This review contains selected values of thermodynamic quantities for the aqueous ionization reactions of 64 buffers, many of which are used in biological research. Since the aim is to be able to predict values of the ionization constant at temperatures not too far from ambient, the thermodynamic quantities which are tabulated are the pK, standard molar Gibbs energy ⌬ r G ‫ؠ‬ , standard molar enthalpy ⌬ r H°, and standard molar heat capacity change ⌬ r C p ‫ؠ‬ for each of the ionization reactions at the temperature T ϭ298.15 K and the pressure pϭ0.1 MPa. The standard state is the hypothetical ideal solution of unit molality. The chemical name͑s͒ and CAS registry number, structure, empirical formula, and molecular weight are given for each buffer considered herein. The selection of the values of the thermodynamic quantities for each buffer is discussed. © 2002 by the U.S. Secretary of Commerce on behalf of the United States. All rights reserved.Of primary interest to this review is the ͑thermodynamic͒ equilibrium constant K a,m ϭa͕H ϩ ͑aq͖͒•a͕A Ϫ ͑aq͖͒/a͕HA͑aq͖͒ ͑1͒ for the ionization reaction HA͑aq͒ϭH ϩ ͑aq͒ϩA Ϫ ͑aq). ͑2͒The above equilibrium constant has been defined in terms of activity and has been denoted as such by affixing a subscript a to the equilibrium constant. It is also necessary to specify the standard state. In this review the principal standard state used for the solute is the hypothetical ideal solution at the standard molality (m°ϭ1 mol kg Ϫ1 ). The standard state for the solvent is the pure solvent. This choice of a molality standard state has been indicated by attaching a subscript m to the equilibrium constant. Eq. ͑1͒ can also be written in terms of the molalities and activity coefficients ͑molality ba-sis͒ ␥ m of the solute species:The standard molality m°has been used in Eq. ͑3͒ to keep the equilibrium constant dimensionless. An alternative way of writing Eq. ͑3͒ iswhere K m ϭm͕H ϩ ͑aq͖͒•m͕A Ϫ ͑aq͖͒/͓m͕HA͑aq͖͒•m°], ͑5͒andHere K m is the equilibrium constant ͑molality basis͒ that pertains to an actual real solution as distinct from the equilibrium constant ͑activity basis͒ that relates to a hypothetical standard state that is approached by real solutions as the sum of the molalities of all solute species approaches zero. One can also formulate equilibrium constants in terms of concentrations c:andHere, c°is the standard concentration ͑1 mol dm Ϫ3 ͒ and ␥ c is the activity coefficient on a concentration basis. The density of the pure solvent is needed to calculate values of K a,m from K a,c and vice versa:On a logarithmic scale the difference between K a,c and K a,m in water at Tϭ298.15 K is very small (pK a,c ϭpK a,m 236 236 GOLDBERG, KISHORE, AND LENNEN

The thermodynamics of binding of concanavalin A (Con A) with a series of linear and branched chain oligosaccharides including certain N-linked complex type and oligomannose type carbohydrates and a fraction of quail ovalbumin containing Man7 and Man8 oligomannose chains have been determined using titration microcalorimetry. Methyl3,6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside, a branch chain trisaccharide moiety found in all N-linked carbohydrates which possesses approximately 60-fold higher affinity than methyl alpha-D-mannopyranoside, exhibited a change in enthalpy of binding (delta H) of -14.4 kcal mol-1 as compared to -8.2 kcal mol-1 for the monosaccharide. This demonstrates that Con A possesses an extended binding site for the trimannoside. However, a biantennary complex type carbohydrate with terminal beta (1,2)-GlcNAc residues which binds with 3-fold higher affinity than the trimannoside possesses a delta H of only -10.6 kcal mol-1. A plot of -delta H versus -T delta S for the carbohydrates in the present study showed positive deviations in -T delta S for the complex type carbohydrate, as well as alpha (1,2)-di- and trimannosyl oligosaccharides which are part of the structures of oligomannose type carbohydrates. The relative favorable changes in binding entropies of these compounds are attributed to the presence of multiple internal and terminal residues in each molecule which can independently bind to the monosaccharide binding site of the lectin. The delta H values for the complex type carbohydrate and the alpha (1,2) mannose oligosaccharides were also approximately -2.5 kcal mol-1 greater than that of methyl alpha-D-mannopyranoside, indicating some extended binding site interactions.(ABSTRACT TRUNCATED AT 250 WORDS)

We have studied here using a number of biophysical tools the effects of osmolytes, betaine, citrulline, proline and sorbitol which differ significantly in terms of their physical characteristics such as, charge distribution, polarity, H-bonding abilities etc, on the fibrillation of insulin. Among these, betaine, citrulline, and proline are very effective in decreasing the extent of fibrillation. Proline also causes a substantial delay in the onset of fibrillation in the concentration range (50–250 mM) whereas such an effect is seen for citrulline only at 250 mM, and in case of betaine this effect is not seen at all in the whole concentration range. The enthalpies of interaction at various stages of fibrillation process have suggested that the preferential exclusion of the osmolyte and its polar interaction with the protein are important in inhibition. The results indicate that the osmolytes are most effective when added prior to the elongation stage of fibrillation. These observations have significant biological implications, since insulin fibrillation is known to cause injection amyloidosis and our data may help in designing lead drug molecules and development of potential therapeutic strategies.

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The interaction of the nonionic surfactant Triton X-100 (TX-100) with two proteins (bovine serum albumin (BSA) and alpha-lactalbumin (alpha-LA)) has been investigated by using a combination of differential scanning calorimetry, isothermal titration calorimetry, and fluorescence and circular dichroism spectroscopies. All of the calorimetric transitions in BSA were partially reversible, while being two-state and reversible in the case of alpha-LA. TX-100 molecules do not reduce the thermal stability of the protein in the monomeric form. However, in the micellar form the protein might become thermally destabilized by the micelles depending upon the nature of the protein. Isothermal titration calorimetry has been used to demonstrate that TX-100 binds to BSA at two sets of sites with 4:1 stoichiometry in each case. The van't Hoff enthalpy calculated from the temperature dependence of the binding constant did not match with the calorimetric enthalpy indicating conformational change in the protein upon surfactant binding. The surfactant binds to alpha-LA with one class of binding site, and the thermal unfolding results indicate it to be a stronger destabilizer than BSA. The fluorescence, circular dichroism, and differential scanning calorimetric results corroborate well with each other. The effect of ionic strength on the binding parameters suggests that TX-100 can bind to the protein surface via both hydrophobic and polar interactions depending upon the nature of the protein. The physical chemistry underlying the interactions between TX-100 and proteins has been presented. The mode of interaction of TX-100 with proteins is via direct binding, which has been discussed quantitatively in this work.

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The apparent molar volumes (V 2, φ) have been determined for glycine, l-alanine, and l-leucine in water and in aqueous magnesium chloride solutions with concentrations ≈ (0.05 to 0.80) mol·kg-1 by measuring the densities at (288.15, 298.15, and 308.15) K. The apparent molar heat capacities (C p2, φ) have also been determined for glycine and l-alanine in aqueous magnesium chloride solutions with concentrations ≈ (0.05 to 0.40) mol·kg-1 by measuring heat capacities in the temperature range (298.15 to 328.15) K. These properties show a peculiar dependence upon the concentration of magnesium chloride. The standard partial molar volumes at infinite dilution ( ) obtained from these data have been used to calculate the partial molar volumes of transfer of amino acids from water to aqueous magnesium chloride solutions at infinite dilution (Δt ), which are positive for the presently studied amino acids at all temperatures and concentrations. The partial molar expansibilities (∂ /∂T) P at infinite dilution and the (∂2 /∂T 2) P values have also been determined from the data at various concentrations of the salt. The volumetric interaction parameters have been calculated from Δt data. The results have been discussed in terms of various interactions operating in these systems.