Torsion angles conformations

The lactam restriction of peptide conformation (20) (Ci -o-Ni+i) was proposed first by Freidinger et al. (58) with the aim at forcing the peptide bond (coi) to be trans and placing a severe constraint on the rj/i torsion angle. Conformational... 

Usually, two conformations are regarded as geometrically different if their minimized RMS deviation is equal to or larger than 0.3 A in Cartesian space RMSx z)> or 30 " in torsion angle space (RMSja). respectively. 

Fig. 4.S Variation in torsional energy (AMBER force field) with O-C-C-0 torsion angle (to) for OCH -CHjO fragment. The minimum energy conformations arise for to = 60° and 300°.

The trans conformation corresponds to a torsion angle of 180°, the gauche(+) conformation to oni + 60° and the gauche -) conformation to -60°. These approximately correspond to the torsion anj of the three minimum energy conformations of butane. 

The dimensionality of a data set is the number of variables that are used to describe eac object. For example, a conformation of a cyclohexane ring might be described in terms c the six torsion angles in the ring. However, it is often found that there are significai correlations between these variables. Under such circumstances, a cluster analysis is ofte facilitated by reducing the dimensionality of a data set to eliminate these correlation Principal components analysis (PCA) is a commonly used method for reducing the dimensior ality of a data set. 

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