Bader's theory of Atoms in Molecules [1] allows the derivation of bonding, non bonding and atomic properties from the topological analysis of an electron density distribution. One key concept of Bader's theory is the transferability of submolecular or atomic electronic properties providing a tool to enter these fragments as building blocks for the additive generation of electron densities of macromolecules which are otherwise obtainable only in exceptional cases.

This study is an experimental verification of reproducibility and transferability in the oligopeptide field. We entered into comparative charge density studies of tripeptides of the type L-Ala-Xxx-L-Ala, where Xxx was to be varied among the 20 naturally accuring amino acids. We present a comparison of the experimental charge densities of two modifications of L-Ala-L-Tyr-L-Ala with different solvent molecules in the crystal lattice, water for modification 1 and ethanol for modification 2.

From the conventional - spherical - structure analyses [2] it was found that the molecular structures in the two modifications were very similar and that even the intermolecular interactions in terms of hydrogen bonds were in most cases comparable with only few exceptions in the contacts to the different solvent molecules. Hence this study represents a favorable case where the reproducibility of the charge density of a given molecule in different crystal structures but widely comparable crystalline environment can be studies. In addition, concerning transferability, the atomic and bond topological properties of the main peptide chain can be compared to the corresponding values of tri-L-alanine [3].

[1] R. F. W. Bader, Atoms In Molecules- A Quantum Theory, Clarendon Press: Oxford, U.K., 1990.

[2] L. Chęcińska, D. Förster, W. Morgenroth, P. Luger, Acta Crystallogr. 2006, accepted.

[3] E. Rödel, M. Messerschmidt, B. Dittrich, P. Luger, Org. Biomol. Chem. 2006, 4, 475-481.

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