Structural Studies of Ribonuclease. II. Deuterium Exchange Studies of Two Crystalline Forms of Ribonuclease1,2

Deuterium exchange studies were carried out on Armour ribonuclease 381-059 and two samples (ribonuclease II and III) prepared by King and associates. Complete deuteration cannot be achieved unless the molecule is above the transition temperature. A significant fraction of the hydrogens exchange slowly, in agreement with the results of Hvidt and Haggis. Below the transition temperature, e.g., at 38°, about 20 hydrogens are sufficiently shielded to prevent exchange. Two groups of 25 hydrogens each exchange more rapidly at 0° but still slowly enough to suggest that they are involved in some folding of the molecule. The remaining 175 exchange essentially instantaneously at 0°, suggesting that they are in random-coil-like parts of the molecule. Ribonuclease II and Armour ribonuclease 381-059 are indistinguishable by the method of deuterium-hydrogen exchange. By this criterion, ribonuclease III appears to have a configuration in solution in which a greater fraction of its hydrogens are shielded.