Molecular structure of the molybdenum oxo-diperoxo compound MoO(O(2))(2)(OPy)(H(2)O): a computational and X-ray study.

We have carried out a combined experimental and theoretical study of the molecular structure of the MoO(O(2))(2)(OPy)(H(2)O) coordination compound using X-ray crystallography and DFT-B3LYP computational method, respectively. The MoO(O(2))(2)(OPy)(H(2)O) complex crystallizes in the orthorhombic space group Pmna with Z = 4, a = 6.9001(9) A, b = 8.0471(1) A, c = 16.227(2) A, V = 901.0(2) A(3), and the X-ray data analysis yields a bipyramidal-pentagonal coordination polyhedron for the Mo atom. The pyridine N-oxide (OPy) ligand occupies the equatorial position, with the oxygen atom of this ligand being located in the same plane as the four peroxo oxygen atoms. The H(2)O ligand is situated trans to the oxo group, forming intermolecular hydrogen bonds with peroxo groups belonging to two adjacent complexes. In our theoretical approach these intermolecular interactions were taken into account by including two methanol molecules which form hydrogen bonds with the water ligand leading to a good agreement between the calculated and the experimental geometry. Our results suggest that it is necessary to take into account the presence of these interactions in order to reconcile the theoretical results to the experimental data, in particular the distance between Mo and the oxygen of water ligand. These results seem to be a general feature for analogous bis-peroxo complexes that have been reported in the literature.

Solvent accessibility to aspartyl and succinimidyl residues at positions 7 and 23 in the amyloid beta 1-28 peptide.

The water accessibilities to aspartyl residues at positions 7 and 23 in the amyloid beta 1-28 peptide associated with Alzheimer's Disease have been calculated using different techniques. These accessibilities of water were compared to those of the succinimidyl residues (SUC) replacing the aspartyl ones (ASP). It has been possible to ascertain that these modifications (ASP--->SUC) lead to a significant increase in the water accessibility to the backbone and alpha-carbon atom of the SUC7 and SUC23 residues. It is suggested that the spontaneous transformation of the ASP--->SUC might lead to an increase of the racemization rates due to the higher accessibility of water at these sites. It is also proposed that the behavior of the adjacent residues in the selectivity of the racemization is to control the water accessibility at the reactive residue.