Experimental charge density of L-alanyl-L-prolyl-L-alanine hydrate: classical multipole and invariom approach, analysis of intra- and intermolecular topological properties.

A high-resolution dataset of the tripeptide L-alanyl-L-prolyl-L-alanine hydrate was measured at 100 K using synchrotron radiation and CCD area detection. Electron densities were obtained from a full multipole refinement of the X-ray experimental data, from an invariom transfer and from a theoretical calculation. Topological and atomic properties were derived via an AIM analysis [Atoms in Molecules; see Bader (1990). Atoms in Molecules: A Quantum Theory, No. 22 in International Series of Monographs on Chemistry, 1st ed. Oxford: Clarendon Press] of these densities and compared with each other, as well as with results from the literature of other oligopeptides and amino acids. By application of the invariom formalism to a dataset of limited resolution, its performance was compared with a conventional spherical refinement, highlighting the possibility of aspherically modelling routine structure-determination experiments. The hydrogen-bonding scheme was subject to a detailed analysis according to the criteria of Koch & Popelier [(1995), J. Phys. Chem. 99, 9747-9754] as well as to the characterization of Espinosa et al. [(1998), Chem. Phys. Lett. 285, 170-173; (1999), Acta Cryst. B55, 563-572; (2002), J. Chem. Phys. 117, 5529-5542] using the results from the refined and invariom multipole densities as well as the spherical-density model, which are critically compared.