Fig. 6

The active site of (a) Orthorhombic RNase A and (b) monoclinic RNase A 3RN3 [5] showing the \({\text{SO}}_{4}^{2 - }\), Gln-11 and His-12 groups and waters. H-bonds are dashed. In Orthorhombic RNase A His-12 H-bonds to the \({\text{SO}}_{4}^{2 - }\) group both directly and via a water molecule. Gln-11 H-bonds via the same water molecule and directly via the other NH group. In 3RN3 His-12 forms similar H-bonds and Gln-11 uses Oδ instead of Nδ. The conformations are similar. The \({\text{SO}}_{4}^{2 - }\) group is disordered into two groups in the orthorhombic structure. The electron density map gives clear definition of the position of His-12 in the orthorhombic model and is very close to that in 3RN3 (b). (Drawn with Biovia [19])