From a drastic decrease in the phosphorescence lifetime of tryptophan residues buried in compact rigid cores of globular proteins, it was possible to demonstrate that freezing of aqueous solutions is invariably accompanied by a marked loosening of the native fold, an alteration that entails considerable loss of secondary and tertiary structure. The phenomenon is largely reversible on ice melting although, in some cases, a small fraction of macromolecules recovers neither the initial phosphorescence properties nor the catalytic activity. The variation in the lifetime parameter was found to be a smooth function of the residual volume of liquid water in equilibrium with ice and to depend on the morphology of ice. The addition of cryoprotectants such as glycerol and sucrose profoundly attenuates or even eliminates the perturbation. These results are interpreted in terms of adsorption of protein molecules onto the surface of ice.

Freeze-induced perturbations of the protein native fold are poorly understood owing to the difficulty of monitoring their structure in ice. Here, we report that binding of the fluorescence probe 1-anilino-8-naphthalene sulfonate (ANS) to proteins in ice can provide a general monitor of ice-induced alterations of their tertiary structure. Experiments conducted with copper-free azurin from Pseudomonas aeruginosa and mutants I7S, F110S, and C3A/C26A correlate the magnitude of the ice-induced perturbation, as inferred from the extent of ANS binding, to the plasticity of the globular fold, increasing with less stable globular folds as well as when the flexibility of the macromolecule is enhanced. The distortion of the native structure inferred from ANS binding was found to draw a parallel with the extent of irreversible denaturation by freeze-thawing, suggesting that these altered conformations play a direct role on freeze damage. ANS binding experiments, extended to a set of proteins including serum albumin, alpha-amylase, beta-galactosidase, alcohol dehydrogenase from horse liver, alcohol dehydrogenase from yeast, lactic dehydrogenase, and aldolase, confirmed that a stressed condition of the native fold in the frozen state appears to be general to most proteins and pointed out that oligomers tend to be more labile than monomers presumably because the globular fold can be further destabilized by subunit dissociation. The results of this study suggest that the ANS binding method may find practical utility in testing the effectiveness of various additives employed in protein formulations as well as to devise safer freeze-drying protocols of pharmaceutical proteins.

Although freeze-induced perturbations of the protein native fold are common, the underlying mechanism is poorly understood owing to the difficulty of monitoring their structure in ice. In this report we propose that binding of the fluorescence probe 1-anilino-8-naphthalene sulfonate (ANS) to proteins in ice can provide a useful monitor of ice-induced strains on the native fold. Experiments conducted with copper-free azurin from Pseudomonas aeruginosa, as a model protein system, demonstrate that in frozen solutions the fluorescence of ANS is enhanced several fold and becomes blue shifted relative free ANS. From the enhancement factor it is estimated that, at -13 degrees C, on average at least 1.6 ANS molecules become immobilized within hydrophobic sites of apo-azurin, sites that are destroyed when the structure is largely unfolded by guanidinium hydrochloride. The extent of ANS binding is influenced by temperature of ice as well as by conditions that affect the stability of the globular structure. Lowering the temperature from -4 degrees C to -18 degrees C leads to an apparent increase in the number of binding sites, an indication that low temperature and /or a reduced amount of liquid water augment the strain on the protein tertiary structure. It is significant that ANS binding is practically abolished when the native fold is stabilized upon formation of the Cd(2+) complex or on addition of glycerol to the solution but is further enhanced in the presence of NaSCN, a known destabilizing agent. The results of the present study suggest that the ANS binding method may find practical utility in testing the effectiveness of various additives employed in protein formulations as well as to devise safer freeze-drying protocols of pharmaceutical proteins.

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