Structures of the Tripeptides L-Alanyl-L-phenylalanyl-L-alanine (AFA) and Tri-L-aspartic acid (DDD) at Low Temperatures

Amino acids and small peptides as building blocks for biologically active proteins are subject of numerous studies in the field of electron density.[1] The tripeptide AFA is an example for a study on the systematic row Ala–Xxx–Ala consisting of measurements of some of these tripeptides and calculations for all 20 possible combinations.[2][3] The tripeptide DDD is an example within a series of homo-tripeptides containing three times the same amino acid. So far only Trialanine could bemeasured in this series.[4] Once the electron density distributions for these systematic series have been determined and atomic and bond properties have been calculated, the concept of transferability of atomic and molecular fragments within the QTAIM approach of Bader [5] can be studied in detail. Database applications as having been developed in the groups of Lecomte,[6] Coppens [7] and Luger, [8] that are intended to make electron-density determination on large proteins possible, essentially rely on studies like these. Figure 1: ORTEP representations of the spherical structures of AFA (left) and DDD (right), 50 % probability of displacement ellipsoids, hydrogen atoms represented by spheres of arbitrary radii. A dataset of the tripeptide AFA has been measured at beamline F1 of HASYLAB at 100K. 187215 reflections were collected of which 18855 were unique with a completeness of 96.7 % up to