Glycine, dipeptide allowed conformations

What Figure 2.11 tells us is that a conformational map for a dipeptide of glycine (the side chain in glycine is very small, just a hydrogen) has mostly allowed or partially allowed conformations and therefore polyglycine is flexible. One question that you might ask is how do we know that the conformational plot for a polypeptide is the same as for a dipeptide The answer is that because the side chain points away from the backbone for most conformations the atoms in the side chains are separated by more than the sum of the van der Waals radii. However, below we discuss several highly observed conformations of proteins in which the conformational map is an overestimation of the flexibility because of interactions between atoms more than two peptide units apart in space. 

Figure 2.14. Conformational plot for glycine-proline. Addition of proline to a dipeptide further reduces the number of allowable conformations when compared to Figures 2.12 and 2.13.

Fig. II, Allowed areas of the steric map for various dipeptides. In area 0 no conformations are allowed. Conformations in areas 1 to 4 are allowed for glycyl glycine, in areas 2 to 4 for glycyl-L-alanine, in areas 3 to 4 for higher straight-chain homologs, while only area 4 is allowed for glyeyl-L-valine and glycyl -n -isoleucine. The circles marked R and L indicate the location of the standard right- and left-handed a-helix on the steric map (Nemethy etal., 1900a,b).

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