Test180919
ContentsPeptide bondThe peptide bond (highlight in green) formation is a condensation reaction between the carboxyl group of the amino acid i and the amino group of the amino acid i+1. The peptide bond is a resonance structure between two limiting states. Therefore the N-C bond has a partial double bond character and the atoms/groups Cα, HN, C, C=O are within one plane. Peptide bonds are usually in trans conformation (ω torsion angle= 180°). In cis-conformation, there would be steric hindrance between Cα,i and Cα,i+1.
Cis conformation has been observed almost exclusively for peptide bonds preceding a Pro residue (Xaa - Pro), see below.
Phi torsion angle is defined by the four atoms φ = C - N - Cα - C (in green). Psi torsion angle is defined by the four atoms ψ = N - Cα - C - N (in green). Omega torsion angle is defined by the four atoms ω = Cα - C - N - Cα (in green). Cis peptide bondsThe ω torsion angle can adopt a value close to 0° (cis-conformation), when a Pro residue is the following residue (Xaa-Pro peptide bond). In this situation a cis-Pro and a trans-Pro are similarily unfavorable, since there is a steric clash between Cα,i with Cα,i+1 or Cδ,i+1, respectively. Conversely, the carbonyl O of residue i is in tight juxtaposition with Cδ,i+1 or Cα,i+1 (note that latter tight contact occurs in any trans peptide bond).
Torsion angle φ restricted for ProDue to the ring closure of the Pro side-chain the torsion around the N - Cα is considerably restricted to a value between -50° to -90°.
References
Next TopicSecondary structure of proteins https://proteopedia.org/wiki/index.php/User:Tilman_Schirmer/Sandbox_100 | |||||||||||
