V. A. Gindin

The role of short hydrogen bonds in mechanisms of enzymatic action

Abstract 1 H and 13 C NMR spectra of trypsin and ribonuclease, stabilized by chemical modification with a hydrophilic polymer, have been obtained over a wide pH range (1–11). The spectral features, referred to some nuclei of the catalytic sites (the “catalytic triad” for trypsin and the His-12—His-119 pair for ribonuclease), have been identified using different NMR techniques as well as chemical modification with selective reagents. It is found that monoprotonation of these systems leads to symmetrical (or quasi-symmetrical) H-bonds formed between the basic groups. This allows us to explain the discrepancies between experimental data obtained by different authors on the protonation sites in these catalytic systems. The simulation of the catalytic triad by a 15 N labeled low molecular weight model has led us to the conclusion that external agents do not cause any discrete proton transfers but do cause a smooth shift of the bridging protons from one basic atom to another, with the quasi-symmetrical H-bonds being formed in intermediate cases. On the basis of these experimental data, a new concept has been proposed for the mechanism of acid—base catalysis performed by the pairs of weak basic groups like His—Im and Asp(Glu)—COO − (p K a 3—7) which are not capable of proton abstraction from alcoholic or water OH groups (p K a > 13). This catalysis may consist on the one hand of changing the charge densities on reacting groups due to strong H-bonding and, on the other hand, of facilitating the free movement of a proton in the field of several basic atoms when going along the reaction coordinate. The energy of the very strong H-bonds thus formed diminishes the activation energy of the reaction.

NMR study of proton location in strongly hydrogen bonded complexes of pyridine as influenced by solvent polarity

1H,15N and 13C NMR spectra of H-bonded complexes of 15N-enriched pyridine (Y) with various strong proton donors (XH) were obtained in a range of solvents with different dielectric constant at low temperature. Chemical shifts and nuclear magnetic relaxation times, T1, were measured. A procedure for evaluating the mean NH distance in the complexes using T1 values for 15N and 13C nuclei treated as determined by dipole-dipole coupling with nearest protons is proposed. The results point to a gradual shift of the average proton position towards the N atom, with increasing the XH acidity. For a given complex, the NH distance falls with the e dielectric constant of a solvent, linear correlation between rNH and the reactive field factor, F=(e-1)/(2e+1), being observed. The analogous correlation was found for the Δδ values. For interpretation of the effect, an anharmonic two-dimensional model of H-bond in connection with the Onsager-Butcher potential was used. It was found that the reactive field of a solvent causes the NH bond to shorten due to the specific feature of the dipole moment function common for nearly all XH…Y and X−…HY+ complexes (∂μ/∂RY-H<0). With the required parameters taken from IR spectra, this model gives a reasonable coincidence with the experimental ArNH values.

Direct 13C-31P coupling constant of coordinated triphenylphosphine as a characteristic of electron-withdrawing power of the metal center

It is known that C signals of phenyl groups attached to a phosphorus atom are split due to coupling with the P nucleus. The direct C–P coupling constant (JCP) sharply increases in going from aromatic phosphines to the corresponding phosphine oxides. In keeping with our and published data [1–10], the JCP value of PPh3 is negative and is –11 Hz, and the JCP value of Ph3P=O is positive (104 Hz). It is reasonable to rationalize increase of JCP by change of the valence state of the phosphorus atom. The phosphorus atom in the triphenylphosphine molecule possesses a lone electron pair (LEP), whereas the latter is involved in interaction with a strong electron acceptor (oxygen atom) in the phosphine oxide molecule.

Reaction of chiral pyrrolylphosphine with polynuclear carbonyl complexes of osmium and rhodium

Reactions of labile Os3(CO)10(NCMe)2 and Rh6(CO)15(NCMe) clusters and dinuclear Rh2(CO)4Cl2 complex with chiral (S)-[2-(4-isopropyl-4,5-dihydro-1,3-oxazol-2-yl)phenyl]di(1H-pyrrol-1-yl)phosphine were studied. The reaction of Os3(CO)10(NCMe)2 with the chiral phosphine was diastereoselective, and it afforded only one diastereoisomer with ee 100%. All isolated compounds were completely characterized by NMR and mass spectra and X-ray diffraction data.