Incorporation of diacids into the polyglycine II structure: model studies.

Aliphatic diacids are often incorporated into polypeptide structures in order to obtain model compounds for hormones, protein turns, etc. They are also fundamental components of many commercial polyamides. On the other hand glycine, the simplest amino acid, shows unique conformational features. In order to better understand the structure of such compounds, we have synthetized and determined the molecular structure of three models represented by the general formula CH3-CH2-CH2-NH-CO-CH2-NH-CO-(CH2)n-2-CO-NH-CH2-CO-NH-CH2-CH2-CH3, with n = 3, 4, or 6. Conformational differences have been found in the dicarboxylic moiety, whereas glycine always has the polyglycine II conformation. The -CO-(CH2)n-2-CO-segment adopts a folded conformation: SS, TGT, and SGTGS for n = 3, 4, and 6, respectively. Molecular packing is always pseudohexagonal and a network of hydrogen bonds oriented in three directions at 120 degrees is formed. The results are of interest in order to provide information about polyamides in which glycine residues are incorporated. Our results confirm the tendency of glycine residues to adopt the polyglycine II conformation in its copolymers with aliphatic compounds.