M.J. Pallansch

Changes in state of water in proteinaceous systems.

Abstract When lyophilized preparations of β-lactoglobulin, bovine serum albumin, and calfskin collagen sorbed at least 0.18 gm H2O per gram of dried protein, it was observed through differential scanning calorimetry (DSC) that the heat of vaporization of the sorbed water was 80–125 cal/gm higher than the ΔHv of liquid water. When less H2O is sorbed, at lower values of P P o for the sorbed water was equivalent to that of free water. These differences in strength of H2O-protein binding may be attributed to the availability of protein surfaces or suitable H2O binding sites. At the higher moisture levels the solid protein matrix has become swollen and possibly conformational changes have occurred in the protein molecules permitting more H2O-surface contacts and the formation of an “icelike” structure. Accordingly extensive water binding was observed in completely wet systems by measuring the heat of fusion of the water associated with wet pellets of ultracentrifugal casein. Water bound in an “ice” structure will not freeze on cooling to low temperatures (−70°C) and may therefore be assessed through DSC. Such bound water was found to correspond to 50%–60% of the dry weight of the protein present.

Calorimetric measurement of the heat of desorption of water vapor from amorphous and crystalline lactose

Abstract The heat required to release and vaporize bound H 2 O from crystalline α-lactose monohydrate and from lactose glass, as determined by differential scanning calorimetry is 12.3±0.7 and 10.8±0.5 kcal·mole −1 of H 2 O, respectively. Water vapor sorption by anhydrous α-lactose leads to the formation of the α-monohydrate. The isotherm, obtained gravimetrically for this process is Langmuir type. β-Lactose is completely non-hygroscopic below 97% relative humidity. Thereafter, it sorbs H 2 O rapidly to form a concentrated solution wherein the lactose is capable of mutarotation. Densites of lactose forms determined pycnometrically by helium displacement are: 1.535 g/cm 3 for α-lactose·H 2 O; 1.547 g/cm 3 for anhydrous α-lactose; and 1.576 g/cm 3 for β-lactose.

Effect of sorbed water on the heat capacity of crystalline proteins

Abstract Heat capacity measurements by differential scanning calorimetry (DSC), performed with anhydrous samples of ovalbumin, yielded a value for Cp of 0.267 ±0.033 cal/g/°C at 12°C; and the extrapolation of data from hydrated samples to zero water content yielded a similar value for Cp of 0.244±0.011 cal/g/°C. The heat capacity of anhydrous β-lactoglobulin is 0.273 ±0.027cal/g/°C, and the value obtained by extrapolation of data for hydrated samples is 0.284±0.019 cal/g/°C. A linear relation between specific heat and moisture content was observed with the hydrated samples which contained 0.03–0.21 g sorbed water per gram of protein. No temperature dependence of specific heat was observed in the interval scanned (0–25°C). The computed, apparent, partial specific heats of the proteins are 0.245 ±0.010 cal/g/°C for ovalbumin and 0.283±0.02 cal/g/°C for β-lactoglobulin; and the partial specific heat of the sorbate is 1209±0.103 cal/g/°C for water sorbed by ovalbumin, and 0.947±0.137 cal/g/°C for water sorbed by β-lactoglobulin.

Contraction of dried casein particle surfaces effected by sequential H2O vapor sorption and desorption cycles

Abstract When casein micelles, isolated from milk by high-speed centrifugation, are washed with water and dehydrated by serial transfer through liquids of decreasing polarity before final solvent removal under vacuum, the resulting dry material has a specific surface area 10 times higher than washed micelles dried directly from aqueous systems by lyophilization. Subjecting the dried proteinaceous material to cyclic H 2 O vapor sorption and desorption resulted in contraction and loss in BET surface area as measured by low temperature N 2 adsorption. The observed surface area decrease is proportional to the amount of water sorbed by the casein before removal. The data indicate that the micelles in milk may be subject to shrinkage and loss of porosity during drying by procedures based on direct water vapor transfer.

Influence of dehydration method on the adsorption of benzene vapor by dried casein

Washed casein micelles were dehydrated by serial transfer through solvents of decreasing polarity followed by drying in vacuo. The product adsorbed approximately three times as much benzene as lyophilized casein for all PP0 values at 24 and 30°C. Adsorbed C5H6 molecules are very mobile on the surfaces of the lyophilized particles, −qst = 8.7 kcal/mole over the entire isotherm. However, much of the benzene is in a more tightly bound adsorbed state on solvent transfer dehydrated casein, with −qst gradually decreasing from a high of 20 kcal/mole to a constant value near 8 kcal/mole. Both types of casein adsorbed more C6H6 than anticipated from BET surface areas calculated from low temperature N2 adsorption data. The results suggest that the apolar amino acid residues of the casein subunits, which are normally squeezed into cavities in water according to theories of hydrophobic bonding, may be separated when water is replaced with a less polar solvent. Ultimately a porous dried product is thus obtained with localized binding sites for benzene, rather than hydrophobic regions in the macromolecule.

Hydrocarbon binding by particles of bovine casein

Abstract Casein, isolated from milk by high-speed centrifugation, was washed with water and dehydrated by serial transfer through liquids of decreasing polarity to provide a system consisting of protein dispersed in anhydrous, liquid hydrocarbon. Heats of fusion of solvents associated with casein were measured to determine how much hydrocarbon is free and can be frozen and what fraction is bound to the protein and immobilized. Ring substitution decreased binding in the aromatic compounds studied, i.e. : 8.4 mmoles benzene, 4.7 mmoles p -xylene, and 3.3 mmoles mesitylene bound per gram casein. Binding of even n-paraffins was significantly less than that of the aromatic compounds and decreased with increasing chain length, i.e. : 2.0 mmoles C 8 H 18 , 1.9 mmoles C 10 H 22 , 1.6 mmoles C 12 H 26 , 1.1 mmoles C 14 H 30 , and 1.0 mmoles C 16 H 34 bound per gram casein. The observation of unfreezable hydrocarbon indicates that some solvents, commonly considered as hydrophobic, attain preferred orientation at the protein-solvent interface.