Conformational studies on histone H3 and its CNBr peptides.

Histone H3 and H3 peptides 1--120, 1-90, 91-135, 91-120 and 121-135 have been prepared and examined for salt-induced conformational changes by circular dichroism measurements. It was found that reduced histone H3 and the reduced peptides 1-120, 91-135 and 91-120 exhibit biphasic changes with the formation of alpha-helix and beta structures. H3 peptide 1-90, on increasing the ionic strength to moderately high levels, monophasically formed appreciable quantities of alpha-helix and beta structures, while peptide 121-135 remained unfolded under all ionic strengths examined. All the above peptides except 121-135 also aggregate when the ionic strength is raised. The salt-induced near-ultraviolet circular dichroic spectra of histone H3 and peptide 1-90 were found to be very similar, suggesting that the conformational changes induced in the peptide 1-90 are essentially the same as those observed for the intact histone. These results support the contention that the polypeptide segments of this histone interact initially by parallel self-association followed by the formation of even larger aggregates on a longer scale.