Thermodynamic models for water–protein sorption hysteresis

Abstract

SynopsisSorption isotherms of water by proteins show hysteresis that appears to be due to interactions at the molecular level. Four thermodynamically consistent models for this irreversible process are presented. Hysteresis could be the result of slow, incomplete conformational changes occurring upon addition and removal of water. Conformational hysteresis would occur if a number of different conformations, each corresponding to a local free energy minimum, could be present at each pressure of water vapor. Hysteresis might result from an incomplete process of intermolecular phase annealing. Finally, hysteresis might be due to incomplete phase change if two different protein phases are present. Experimental tests for these models are presented. Further study should lead to more insight into the effects of the presence of water on protein conformation and dynamics.

Hydration Potential of Lysozyme: Protein Dehydration Using a Single Microparticle Technique

“…It is known that protein sorption isotherms exhibit hysteresis below a w 0.9 (27)(28)(29)(30)(31). For a given a w , proteins are typically more hydrated during desorption than absorption.…”

Influences of the Glassy and Rubbery States on the Thermal, Mechanical, and Structural Properties of Doughs and Baked Products

“…Lillford (1988) has expressed a similar opinion, crediting (as others had previously [Levine and Slade, 1986]) the onset of enzymatic activity in lowmoisture, amorphous lysozyme powders to plasticization by sorbed water, leading to sufficient segmental mobility for diffusion-limited enzyme-substrate interactions to occur. With regard to water-protein sorption hysteresis, Bryan (1987b) has suggested that hysteresis "could result from slow, incomplete conformational changes occurring upon addition of water" to proteins in the solid state, and its subsequent removal. These changes could result from incomplete "intermolecular phase annealing" or "phase changes," and the resulting hysteresis "might be related to the physical state and prior history of the sample" (Bryan, 1987b).…”

“…With regard to water-protein sorption hysteresis, Bryan (1987b) has suggested that hysteresis "could result from slow, incomplete conformational changes occurring upon addition of water" to proteins in the solid state, and its subsequent removal. These changes could result from incomplete "intermolecular phase annealing" or "phase changes," and the resulting hysteresis "might be related to the physical state and prior history of the sample" (Bryan, 1987b). Again, Lillford (1988) has expressed similar sentiments, pointing out (again as others had previously [Levine and Slade, 1987]) that starch (van den Berg, 1981) and other hydrophilic polymers plasticized by water "are far from inert in the adsorption process," should not be modeled as "an immobile and unaffected substrate," manifest a "nonequilibrium desorption process responsible for hysteresis," and probably are "usually in a metastable kinetic state ... where the interaction of polymers and water cannot be treated in terms of equilibrium thermodynamics."…”

“…Again, Lillford (1988) has expressed similar sentiments, pointing out (again as others had previously [Levine and Slade, 1987]) that starch (van den Berg, 1981) and other hydrophilic polymers plasticized by water "are far from inert in the adsorption process," should not be modeled as "an immobile and unaffected substrate," manifest a "nonequilibrium desorption process responsible for hysteresis," and probably are "usually in a metastable kinetic state ... where the interaction of polymers and water cannot be treated in terms of equilibrium thermodynamics." Bryan (1987b) has also noted the fact that freeze-dried protein samples to be used as substrates in sorption experiments "might form an amorphous [solid] phase," a possibility overlooked by Lioutas and co-workers (1986) in their discussion of the water sorption behavior of lysozyme.…”

“…In a recent review that included this subject (Levine and Slade, 1987), an explanation (subsequently endorsed by Lillford [1988]) for the sorption behavior described by Bryan (1987aBryan ( , 1987b has been offered. Water vapor absorption in food systems can be treated as a diffusion-controlled transport process involving structural relaxations in glassy and rubbery polymers.…”