Physicochemical properties of chromatin digested with trypsin

Chromatin was prepared from calf thymus and digested with trypsin. Some physicochemical properties of chromatin were examined in connection with the time-course of the tryptic digestion. As the tryptic digestion proceeded, chromatin showed increases in viscosity and susceptibility to DNAase II and exhibited considerable alteration in thermal denaturation. A monophasic melting profile was found in the trypsic (digested) chromatin, but a biphasic one in the native (undigested) chromatin. The melting temperature descended from 78.2 degrees C for the native chromatin to 70.2 degrees C for the chromatin after only 10 min and further to 65.3 degrees C after 180 min tryptic digestions. The molar percent of total basic amino acid or lysine plus arginine in the chromatin increased with the time-course of the tryptic digestion whereas that of the total hydrophobic amino acid decreased. The molar ratio of hydrophobic amino acids to basic amino acids thus descended from 1.46 for the native chromatin to 1.05 for the chromatin after a 180-min tryptic digestion. These findings suggest that the neutral or hydrophobic portions in chromatin protein might play a role in the maintenance of the tertiary structure of chromatin.