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Abstract
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.