Method of high-sensitivity modulation differential scanning calorimetry was applied for investigation of the kinetically controlled irreversible thermal denaturation of the trypsin inhibitor from soybeans (Kunitz inhibitor, KI) in diluted solution. The measurements were carried out with a temperature-modulation capillary nanocalorimeter designed and produced by the Institute of Biological Instrumentation of the RAS (Pushchino, Russia). An algorithm of the experimental data processing and corresponding software were developed. It was shown that the modulation nanocalorimetry allows one to obtain in one experiment the temperature dependence of the rate constant for irreversible protein denaturation. The temperature dependence of the rate constant and the activation energy of the irreversible denaturation of Kunitz inhibitor were determined. The obtained value of the activation energy (E a = 206 ± 6 kJ mol−1) agrees with independent estimates of this kinetic parameter.

The order-disorder conformational transition in the tetramethylammonium salt of gellan has been studied by high-sensitivity differential scanning calorimetry as a function of concentration of the background electrolyte, Me4NCl (0.0025-0.5 M). The transition temperature and enthalpy increase with increasing salt concentration following a logarithmic relation. At the same time, there is a linear correlation between the transition enthalpy and temperature with a positive slope of 0.18 J g(-1) K-1. According to Kirchhoff's law, the slope can be considered as a heat capacity increment of the transition. This value is much larger than that expected to arise from electrostatic effects. Thus, it has been suggested that the transition is accompanied by an increase in water accessibility to some less hydrophilic fragments of the primary structure of gellan buried in its ordered conformation. The transition has an lambda-like heat capacity profile and can be therefore defined as a second-order phase transition. This assignment is in agreement with predictions of a matching model of the double helix-coil transition. The model fits well to the integral transition curve at 0.5 M Me4NCl, assuming a cooperative length of gellan to be about eight repeating units. This value is close to the size of chain corresponding to the persistent length of gellan in the coil state. The cooperativity parameter in the matching model (sigma = 0.62 +/- 0.01) indicates a small contribution of the stacking effect into the cooperativity of the transition, while a larger contribution apparently originates from the loop factor. The free energy of transition at a reference temperature (273 K) is a logarithmic function of the concentration of the salt, Delta(t)G(C-s), as predicted by the counter condensation theory and the Poisson-Boltzmann (PB) model. The latter is preferable because of the low linear charge density parameter (xi < 1) for the double helix of gellan. Analysis of the Delta(t)G(C-s) function in terms of the PB model showed that the linear charge density of gellan in the coil conformation needs to be higher than that estimated for the fully stretched chain.

The effect of vanillin on thermal denaturation of ovalbumin was studied using high-sensitivity differential scanning calorimetry. At neutral and acidic (6.7 and 3.0) pH values thermal denaturation of ovalbumin depends on the heating rate and is irreversible. The experimental denaturation thermograms of ovalbumin were compared with thermograms simulated according to the Lumry–Eyring model of irreversible protein denaturation (N↔D→A). It was shown that general tendencies of changes in the calorimetric parameters of ovalbumin denaturation as a function of the heating rate are in accordance with those predicted by this model. It was concluded that the heating rate could be considered as a factor shifting the denaturation equilibrium of ovalbumin (N↔D) similarly to its shifts caused by such physicochemical factors as pH, ionic strength or ligand concentration. Both the temperature and the enthalpy of ovalbumin denaturation decrease with an increase in the vanillin concentration within the pH range from 3.0 to 9.0. This effect of vanillin binding does not depend on the heating rate. The equilibrium parameters of ovalbumin denaturation have been determined by an extrapolation procedure at different vanillin concentrations. The equilibrium denaturation parameters have been used for calculation of the excess free energy of denaturation as a function of vanillin concentration. The vanillin binding constant Kb equals 5M−1 at pH 6.7 and 33M−1 at pH 3.0. The denaturation increment of the numbers of binding sites is Δdν=20 at pH 6.7 and Δdν=3 at pH 3.0. Possible mechanism of the vanillin binding to ovalbumin and its applied aspects are discussed.

The release of 2-octanone bound by bovine serum albumin (BSA)/pectin complexes in aqueous solutions was induced by shift of pH from neutral (pH 6.4) to acid (pH 4.3). The release of 2-octanone was due to BSA unfolding on the oppositely charged rigid polysaccharide matrix at pHs below BSA’s isoelectric point (IEP). The amount of released and freely available flavour was estimated by the effect of 2-octanone on the conformational stability of BSA, i.e. using BSA added to 2-octanone solution as an internal molecular detector. DSC was used to study the effects of: (i) protein denaturation on flavour binding capacity, (ii) binding of flavour on conformational stability of the protein and (iii) to develop an instrumental technique to measure the flavour release.

Structural cooperative transitions in thermosensitive nonionic hydrogels of poly(N-vinylcaprolactam) (PVCa) were studied using high-sensitivity differential scanning calorimetry (HS-DSC). The thermoshrinking of the PVCa gel is endothermic, much like first-order phase transitions. According to the HS-DSC data, the gel system undergoes two successive cooperative transitions between 25 and 50 degrees C. The low-temperature transition (31.5 degrees C) is proposed to be associated with the microsegregation resulting in formation of hydrophobic domains (micromicelles) in the gel whereas the high-temperature transition (37.6 degrees C) is due to the gel volume collapse. The transition temperatures decrease in the presence of NaCl and increase with increasing sodium dodecyl sulfate (SDS) concentration. Dependences of the transition enthalpies and entropies on NaCl and SDS concentrations display a more complex character. The influence of sodium chloride and SDS on the transition temperatures is in agreement with the general expectations taking into account their effect on hydrophobic interactions and the ability to form micromicelles.

Thermally induced order-disorder conformational transition in succinoglycan was studied using the method of high-sensitivity differential scanning microcalorimetry within the range of polysaccharide concentrations from 0.1 to 3.5 mg mL(-1) at NaCl concentrations 0, 0.01, and 0.1M. The positions and shapes of the excess heat capacity curves depended substantially on both the NaCl and polysaccharide concentrations. At low polysaccharide concentrations in salt-free solution the experimental curves were closely approximated by the two-state model suggesting the transition mechanism to be of the single helix-coil type. With increasing polysaccharide and/or NaCl concentration, the experimental curves changed significantly in symmetry, which indicated a changing transition mechanism. At high polysaccharide concentrations or in the presence of the salt, the order-disorder transition of succinoglycan was shown to include two stages: the cooperative dissociation of the helix dimer and subsequent two-state melting of the helix monomer. The dependence of thermodynamic parameters for the dissociation and melting of helix structures in succinoglycan on NaCl and polysaccharide concentrations was obtained by fitting the experimental excess heat capacity curves. The cooperativity parameter sigma for the single helix-coil transition as well as the average length of the helix segment of succinoglycan were calculated. Some features of succinoglycan ordering in solution are discussed. (C) 1996 John Wiley & Sons, Inc.

This review paper is an attempt to summarize the results of investigations over several years in the field of thermotropic gelation of proteins which have been carried out in the Laboratory of Novel Food Forms of the A.N.Nesmeyanov Institute of Organo-Element Compounds in Moscow. The gelling behavior of food proteins [soybean globulin fraction, 11S globulin from broad beans (legumin) and ovalbumin] is studied. Heat-setting mechanisms and general physical approaches for the phenomenological description of gel formation from globular proteins are discussed. Behavior of gelling systems with protein concentrations below and above the gelation threshold (the gel-point) is analysed in detail. The classical and modern theories of gelation, and their use in the description of concentration dependences of the average size of finite clusters in the sol-fraction, and gel equilibrium elasticity modulus are compared. The modern theory of gelation based on the 3d percolation model has been shown to reflect more correctly the main features of the thermotropic gelation of proteins. Detailed consideration is given to viscoelasticity of protein thermotropic gels under small deformations. It is shown that, due to microheterogenéity of the structure, protein gels display some characteristic features of the rheological behavior of both amorphous and crystalline polymer solids.