PROTEINS IN FROZEN-SOLUTIONS - EVIDENCE OF ICE-INDUCED PARTIAL UNFOLDING

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