TEMPERATURE-INDUCED CHANGES IN PROTEIN STRUCTURES STUDIED BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY AND GLOBAL ANALYSIS

Fourier transform infrared (FTIR) spectroscopy has been used to study temperature-induced structural changes which occur in albumin, immunog lobulin G, fibrinogen, lysozyme, alpha-lactalbumin, and ribonuclease S when dissolved in (H2O)-H-2. In order to analyze the data, a new meth od was developed in which the data were analyzed globally with the aid of a spectral model. Seven or eight bands were sufficient to fit the full data set of spectra ranging from 1420 to 1760 cm(-1) with a root mean square error of 1-2% of the maximum. Subsequently, the estimated band amplitude curves which showed a sigmoidal progression with increa sing temperature were (globally) fitted with a two-state thermodynamic model. In this way, information on structural changes as well as on t he thermal stability of the proteins was obtained. In all proteins inv estigated, enhanced H-1-H-2 exchange occurred at temperatures well bel ow the unfolding of the secondary structure. This was interpreted as a change in tertiary structure leading to enhanced solvent accessibilit y. In all the proteins investigated, except for ribonuclease S, an int ermolecular beta-sheet band indicative of aggregation appeared concomi tant with the denaturation of the secondary structure. The results are compared with data from other techniques and discussed in terms of lo cal unfolding and folding intermediates.