Psi angle

Regions of ordered secondary structure arise when a series of aminoacyl residues adopt similar phi and psi angles. Extended segments of polypeptide (eg, loops) can possess a variety of such angles. The angles that define the two most common types of secondary structure, the a helix and the (5 sheet, fall within the lower and upper left-hand quadrants of a Ramachandran plot, respectively (Figure 5-1). 

Figure 5-1. Ramachandran plot of the main chain phi (< ) and psi (T) angles for approximately 1000 nonglycine residues in eight proteins whose structures were solved at high resolution. The dots represent allowable combinations and the spaces prohibited combinations of phi and psi angles. (Reproduced, with permission, from Richardson JS The anatomy and taxonomy of protein structures. Adv Protein Chem 1981 34 167.)...

For polysaccharide, choose connectivity (01-C[l-6]), anomer (alpha or beta), isomer (l or d), and conformation (define phi and psi angle with omega = 180). Add sugars (alsohexoses or aldopentoses) to build polysaccharide chain. 

Each atom of every individual amino acid present in the complexes was processed to extract the associated sets of phi-psi angles to generate a statistically significant cumulative Ramchandran plot for chain A of all proteins [60] (Fig. 5.62). 

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