Peptide unit geometry

This unit is called the peptide unit. Figure 13.1 shows the geometry of the peptide backbone. Due to mesomery, the peptide unit is stabilized by a resonance energy of 80-90 kJ mol-i ... 

In the peptide complex (S3) the peptide units are packed in antiparallel pleated sheets, the geometry of which is only slightly affected by chelation to the metal atoms. The metal atoms make a number of short contacts with the carbon atoms of the aromatic ring individual Cu C contacts range from 3.17—3.34 A. ... 

The crystal structure of [Cd(GlyGly)(Im)Cl] has been reported72 and shown to involve dimeric units with a distorted octahedral geometry about each metal atom this complex has also been studied in a 13C NMR investigation of a range of nucleoside and peptide complexes of cadmium(II).430... 

Molecular Modeling Studies and Conformational Energy Calculations on Dithiopeptides Mean Geometry of the Thiopeptide Unit and Conformational Flexibility of Peptide-Thiopeptides and Dithiopeptides. ... 

Fig. 1.3. The dependences on the dihedral angles (0, i/(), of the chemical shielding constants for the L-alanine residue C and Cp carbons in peptides. Chemical shielding calculations were carried out using the GIAO-CHF method with 4-31G ab initio basis set. The 4-31G optimized geometries for the model molecules, A-acetyl-A -methyl-L-alanineamide, were employed (a) the isotropic (b) cru, (c) 0-22 (d) cr, for the C/3 carbon (unit in ppm) (e) the isotropic (f) ctu (g) 0-22 and (h) o-j, for the C carbon (unit in ppm).

A characteristic feature of peptides is the repetitive unit of the three directly bonded atoms N-Ca-Ccarbonyi, which form the backbone of a peptide structure. Along the backbone, the geometry of the peptide is mainly determined by the torsion angles a>, (f>, and ijj. In most cases of naturally occurring peptides, the dihedral angle of a> exhibits a value of 180° (see Fig. 34). 

In addition to its planar geometry, the amide bond affects the structure of peptides in another important way. The N—H and the C=0 units are candidates for hydrogen bonding with other peptide linkages both within the same and with adjacent polypeptide chains. 

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