Electron density and electrostatic properties of two peptide molecules: tyrosyl-glycyl-glycine monohydrate and glycyl-aspartic acid dihydrate.

The electron density and electrostatic properties of Tyr-Gly-Gly and Gly-Asp molecules have been determined from high-resolution X-ray diffraction data at 123 K. Topological properties of the charge distribution are discussed and compared with those derived from other experimental studies on peptide molecules, and the characteristics of the (3,-1) critical points of the C=O, C-N, C-C bonds are analysed. Crystal data for Tyr-Gly-Gly: C13H17N3O5.H2O, Mr = 313, orthorhombic, P2(1)2(1)2(1), Z = 4, T = 123 +/- 2 K; lattice parameters: a = 7.984 (2), b = 9.535 (3), c = 18.352 (5) A, V= 1397.1 (6) A3, Dx = 1.49 g cm(-3), mu = 1.2 cm(-1) for lambdaMo = 0.7107 A. Crystal data for Gly-Asp: C6H10N2O5.2H2O, Mr = 212, orthorhombic, P2(1)2(1)2(1), Z = 4, T = 123 +/- 2 K; lattice parameters: a = 9.659 (1), b = 9.672 (1), c = 10.739 (1) A, V= 1003.3 (4) A3, Dx = 1.40 g cm(-3), mu = 1.3 cm(-1) for lambda(Mo)= 0.7107 A.

Topological analysis of the electron density in hydrogen bonds.

Topological analysis of the experimental electron density rho(r) in hydrogen-bonding regions has been carried out for a large number of organic compounds using different multipole models and techniques. Relevant systematic relationships between topological properties at the critical points and the usual geometric parameters are pointed out. Results involving X-ray data only and joint X-ray and neutron data, as well as special hydrogen bonding cases (symmetric, bifurcated, peptide bonds, etc.) are included and analysed in the same framework. A new classification of hydrogen bonds using the positive curvature of the electron density at the critical point [lambda(3)(r(CP))] is proposed.