Section 18.3.2.5. Torsion angles

Since torsion angles are generally more adequately determined by protein structures of typical resolutions, there is less need to derive parameters from the CSD for refinement purposes. Further, distributions derived from small molecules may not be representative of torsion angles among proteins, since typical fragments lack `typical' protein secondary structure. This kind of environmental dependence will affect softer parameters such as dihedral angles more than bonds or angles. However, since the torsion-angle distribution is a function not only of potential interactions of the peripheral groups but of the electronic character of the bond itself, some questions may be best examined by the study of chemical fragments. One such feature might be the distributions for aromatic residues. Statistical distributions of side-chain orientations show the apparent stability of an eclipsed conformation, particularly for ( conformer, Fig. 18.3.2.3). If the relative dearth of secondary structure leads to atypical torsion-angle distributions in proteins, it must be conversely expected that torsion-angle statistics derived from the set of all proteins will be less applicable, for example, to helical proteins than statistics derived from mostly helical proteins. This illustrates the dilemma of selecting the ideal statistical database for protein refinement.

Figure 18.3.2.3| top | pdf | Tryptophan dihedral angle distribution. The 36 tryptophan fragments in the CSD show several apparent minima, including an eclipsed Cα—Cβ—Cγ—Cδ1 dihedral conformation. This is apparent in , tryptophan distribution plots from protein structures as well (Laskowski, MacArthur et al., 1993).