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The effect of surface adsorption on the structure and stability of proteins is a matter of increasing interest in biotechnology. Therefore, we have examined the effect of adsorption to silica on the thermal stability of 7 proteins employing differential scanning calorimetry (DSC) and front surface fluorescence (FSF) spectroscopy. In general, it was found that surface adsorption decreased the thermal stability of the bound protein. Using lysozyme for further studies, DSC, FSF, and FTIR spectroscopies, as well as enzymatic activity measurements, were used to explore the effect of decreasing surface apolarity on stability. It was observed that increasing surface apolarity produced decreasing stability and increasing structural alteration of the adsorbed protein.

Differential scanning calorimetry was performed on the five major lens crystallin fractions [HM-alpha, alpha, beta H, beta L, and (beta s + gamma)] of the bovine lens as well as on more purified forms of alpha- and gamma-crystallins. All were found to be relatively thermally stable although the alpha-crystallin were found to at least partially unfold at an approximately 10 degrees C lower temperature than the beta and gamma fractions. Increasing protein concentration had little effect on gamma-crystallin thermograms but had marked effects on those of the alpha- and beta-crystallins. Increases in the thermal stability with increasing protein concentration for the beta-crystallins can be explained most simply by the known beta L/beta H equilibrium, but, in the case of the alpha-crystallins, excluded volume effects may be an important factor. In both cases, the increased stability at high concentrations could be of physiological relevance. As well as the expected endothermic unfolding transitions, all of the lens crystallins revealed exothermic peaks that correlate with protein precipitation. Interestingly, this phenomenon occurs only after extensive structural alteration in the case of the alpha-crystallins but is present very early in the initial stages of structural perturbation of the beta- and gamma-crystallins.

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