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Changes are observed in the aromatic region of the nuclear magnetic spectrum of bovine pancreatic ribonuclease A on addition of the inhibitors cytidine-2′-monophosphate and cytidine-5′-monophosphate. These are described and compared with those previously observed in the presence of cytidine-3′-monophosphate. Addition of these inhibitors causes the pK values of histidine 12 and histidine 119 to increase and their C(2)-H peaks to shift. The magnitudes of the peak shift and pK increase of histidine 119 vary with the position of the phosphate group and can be correlated with the binding strength of the inhibitor (2′ > 3′ > 5′). Both of these histidine residues are positively charged in the complexes with cytidine-2′-monophosphate and cytidine-3′-monophosphate. The pK value and chemical shift of the C(2)-H peak of histidine 105 are unaffected by inhibitor binding. The histidine 48 peak shifts slightly in the presence of inhibitor, indicating that inhibitor binding effects the conformational equilibrium of the enzyme. The nature of this equilibrium is discussed. In order to specify the interactions of the different chemical groups of the mononucleotide inhibitors with ribonuclease, the effects of the binding of the inhibitors cytidine and phosphate on the nuclear magnetic resonance spectrum of the enzyme were also observed. Further information concerning the environment of the inhibitor in the active site was obtained from the shifts of the C(6)-H, C(5)-H and C(1′)-H peaks of the nucleotide and nucleoside inhibitors. Models of the structures of the ribonuclease-mononucleotide complexes are postulated which are consistent with the nuclear magnetic resonance data and with the results of studies of the structure of ribonuclease by other methods.

High-resolution proton magnetic resonance spectra along with chemical shift and line-width data are presented and discussed for the following polyamino acids throughout their helix-coil transition regions: poly- l -alanine (2 molecular weights), poly-β-benzyl- l -aspartate, poly-γ-benzyl- l -glutamate, poly- l -glutamic acid (2 molecular weights), and poly- l -methionine. The sharpness of the spectra indicate that there is a rapid interconversion between helical and non-helical regions of the polymers. The lifetime of a residue in either state has an upper limit of about 6 × 10 −3 sec. The peak corresponding to the proton on the α-carbon of the polypeptide backbone shifts upfield on helix formation. The magnitude of this shift, which is attributed to the magnetic anisotropy of the peptide bond, appears to be a sensitive measure of per cent helicity. The chemical shift on helix formation of the peak corresponding to the proton on the peptide nitrogen is determined by two opposing factors: (1) differences in hydrogen bonding; (2) magnetic anisotropy of the adjacent peptide bond. Side-chain resonances are not shifted appreciably on helix formation.