On the role of strain in blue copper proteins.

Theoretical investigations of the structure and function of the blue copper proteins are described. We have studied the optimum vacuum geometry of oxidised and reduced copper sites, the relative stability of trigonal and tetragonal Cu(II) structures, the relation between the structure and electronic spectra, the reorganisation energy, and reduction potentials. Our calculations give no support to the suggestion that strain plays a significant role in the function of these proteins; on the contrary, our results show that the structures encountered in the proteins are close to their optimal vacuum geometries (within 7 kJ/mol). We stress the importance of defining what is meant by strain and of quantifying strain energies or forces in order to make strain hypotheses testable.

The cupric geometry of blue copper proteins is not strained.

The geometry of several realistic models of the metal coordination sphere in the blue copper proteins has been optimised using high-level quantum chemical methods. The results show that the optimal vacuum structure of the Cu(II) models is virtually identical to the crystal structure of oxidised blue copper proteins. For the reduced forms, the optimised structure seems to be more tetrahedral than the one found in the proteins, but the energy difference between the two geometries is less than 5 kJ/mol, i.e. within the error limits of the method. Thus, the results raise strong doubts against hypotheses (entatic state and the induced-rack theory) suggesting that blue copper proteins force the oxidised metal coordination sphere into a structure similar to that preferred by Cu(I) in order to minimise the reorganisation energy of the electron transfer reaction. Instead, a small reorganisation energy seems to be reached by an appropriate choice of metal ligands. In particular, the cysteine thiolate ligand appears to be crucial, changing the preferred geometry of the oxidised complexes from square-planar to a more trigonal geometry.

A Density Functional Study of the Structure and Stability of CrF(4), CrF(5), and CrF(6).

The structure and stability of VF(5) and the higher chromium fluorides CrF(4), CrF(5), and CrF(6) have been investigated using density functional theory. The local density approximation (LDA) was used to obtain geometries and vibrational frequencies, while nonlocal corrections were added in order to obtain more accurate binding energies. The results obtained for CrF(4) and VF(5) are in good agreement with the available experimental data, indicating the quality of the method used. Both CrF(5) and CrF(6) are found to be stable with respect to Cr-F dissociation. The calculated binding energies are 49.7 and 40.7 kcal/mol, respectively. In agreement with recent ab initio work, the octahedral isomer is found to be the most stable for CrF(6). An activation barries of 16.9 kcal/mol is calculated for pseudorotation to a trigonal prism transition structure. CrF(5) is found to be dynamically Jahn-Teller distorted from D(3h) to C(2v) symmetry.