Publisher Summary This chapter discusses crystal structures of metal–peptide complexes. Most of the crystal-structure analyses of metal–amino acid and metal–peptide complexes have been carried out on the assumption that such complexes act as models for the metal-binding sites on proteins. Crystal-structure analyses show that the geometrical features of metal complexes of amino acids, peptides and imidazole are related in systematic ways to the chemical structure, which the complexes have in the crystalline state. Moreover, the transfer of geometrical information from crystal-structure analyses to species that exist in solution depends on the assumption that the complexes found in crystals are present also in the solutions from which the crystals grow. This chapter illustrates that, the existence of cis-trans isomers coordination isomers, optical isomers, and dimeric species in the crystalline complexes emphasizes the variety of species, which must be considered when equations are written to represent metal–peptide equilibria in solution.

Crystal structures of metal-peptide complexes.

The nature of Cu bonding to natural organic matter

An analysis of the various parameters associated with N-H… O type of hydrogen bonds has been made using data from reported crystal structures of amino acids and simple peptides. The different parameters at the donor and the acceptor ends have been suitably defined and evaluated. In some cases the analysis is done depending upon the chargedness or other characteristics of the donor and acceptor groups. Histograms giving the distribution of these parameters have been drawn and possible conclusions arrived at:



1. The distribution shows a maximum between 2.8 A and 2.9 A for the charged donor group and 2.9 A and 3.0 A for the uncharged donor group and is probably not dependent upon the charge on the acceptor group.



2. The angle between the directions CO and O. N tends to lie between two cones about C O with semi-vertical angles 40 and 70°. The orientation of the directions O. N and O. H with respect to the lone pair orbital directions on the acceptor oxygen atoms are analysed in detail using spherical polar coordinates. The analysis indicates that the group NH has a very strong tendency to point towards the acceptor oxygen atom. A general feature has been found in that the direction N-H tends to be closer to an orbital if the oxygen is an acceptor of two hydrogen bonds, while the direction tends to lie in between the orbitals when the acceptor oxygen is the receipient of only one hydrogen bond. The possible explanation of this on the basis of lone pair interaction is briefly discussed.

Study of hydrogen bonds in amino acids and peptides.

Structural isomerism of copper(II) compounds

A theoretical and computational study of the nitrogen superhyperfine structure in Cu(II) complexes is reported. The determination of hybridization parameters for nitrogen donor orbitals from the data is examined. For most Cu(II) complexes the results deviate substantially from pure “sp2” or “sp3” hybridization. Semiempirical INDO/S calculations for five Cu(II) complexes were carried out at the UHF and ROHF level. The results suggest that the small anisotropy in the nitrogen hyperfine parameters is caused by spin polarization of the nitrogen valence shell orbitals. A simple, approximate way for the determination of the π-spin density from experimental data is outlined. A density functional study using various basis sets and functionals is reported for the same five complexes. Hybrid functionals, such as B3LYP and PWP1, give better predictions than functionals based on the generalized gradient approximation like BP or BLYP. Provided that at least a polarized triple-ζ basis is used, the hybrid functionals B3...

H. C. Freeman

Papers

A refinement of the structure of bisglycinocopper(II) monohydrate, Cu(NH2CH2COO)2.H2O