydrogen bonds in the co-operative unit are broken and the separate chains in this short region are free to flail around under the action of thermal collisions relatively unimpeded by intermolecular interactions.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1996

Hyperthermophile protein folding thermodynamics: differential scanning calorimetry and chemical denaturation of Sac7d.

Recombinant Sac7d protein from the thermoacidophile Sulfolobus acidocaldarius is shown to be stable towards acid, thermal and chemical denaturation. The protein maintains a compact native fold between pH 0 and 10 in 0.3 M KCl and 25 degrees C as indicated by near and far UV circular dichroism spectra. Thermal unfolding followed by differential scanning calorimetry (DSC) occurs as a reversible, two-state transition from pH 0 to 10, with a maximal Tm of 90.7 degrees C between pH 5 and 9. At pH 0 the protein unfolds with a Tm of 63.3 degrees C. Plots of the enthalpy of unfolding as a function of Tm are linear and yield an anomalously low delta Cp of 497 (+/-20) cal deg-1 mol-1 using the Kirchhoff relation. Guanidine hydrochloride and urea-induced chemical denaturation of Sac7d occur reversibly and can be followed by circular dichroism. Global non-linear regression of the chemical denaturation data constrained by DSC determined values for delta Hm and Tm yields a delta Cp of unfolding of 858 (+/-21) cal deg-1 mol-1. The higher delta Cp is in good agreement with that predicted from the buried polar and apolar surface areas using the NMR solution structure. It is similar to values reported for mesophile proteins of comparable size, indicating that the packing and change in solvent-accessible surface area on unfolding are not unusual. Similarly, guanidine hydrochloride and urea m-values are in good agreement with those expected for a protein of 66 residues. Possible explanations for the difference in delta Cp determined by application of the Kirchhoff relation to DSC data and that determined by the global fit are discussed. Protein stability curves defined by either delta Cp values are similar to those observed for small mesophile proteins. Although the protein is thermally stable, it is marginally stable thermodynamically with a free energy of unfolding of 1.6 (+/-0.1) kcal mol-1 at the growth temperature of 80 degrees C. The large number of potential ion pairs on the surface of this hyperthermophile protein do not result in an inordinate increase in stability. Post-translational modification, possibly lysine monomethylation, appears to be the single most important stabilizing factor that distinguishes the native hyperthermophile protein from small mesophile proteins. Additional stabilization in vivo is expected from compatible osmolytes (polyamines) and DNA-binding.

Archaeal Proteins↗
PubMed2000

Thermodynamics of DNA binding and condensation: isothermal titration calorimetry and electrostatic mechanism.

The thermodynamics of binding of the trivalent cations cobalt hexammine and spermidine to plasmid DNA was studied by isothermal titration calorimetry. Two stages were observed in the course of titration, the first attributed to cation binding and the second to DNA condensation. A standard calorimetric data analysis was extended by applying an electrostatic binding model, which accounted for most of the observed data. Both the binding and condensation reactions were entropically driven (TDeltaS approximately +10 kcal/mol cation) and enthalpically opposed (DeltaH approximately +1 kcal/mol cation). As predicted from their relative sizes, the binding constants of the cations were indistinguishable, but cobalt hexammine had a much greater DNA condensing capacity because it is more compact than spermidine. The dependence of both the free energy of cobalt hexammine binding and the critical cobalt hexammine concentration for DNA condensation on temperature and monovalent cation concentration followed the electrostatic model quite precisely. The heat capacity changes of both stages were positive, perhaps reflecting both the temperature dependence of the dielectric constant of water and the burial of polar surfaces. DNA condensation occurred when about 67 % of the DNA phosphate charge was neutralized by cobalt hexammine and 87 % by spermidine. During condensation, the remaining DNA charge was neutralized.

Binding, Competitive↗