Studies on cyclic peptides. I. Cyclodisarcosyl–metal salts complexes

Abstract

SynopsisThe complexes of cyclodisarcosyl (N,N'-dimethyldiketopiperazine) have been synthesized with copper perchlorate, lithium perchlorate, barium perchlorate, silver perchlorate, silver nitrate, and boron trifluoride. It has been shown by infrared absorption spectra that the metal cations coordinate to carbonyl groups of the peptide to form insoluble complexes. It has been suggested that the complexation takes place by iondipole interaction. Since a linear analog of the peptide formed no insoluble complex with the metal salts, conformation of cyclodisarcosyl seems to play an important role in the complex formation. Examination of the molecular model and X-ray analysis suggest that the copper perchlorate complex of the cyclic peptide has a "2: 1-sandwich" structure.

The crystal structures of complexes of cyclodisarcosyl with metal salts. I. The 2:1 complex of cyclodisarcosyl with lithium perchlorate

“…This seems to show that as the coordination bond of Li-O becomes strong, the double-bond character of the C(1)-N bond increases. There is, however, no quantitative correlation between the shifts of the stretching frequencies (Sugihara et al, 1973) and the bond distances of the C(1)-O and C(1)-N bonds.…”

“…Cyclodisarcosyl (N,N'-dimethyldiketopiperazine) is known to form crystalline complexes with metal salts (such as copper perchlorate, lithium perchlorate, barium perchlorate, silver perchlorate and silver nitrate) and with boron trifluoride (Sugihara, Imanishi & Higashimura, 1973). It has been shown by infrared absorption spectra that the carbonyl O atoms of the peptide coordinate to the metal cations in these metal complexes.…”

Synthesis, Conformation, and Interactions with Small Molecules of Bis (Cyclic Dipeptides)

“…The temperature dependence of NMR.-chemical shifts of three amide protons of bis (I) in D,-dimethylsulfoxide (D6-DMSO) were measured; these were 3 . 3~ lo-,, 4 . 8~ lop3 and 6 .…”

“…In cyclic dipeptides, the carbonyl groups cannot cooperate to form intramolecular complexes. However, cyclo (Sar), forms insoluble complexes with various metal ions in ethyl acetate [4]. The crystalline complex between cyclo (Sar), and LiC104 has a network structure, in which four carbonyl groups of four cyclic dipeptides coordinate to Lif [ 5 ] .…”

“…In cyclic dipeptides, the carbonyl groups cannot cooperate to form intramolecular complexes. However, cyclo (Sar), forms insoluble complexes with various metal ions in ethyl acetate [4]. The crystalline complex between cyclo (Sar), and LiC104 has a network structure, in which four carbonyl groups of four cyclic dipeptides coordinate to Lif [ 5 ] .Therefore a corresponding bis (cyclic dipeptide) might be an efficient ligand for metal ions because its complexation should be favored over that of simple-cyclic dipeptides by the entropy term.…”