Atom-plus-ligand approximation

D. The Atom-plus-Ligand Local-Term Approximation... 

The atom-plus-ligand local-term approximation has been justified a posteriori for 30) and 57) shielding, since certain expected relationships of chemical shifts, such as additivity of substituent effects, were found to be absent for the observed shifts, but were fulfilled for i.e., after correction for the atom-plus-ligand diamagnetic term. Some examples are given in the following sections. 

Going beyond the one-center approximation, the atom-plus-ligand model gives... 

An interesting and useful method of theoretical treatment of certain properties of complexes and crystals, called the ligand field theory, has been applied with considerable success to octahedral complexes, especially in the discussion of their absorption spectra involving electronic transitions.66 The theory consists in the approximate solution of the Schrddinger wave equation for one electron in the electric field of an atom plus a perturbing electric field, due to the ligands, with the symmetry of the complex or of the position in the crystal of the atom under consideration. 

Each molecule (molecular weight 30000) contains one zinc(II) atom, which is (approximately) tetrahedrally coordinated to two N atoms and one O atom from amino-acid residues plus a water molecule. The structures of both the enzyme and some enzyme-substrate complexes have been carefully studied and the detailed mechanism of the hydrolysis is now quite well understood. Without going into details, a crucial factor appears to be the pronounced distortion from regular tetrahedral coordination about the Zn(II), apparently imposed by the conformational requirements of the polypeptide chain. The conflict of interest between the needs of the Zn(II) atom - which, when four-coordinate, always assumes tetrahedral coordination - and the ligands induces an entatic state, a condition of strain and tension which enhances the reactivity at the active site. The Zn atom binds the substrate peptide via the O atom of the —CONH— peptide link, and the entatic state of the free enzyme facilitates formation of the enzyme-substrate complex. 

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