Calorimetric,density and circular dichroism studies of the reversible structural transition in Pf1 filamentous bacterial virus

The macromolecular structural transition of Pf1 filamentous bacterial virus detected by X-ray diffraction analysis has been studied in virus solutions by density, circular dichroism, and microcalorimetric measurements. The reversible structural change occurring between 5 °C and 25 °C has a calorimetrically determined transition enthalpy ΔH_t,cal of 14·5 ± 1.5 kJ (mol protein)^?1. The transition curves resulting from the density, circular dichroism, and calorimetric measurements have been analysed in terms of a two-state process to extract the van't Hoff enthalpy. Comparison of the effective transition enthalpy and the calorimetric ΔH_t,cal values gives about 26 protein subunits as the size of the co-operative unit. Parallel heat capacity and density measurements on fd virus show no such transition, in agreement with X-ray diffraction studies.     

Calorimetric measurement of enthalpy change in the isothermal helix-coil transition of poly(L-ornithine) in aqueous solution.

The enthalpy change associated with the isothermal pH-induced uncharged coil-to-helix transition ΔH_h° in poly(L -ornithine) in 0.1 N KCl has been determnined calorimetrically to be ?1530 ± 210 and ?1270 ± 530 cal/mol at 10° and 25°C, respectively. Titration data provided information about the state of charge of the polymer in the calorimetric experiments, and optical rotatory dispersion data about its conformation. In order to compute ΔH_h°, the observed calorimetric heat was corrected for the heat of breaking the sample cell, the heat of dilution of HCl, the heat of neutralization of the OH^? ion, and the heat of ionization of the δ-amino group in the random coil. The latter was obtained from similar calorimetric measurements on poly(D ,L -ornithine). Since it was discovered that poly(L -ornithine) undergoes chain cleavage at high pH, the calorimetric measurements were carried out under conditions where no degradation occurred. From the thermally induced uncharged helix–coil transition curve for poly(L -ornithine) at pH 11.68 in 0.1 N KCl in the 0°–40°C region, the transition temperature T_tr and the quantity (?θ_h/?T)_Ttr have been obtained. From these values, together with the measured values of ΔH_h°, the changes in the standard free energy ΔG_h° and entropy ΔG_h°, associated with the uncharged coil-to-helix transition at 10°C have been calculated to be ?33 cal/mol and ?5.3 cal/mol deg, respectively. The value of the Zimm–Bragg helix–coil stability constant σ has been calculated to be 1.4 × 10^?2 and the value of s calculated to be 1.06 at 10°C, and between 0.60 and 0.92 at 25°C.     

Solution properties of synthetic polypeptides. XII. Enthalpy changes accompanying helix-coil transition of polypeptide

For helix-coil transitions of polypeptide in binary mixtures consisting of helix-forming solvent and coil solvent, the transition enthalpy ΔH(T,x) has been found to depend significantly on temperature (T) and solvent composition (x). For such systems, calorimetric measurements may yield some averages of ΔH(T,x) which are no longer amenable to direct comparison with ΔH itself. Theoretical equations relating calorimetric data to ΔH(T,x) are derived and tested favorably with experimental data. It is demonstrated that the transition enthaply from heat capacity measurements is approximately equal to ΔH_cfm, while those from heat of dilution and heat of solution measurements are equal to ΔH_c. Here ΔH_c denotes the value of ΔH at the transition point and f_m represents the maximum helical content attained in a thermally induced transition. The discrepancies among calorimetric data are also discussed.     

Calorimetric experiments of Mn2+-binding to α-lactalbumin

We measured by batch microcalorimetry the standard enthalpy change ΔH° of the binding of Mn²⁺ to apo-bovine α-lactalbumin; ΔH° = −90 ± 4kJ·mol⁻¹. The binding constants, K_Mn²⁺, calculated from the calorimetric and circular dichroism titration curves, are (4.6±1) · 10⁵M⁻¹, respectively. Batch calorimetry confirms the competitive binding of Ca²⁺, Mn²⁺ and Na⁺ to the same site. The relatively small enthalpy change for Mn²⁺ binding compared to Ca²⁺ binding favours a model of a rigid and almost ideal Ca²⁺-complexating site, different from the well-known EF-hand structures. Cation binding to the high-affinity site most probably triggers the movement of an α-helix which is directly connected to the complexating loop.     

Thermodynamics of the B to Z transition in poly(dGdC)

The thermodynamics of the B to Z transition in poly(dGdC) was examined by differential scanning calorimetry, temperature-dependent absorbance spectroscopy, and CD spectroscopy. In a buffer containing 1 mM Na cacodylate, 1 mM MgCl₂, pH 6.3, the B to Z transition is centered at 76.4°C, and is characterized by ΔH_cal = 2.02 kcal (mol base pair)^?1 and a cooperative unit of 150 base pairs (bp). The t_m of this transition is independent of both polynucleotide and Mg²⁺ concentrations. A second transition, with ΔH_cal = 2.90 cal (mol bp)^?1, follows the B to Z conversion, the t_m of which is dependent upon both the polynucleotide and the Mg²⁺ concentrations. Turbidity changes are concomitant with the second transition, indicative of DNA aggregation. CD spectra recorded at a temperature above the second transition are similar to those reported for ψ(–)-DNA. Both the B to Z transition and the aggregation reaction are fully and rapidly reversible in calorimetric experiments. The helix to coil transition under these solution conditions is centered at 126°C, and is characterized by ΔH_cal = 12.4 kcal (mol bp)^?1 and a cooperative unit of 290 bp. In 5 mM MgCl₂, a single transition is seen centered at 75.5°C, characterized by ΔH_cal = 2.82 kcal (mol bp)^?1 and a cooperative unit of 430 bp. This transition is not readily reversible in calorimetric experiments. Changes in turbidity are coincident with the transition, and CD spectra at a temperature just above the transition are characteristic of ψ(–)-DNA. A transition at 124.9°C is seen under these solution conditions, with ΔH_cal = 10.0 kcal (mol bp)^?1 and which requires a complex three-step reaction mechanism to approximate the experimental excess heat capacity curve. Our results provide a direct measure of the thermodynamics of the B to Z transition, and indicate that Z-DNA is an intermediate in the formation of the ψ-(–) aggregate under these solution conditions.     

Experimental theromodynamics of the helix-random coil transition. I. Influence of polymer concentration and solvent composition in the PBG-DCA-EDC system

The course of the reversible helix formation of poly(γ-benzyl L -glutamate) (PBG) dissolved in a mixture of dichloroacetic acid (DCA) and 1,2-dichloroethane (EDC) was followed by measuring the heat capacity and the optical rotation of the system through the transition region. The results of these measurements indicate that the transition enthalpy ΔH the transition temperature T_c, and the Zimm-Bragg parameter σ depend considerably on the PBG concentration as well as on the composition of the solvent. For the standard state of infinite dilution, however, a linear extrapolation of the measured ΔH if values results in a standard value ΔH° = 950 cal./mole, independent of the solvent composition. The results of the calorimetric measurements are discussed in relationship to changes in optical rotation. Some peculiarities in the measured thermodynamic and optical properties in solutions with relatively high content of dichloroacetic acid are reported.     

Influence of DPPE surface undulations on melting temperature determination: UV/Vis spectroscopic and MD study

One of the most distinguished quantities that describes lipid main phase transition, i.e. the transition from the gel (L_β(′)) to the fluid (L_α) phase, is its melting temperature (T_m). Because melting is accompanied by a large change in enthalpy the, L_β(′) → L_α transition can be monitored by various calorimetric, structural and spectroscopic techniques and T_m should be the same regardless of the metric monitored or the technique employed. However, in the case of DPPE multilamellar aggregates there is a small but systematic deviation of T_m values determined by DSC and FTIR spectroscopy. The aim of this paper is to explain this discrepancy by combined UV/Vis spectroscopic and MD computational approach. Multivariate analysis performed on temperature-dependent UV/Vis spectra of DPPE suspensions demonstrated that at 55 ± 1 °C certain phenomenon causes a small but detectable change in suspension turbidity, whereas a dominant change in the latter is registered at 63.2 ± 0.4 °C that coincides with T_m value determined from DSC curve. If this effect should be ignored, the overall data give T_m value the same as FTIR spectra data (61.0 ± 0.4 °C). As the classical MD simulations suggest that about 10° below T_m certain undulations appear at the surface of DPPE bilayers, we concluded that certain discontinuities in curvature fluctuations arise at reported temperature which are to some extent coupled with lipid melting. Ultimately, such events and the associated changes in curvature affect T_m value measured by different techniques.     

Influencing factors of dsDNA dye (high-resolution) melting curves and improved genotype call based on thermodynamic considerations

High-resolution melting of dsDNA using suitable dyes is a simple and cost-effective alternative for mutation scanning. Analytical variation can result from salt and template concentration (C_T). To overcome this problem the van’t Hoff transition enthalpy ΔH_vH from dsDNA melting curves was estimated and used for robust genotype calling or mutation scanning. Model calculations show the effect of salt, C_T, and temperature resolution on (1) T_m, (2) difference plots, (3) melting peaks, and (4) calculated ΔH_vH. Using the LightCycler480, the influence of dye (ResoLight) and scanning speed was assessed. The model calculations show that only ΔH_vH is not influenced by salt and C_T. Higher amplicon enthalpy ameliorates the ability to discriminate mutations. Temperature resolution is important for peak- but not for curve-based genotyping. ResoLight increases T_m by 3.4 °C, while lowering ΔH_vH. Using a 4-bp deletion in a 200-bp amplicon as a model, the miscalling rate improved substantially, when using ΔH_vH instead of difference plots. Melting curves of duplex DNA are influenced by dye and salt and less so by duplex concentrations. As predicted from theory, ΔH_vH is a robust measure for mutation detection in two-state melting. The influence of dyes on enthalpy is of general impact for PCR assays.     

Conformation of adenosine 3′,5′-monophosphate in solution as studied by the NMR-desert method II. Self-association and temperature-dependent glycosidic isomerization at pH 7

A study has been made of the association and the temperature-dependent conformation of adenosine 3′,5′-monophosphate (cyclic AMP) in a neutral aqueous (²H₂O) solution by means of proton magnetic resonance chemical shift and relaxation. The concentration and temperature-dependent chemical shifts of H(1′), H(2), and H(8), have enabled us to estimate the self-association constant, K_a = 1.1 ± 0.3 M^?1 at 25°C and thermodynamic parameters ΔH = ?5.8 ± 1.5 kcal/mol and ΔS (25°C) = ?19.0 ± 3 cal/mol per degree.The NMR-DESERT (Deuterium Substitution Effect on Relaxation Times) method has been utilized for the determination of the syn-anti conformational equilibrium in the monomeric state and for the determination of the mutual orientation of the two adenine rings in the dimeric state of cyclic AMP. The molecules were found to coexist with nearly equimolarity of syn-anti conformers and thermal activation of the molecules perturbs the syn-anti conformational equilibrium to comprise the syn form in preference at higher temperature. The glycosidic isomerization (from anti to syn) was found to be characterized both by a positive enthalpy change and by a positive entropy change. The cyclic AMP molecules prefer to take a ‘trans-stacking’ conformation in the dimeric state where the two molecules are arranged in such a way that the H(2) of one molecule is close to the H(8) of the other.     

Experimental thermodynamics of the helix--random coil transition. 3. Determination of the transition enthalpies of the helical complexes poly(I+C) and poly I in solution

The enthalpies of the helix-coil transitions of the ordered polynucleotide systems of poly(inosinic acid)–poly(cytidylic acid) [poly(I + C)], (helical duplex), and of poly (inosinic acid) [poly(I + I + I)], (proposed secondary structure: a triple-stranded helical complex), were determined by using an adiabatic twin-vessel differential calorimeter. Measuring the temperature course of the heat capacity of the aqueous polymer solutions, the enthalpy values for the dissociation of the helical duplex poly (I + C) and the three-stranded helical complex poly(I + 1 + 1), respectively, were obtained by evaluating the additional heat capacity involved in the conformational change of the polynucleotide system in the transition range. The ΔH values of the helix-coil transition of poly (I + C) resulting from the analysis of the calorimetric measurements vary between the limits 6.5 ± 0.4 kcal/mole (I + C) and 8.4 ± 0.4 kcal/mole (I + C). depending on the variation of the cation concentration ranging from 0.063 mole cations kg H₂O to 1.003 mole cations/kg H₂O. The calorimetric investigation of an aqueous poly I solution (cation concentration 1.0 mole/kg H₂O) yielded the enthalpy value ΔH = 1.9 ± 0.4 kcal/mole (I), a result which has been interpreted qualitatively following current models of inter- and intramolecular forces of biologically significant macromolecules. Additional information on the transition behavior of poly(I+ C)Was obtained by ultraviolet and infrared absorption measurements.     

A calorimetric study of the binding of 2'-deoxyuridine-5'-phosphate and 5-fluoro-2'-deoxyuridine-5'-phosphate to thymidylate synthetase.

The thermodynamic parameters, ΔH′, ΔG′, and ΔS′, and the stoichiometry for the binding of the substrate 2′-deoxyuridine-5′-phosphate (dUMP) and the inhibitor 5-fluoro-2′-deoxyuridine-5′-phosphate (FdUMP) to Lactobacillus casei thymidylate synthetase (TSase) have been investigated using both direct calorimetric methods and gel filtration methods. The data obtained show that two ligand binding sites are available but that the binding of the second mole of dUMP is extremely weak. Binding of the first mole of dUMP can best be illustrated by dUMP + TSase + H⁺?(dUMP-TSase-H⁺). [1] The enthalpy, ΔH₁′, for reaction [1] was measured directly on a flow modification of a Beckman Model 190B microcalorimeter. Experiments in two different buffers (I = 0.10 m) show that ΔH₁′ = ?28 kJ mol^?1 and that 0.87 mol of protons enters into the reaction. Analysis of thermal titrations for reaction [1] indicates a free energy change of ΔG₁′ = ?30 kJ mol^?1 (K₁ = 1.7 × 10⁵ m^?1). From these parameters, ΔS₁′ was calculated to be +5 J mol^?1 degree^?1, showing that the reaction is almost totally driven by enthalpy changes. Gel filtration experiments show that at very high substrate concentrations, binding to a second site can be observed. Gel filtration experiments performed at low ionic strength (I = 0.05 m) reveal a stronger binding, with ΔG₁′ = ?35 kJ mol^?1 (K₁ = 1.2 × 10⁶ m^?1), suggesting that the forces driving the interaction are, in part, electrostatic. Addition of 2-mercaptoethanol (0.10 m) had the effect of slightly increasing the dUMP binding constant. Binding of FdUMP to TSase is best illustrated by 2FdUMP + TSase + n_HH⁺?FdUMP₂ ? TSase ? (H⁺)_nH. [2] The enthalpy for this reaction, ΔH₂, was also measured calorimetrically and found to be ?30 kJ mol^?1 with n_H = 1.24 at pH 7.4 Assuming two FdUMP binding sites per dimer as established by Galivan et al. [Biochemistry15, 356–362 (1976)] our calorimetric results indicate different binding energies for each site. Based on the binding data, a thermodynamic model is presented which serves to rationalize much of the confusing physical and chemical data characterizing thymidylate synthetase.     

The enthalpy of oxidation of flavin mononucleotide: Temperature dependence of in vitro bacterial luciferase bioluminescence

The enthalpy of the bioluminescent reaction

$\text{FMNH}{}_2\text{+ RCHO + O}{}_2\text{→}\text{luciferase}\text{FMN + RCOO + H}{}_3\text{O}{}^{\mathrm{+}}\text{+ hv}$

has been studied by direct calorimetric methods. Bacterial luciferase, isolated from Beneckea harveyi (formerly strain MAV) has been used to catalyze the oxidation of reduced flavin mononucleotide (FMNH₂) and a long chain aliphatic aldehyde (dodecanal, RCHO) by molecular oxygen to give the indicated products and blue-green light. The enthalpy measured for this process was found to be ΔH_L = ?338.9 k.J (mol FMN)^?1 (?81.0 kcal) at 25.00 °C and ?402.9 kJ (mol FMN)^?1 (?96.3 kcal) at 7.00 °C. Calculations based on redox electrode potentials indicate a corresponding value of the free energy change, ΔG_L = ?464.8 kJ (mol FMN)^?1 (?111.1 kcal), at 25 °C. Measurements were performed in 0.15 m phosphate buffer, pH 7.0 and the values were arrived at by correcting the observed heats for the heat associated with the autoxidation process: FMNH₂ + O₂ ? FMN + H₂O₂; ΔH_D = ?158.5 kJ (mol FMN)^?1 (?37.8). These data and a detailed thermodynamic analysis have demonstrated the need for two parameters, referred to as the intrinsic free energy, ΔG₁, and intrinsic enthalpy, ΔH₁, which are functionally defined by the relations ΔG_I = ΔG_L ? uhvΔH_I = ΔH_L ? uhv, where u is the quantum yield of the reaction expressed in einsteins mole^?1.These parameters reflect the thermochemistry of the bioluminescent reaction corrected for emitted photons. Thus, they are useful for comparing the thermochemistry of a chemiluminescent process. Their values for the bacterial luciferase system at 25 °C and pH 7.0 are ?391.6 and ?266.9 kJ (mol FMN)^?1 (?93.6 and ?63.8 kcal), respectively, assuming a value of 0.3 for the quantum yield. The calorimetric data also suggest the existence of a long-lived species which persists after photon emission.     

Effects of deuterium on the thermal stability of the poly A-poly U helix

The effect of deuterium on the thermal stability of the polynecleotide double helix formed by the homopolymers polyadenylie acid (poly A)and polyuridylic acid (poly U)has been invertigated by measuring the best capacity as a function of temperature in an automatic scanning adiabatic calorimeter. Hydrogen-bounded and deuterium-bonded helical conformations of the polynecleotides used have been melted in H₂O and D₂O₂ respectively, as solvent. Within the limits of experimental error, there is no dfference in the measured enthallpy change accompanying the helix-random coil transition. The enthalpy change ΔH is 6.6 Kcal/MBP ub any case. The half-conversion temperatures T_m differ by two degrees. T_m for poly (AU) in H₂O is 45.8°C, T_m for poly (AU) in D₂is 47.7.°C.     

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 $\text{kcal}\text{mol}$.     

Chain length and oligonucleotide stability at high pressure

The effect of hydrostatic pressure on the helix-coil transition temperature (T_m) was measured for the DNA oligomers (dA)_n(dT)_n, where n = 11, 15, and 19, in 50 mM NaCl. The data were analyzed in light of previously published data for the polymer, poly(dA)·poly(dT) under the same conditions. As has been observed for DNA polymers, increasing the hydrostatic pressure led to an increase in the T_m of the oligomers; however, the effect of pressure diminished with decreasing chain length. The value of dT_m/dP decreased linearly with the inverse of the chain length varying from 3.15 × 10⁻²°C MPa⁻¹ for the polymer to 0.7 × 10⁻²°C MPa⁻¹ for the 11-mer. The two-state or van't Hoff enthalpy (ΔH_vH) of the helix-coil transition was obtained by analysis of the half-width of the thermal transition. As expected, ΔH_vH decreases with decreasing chain length. In contrast to the behavior of the polymer, poly(dA)·poly(dT), and (dA)₁₉(dT)₁₉, the ΔH_vH of the two shorter duplex oligonucleotides displayed a small pressure dependence dΔH_vH/dP≃−0.4 kJ MPa⁻¹ in both cases. The changes observed in the T_m and ΔH_vH were not sufficient to explain the magnitude of the chain-length dependence of the pressure effect. To interpret the large chain-length dependence of dT_m/dP, we propose that the terminal base pairs contribute a negative volume change to the helix-coil transition. Base pairs distant from the ends exhibit behavior characterized by the polymer where end effects are assumed to be negligible, i.e., a positive volume change for the helix-coil transition. The negative volume change of separating terminal bases may originate from the imperfect interactions these base pairs form with water due to the existence of several energetically equivalent conformations. This is reminiscent of one of the mechanisms proposed to be important in the pressure-induced dissociation of multimeric proteins into their constituent subunits. © 1996 John Wiley & Sons, Inc.     

A microcalorimetric study of the reaction catalysed by pyruvate kinase

The enthalpy change for phosphorylation of ADP^3? by PEP^3? catalysed by pyruvate kinase has been determined at 25°C using flow microcalorimetry. Measurements were made at pH 8 in three buffer systems TRIS, TEA and HEPES and also at pH 8.5 in TRIS buffer. The values of ΔH obtained, ?8.75 kJ mol^?1 in TRIS, ?7.3₉ kJ mol^? in TEA and ?6.1₉ kJ mol^?1 in HEPES surprisingly display a dependence on the buffer system used. The enthalpy change was combined with free energy data to calculate the entropy change for the catalysed reaction.     

Transition temperature and enthalpy change dependence on stabilizing and destabilizing ions in the helix–coil transition in native tendon collagen

The transition temperatures t_t and enthalpy changes ΔH in the helix–coil transition of solid tendon collagen soaked in a solution containing one of the following stabilizing or destabilizing agents, HCHO, NaF, NaCl, NaI, NaBr, NaOH, NH₂CONH₂, CaCl₂, MgCl₂, were measured as a function of molar concentration by a calorimetric method. The temperature and the enthalpy changes accompanying the transition behaved in a similar manner: when the t_t was depressed by the presence of ions, similar behaviour was observed in ΔH. Both parameters (t_t and ΔH) increased for HCHO, and decreased for NaF and NaCl at concentrations lower than 0.2 M. Above 0.2 M they increased for NaF and NaCl, and decreased in the presence of the other reagents listed above. The average t_t and the ΔH observed in collagen soaked in water were 63.5°C and 12.3 cal/g, respectively. In addition to the parameters mentioned above, the molar effectiveness of the various reagents was obtained for the cases where there was a linear relationship between the t_t and molar concentration of the reagent in the solution. Since both the t_t and the ΔH were observed to vary, the entropy change (ΔS) accompanying the transition was calculated using thermodynamic relations. In order to explain the ΔS observed as a function of ionic concentration, the thermodynamic relationships have been obtained from a partition function under suitable assumptions. Since the partition function is dependent on the number of hydrogen bonds responsible for collagen stability, the result obtained has been compared with the values predicted by the two most quoted models for collagen. The present study is in accordance with the Ramachandran model for collagen structure, which predicts more than one hydrogen bond per three residues.     

Effects of sugars and polyols on the sol-gel transition of k-carrageenan: calorimetric study

The effects of sugars and polyols on the sol-gel transition of k-carrageenan were studied in 0.025 m KCl by means of differential scanning calorimetry. Addition of these compounds invariably raised the gelling temperature T_g, with an increase in their concentration, accompanying a decreased (less negative) enthalpy of gelation, ΔH_g. This indicates that it is not ΔH_g but the entropy of gelatin, ΔS_g, which plays an essential role in the gel stabilizatiion by them, thus differing from the enthalpy-driven stabilization by addition by KCl, ethanol or carrageenan. It seemed that such a large change in ΔS_g relative to ΔH_g predominantly occurs in the process of replacing polymer solvent hydrogen bonds by polymer—polymer hydrogen bonds with minor contribution of the conformational entropy of polymer chains. The gel-stabilizing ability of different sugars and polyols is discussed in terms of their different influences on the structure of water.     

Thermodynamics and coordination characteristics of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 extraction complex

The extraction equilibrium of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 complex was carried out in the crown ether1,2-dichloroethaneHCl aqueous solution system at different temperatures. The extraction complex has the overall composition (L)₂·(H₃O⁺·χH₂O)₂·UO₂Cl₄²⁻ (L = dicyclohexano-24-crown-8). The values of the extraction equilibrium constants (K_ex) increase steadily with a decrease in temperature: 13.5 (298 K), 7.96 (301 K), 4.20 (303 K) and 2.07 (305 K). A plot of log K_ex against 1/T shows a straight line. The value of the enthalpy change, ΔH°, was calculated from the slope and equals −212 kJ mol⁻¹. The value of the entropy change, ΔS°, was calculated from ΔH° and K_ex and equals −690 J K⁻¹ mol⁻¹, whereas ΔG° = −6.45 kJ mol⁻¹. Comparing these thermodynamic parameters with those of the dicyclohexano-18-crown-6 isomer A [1] (ΔS° = −314 J K⁻¹ mol⁻¹, ΔH° = −101 kJ mol⁻¹ and ΔG° = −8.37 kJ mol⁻¹), it can be seen that ΔH° and ΔS° are more negative for the former than for the latter, and both are enthalpy-stabilized complexes. The molecular structure of the complex has the feature that there are two H₅O₂⁺ ions in it, in contrast to the H₃O⁺ ions in the dicyclohexano-18-crown-6 isomer A complex [1]. Each of the H₅O₂⁺ ions is held in the crown ether cavity by four hydrogen bonds. The H₅O₂⁺ ion has a central bond. The uranium atom forms UO₂Cl₄²⁻ as a counterion away from the crown ether. The formation of this complex is in good agreement with more negative entropy change and less negative free energy change, as mentioned above.     

Calorimetric studies of the interaction between dna and poly-l-lysine

The transition enthalpy ΔH of the helix—random coil transition of the DNA-polylysine complex was measured as a function of the peptide:nucleotide ratio by the help of an adiabatic scanning differential calorimeter. Furthermore the transition enthalpy of a complex with a specific peptide:nucleotide ratio was determined as a function of the cation concentration of the solution. Finally the reaction enthalpy of the interaction of polylysine with native and denatured DNA was measured with the help of a LKB batch calorimeter. From the results of the calorimetric measurements one can conclude that the transition enthalpy of the DNA—polylysine complexes is linearly dependent on the nucleotide: peptide ratio. The extrapolated value for the 1:1 complex is 14.4 Kcal per mole base pairs.