INTERMOLECULAR BETA-SHEET RESULTS FROM TRIFLUOROETHANOL-INDUCED NONNATIVE ALPHA-HELICAL STRUCTURE IN BETA-SHEET PREDOMINANT PROTEINS - INFRARED AND CIRCULAR-DICHROISM SPECTROSCOPIC STUDY

2,2,2-Trifluoroethanol (TFE)-induced nonnative alpha-helical structure in peptides and proteins has been extensively studied with circular d ichroism (CD) spectroscopy. However, to date, complementary informatio n from infrared (IR) spectroscopy has not been reported. Using both IR and CD spectroscopy, we demonstrate here that the TFE-induced nonnati ve alpha-helical structure in two beta-sheet-predominant proteins, bet a-lactoglobulin and alpha-chymotrypsin, is unstable in comparison with those found in the alpha-helix-predominant proteins myoglobin and cyt ochrome c under identical conditions. IR spectra showed that, immediat ely after dissolution of the beta-sheet proteins in 50% (v/v) TFE, a s trong amide I band component appears at 1654 cm(-1) in H2O and at 1650 cm(-1) in D2O, which is ascribed to alpha-helical structure. However, the intensities of the alpha-helical bands decrease as a function of time, concomitant with the appearance of two new band components near 1620 and 1695 cm(-1) in H2O and 1612 and 1684 cm(-1) in D2O, a typical IR spectral pattern for an intermolecular beta-sheet aggregate. Clear gels begin to develop within 30 min. No similar spectral changes were observed for the alpha-helical proteins. CD spectra suggested initial ly that the TFE-induced alpha-helix was retained in the gelled state. However, further analysis of the spectra, and Gaussian function modeli ng with basic spectra, indicated that the apparent alpha-helix signal was actually due to a combination of signals from intermolecular beta- sheet and residual alpha-helix. These results indicate that the TFE-in duced nonnative a-helix structure in predominantly beta-sheet proteins is unstable and readily converts to an intermolecular beta-sheet aggr egate. (C) 1998 Academic Press.