Secondary structure perturbations in salt-induced protein precipitates.

The secondary structure implications of precipitation induced by a chaotropic salt, KSCN, and a structure stabilizing salt, Na2SO4, were studied for twelve different proteins. alpha-helix and beta-sheet content of precipitate and native structures were estimated from the analysis of amide I band Raman spectra. A statistical analysis of the estimated perturbations in the secondary structure contents indicated that the most significant event is the formation of beta-sheet structures with a concomitant loss of alpha-helix on precipitation with KSCN. The conformational changes for each protein were also analyzed with respect to elements of primary, secondary and tertiary structure existing in the native protein; primary structure was quantified by the fractions of hydrophobic and charged amino acids, secondary structure by x-ray estimates of alpha-helix and beta-sheet contents of native proteins and tertiary structure by the dipole moment and solvent-accessible surface area. For the KSCN precipitates, factors affecting beta-sheet content included the fraction of charged amino acids in the primary sequence and the surface area. Changes in alpha-helix content were influenced by the initial helical content and the dipole moment. The enhanced beta-sheet contents of precipitates observed in this work parallel protein structural changes occurring in other aggregative phenomena.