Calorimetric enthalpy

Naghibi H, Tamura A and Sturtevant J M 1995 Significant discrepancies between van t Hoff and calorimetric enthalpies Proc. Natl Acad. Sc/. USA 92 5597-9... 

H. Naghibi, A. Tamura, J. M. Sturtevant. Significant Discrepancies Between van t Hoff and Calorimetric Enthalpies. Proc. Natl. Acad. Sci. USA 1995, 92, 5597-5599. 

Calorimetric Enthalpy Data for Binary Syngas Mixtures"... 

An Evaporative Film Calorimetric Enthalpy Probe , Cornell Aeronautical Lab Inc Rept ARL 65-47 (March 1965) [Contract AF (33)-657-7774] E) F.A. Vassallo, "Miniature Enthalpy Probes for High Temperature Gas Streams , Cornell Univ Rept ARL 66-0015 (June 1966) [Contract AF 33(615-2461]... 

If the process follows the two-state model, then the calorimetric enthalpy, A //, the calorimetrically obtained van t Hoff enthalpy, AHeff, and, where optical measurements have been made, the optical van t Hoff enthalpy, AHuh, should agree. Figure 16.9 shows the ratio, AH /AHvi obtained from denaturation studies of five globular proteins under various conditions, plotted against the corresponding denaturation temperature. This plot shows that the ratio is very close to unity. Similar results for numerous other small globular proteins have been obtained and most show a similar relationship. It is clear, however, that the ratio is closer to 1.05 than to 1. Freire and Biltonen11 have shown from a consideration of the partition functions associated with the native and the denatured state that this relative excess is... 

Figure 16.9 The ratio of the effective calorimetric enthalpy to the enthalpy calculated by the van t Hoff method from optical measurements, plotted against the temperature of denaturation. The symbols represent the following proteins , metmyoglobin A, ribonuclease O, cytochrome c O, a-chymotrypsin , lysozyme. Reproduced by permission from P. L. Privalov, Adv. Prot. Chem., 33, 167 (1979).

Calorimetric (DSC) measurements yield thermodynamic properties of duplex melting in these oligonucleotides independent of any assumptions concerning the model of melting, such as a cooperative all-or-none process versus a noncooperative, multiple-stage melting process. Comparison of calorimetric enthalpies with van t Hoff enthalpies obtained either from the manipulation of heat capacity curves outlined in equations (16.19) to (16.22), or from optical or NMR measurements [equations (16.14) to (16.17)] allows conclusions to be drawn concerning the size of the cooperative unit. If the two... 

Since a comparison of various van t Hoff enthalpies (calculated by analysis of da/dT at Tm, the slope of /Tm vs In cjot. and the shape of the calorimetric curve)0 with the calorimetric enthalpy shows that A// h A//cai, it was concluded that the melting transition in this duplex is cooperative and well represented by a two-state model. 

The d(CGCGAATTCGCG)2 Duplex. When studied at comparable salt (NaCl) and duplex concentrations, the duplex of d(CGCGAATTCGCG) was found to have about the same Tm by DSC measurements (344 K) and by the temperature-dependent changes in the NMR chemical shifts of nonexchangeable GC and AT protons (345 K). At 0.01 M NaCl, the calorimetric enthalpy was 376 kJmol-1 of duplex, while the van t Hoff enthalpy evaluated from equation (16.28) was found to be 393 kJ-mol-1 of duplex. At a 10-fold higher salt concentration, the calorimetric enthalpy was 427 kJ-mol-1 of duplex, while the van t Hoff enthalpy was 310 kJ-mol-1 of duplex. From the ratio of the van t Hoff enthalpy to the calorimetric enthalpy at the higher salt concentration, it was estimated that 9 out of 12 base-pairs melt together in the cooperative unit, while all melt simultaneously at the low salt concentrations. 

The cooperative unit as expressed here is the number of base-pairs in a given oligonucleotide duplex that melt together as a unit. Consider that the uncertainties of the melting temperatures are about 1 K, and for the calorimetric enthalpies, about ilkJmol-1 of base-pairs. 

This reaction is reversible.16 The temperature at which decomposition occurs depends therefore on the gas phase for example, under vacuum, decomposition becomes noticeable at as low as 500°C. The lack of S° values for this salt prevents any equilibrium calculations for the above reaction, as well as the gef calculation for the paraperiodate. The enthalpy and entropy increments are based on the calorimetric enthalpy increments of David, Mathurin, and Thevenot.12 Thermodynamic data for these salts are given in Tables 9.2 and 9.3. 

A four-parameter equation for predicting the calorimetric enthalpies of acid-base interactions, —Ai/ab, in neutral solvents, proposed by Drago et al. [146], with spectroscopically determined shifts of the OH stretching frequency of phenol when interacting with a variety of bases of dilute solutions in carbon tetrachloride and tetrachloroethylene. 

When the apparent equilibrium constant K for an enzyme-catalyzed reaction depends on pH and pMg, the calorimetric enthalpy of reaction Af//(cal) is given by (2)... 

Table I The melting temperatures (Tm) from the CD thermal denaturation studies are estimated from the temperature at which the slope of the first derivative of the uncorrected data was a minimum ty2 is the temperature of half-completion of the DSC thermal denaturation transition. A//cal is the calorimetric enthalpy. The DSC AAG°u = AG°u (mutant) - AG°u (wild-type), at the value of ti/2 for the wild-type protein (74.6°C).

The adopted values of C° below 300 K are taken from Douglas and Beckett ( ) based on the low temperature C measurements of King (53-296 K, 9). At higher temperatures the adopted C data are based on the drop calorimetric enthalpy data of Christensen et al. (399-1795 K, 1 ). No transitions were detected in either study. 

Interpolation of the calorimetric enthalpies of mixing of Holm and Kleppa ( ) yields A H LlP(t) + BePgCO > LlBePg(i) at 1135 K. Reduction with the estimated JANAP functions yields... 

A crystal II to crystal I transition at 419.6 K with a enthalpy change of 0.085 kcal mol" has been listed (1 ) from Bridgman (11). However, West ( ) found no change in the x-ray powder diffraction pattern over the 298-473 K range. Although not definitive because of the small heat change listed, the drop calorimetric enthalpy measurements of Dworkin and Bredig (6) do not show this transition. Therefore, we do not indicate any solid state transition at 419.6 K. 

The heat capacity was derived from drop calorimetric enthalpy measurements by Dworkin and Bredig (2, 1221-1260 K) and is constant at 24.13 cal K mol" over the range measured. A glass transition temperature is assumed at 820 K with C" = 24.13 above this temperature and C equal to that of the crystal below this temperature. 

Horn, J. R., Russel, D., Lewis, E. A., Murphy, K. P. van t Hoff and calorimetric enthalpies from isothermal titration calorimetry are there significant discrepancies Biochemistry 2001, 40, 1774-1778. 

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