The graph of number of amino acid (AA) residues in a loop Vs the frequency of their occurrence in proteins largely follows a tending-to-zero pattern. However, there appear to be some specific number of AAs where the frequency suddenly drops, leading to bi- or multi-modality in the plot. Is there some molecular explanation for this, or is it just an artifact?
I’ve attached the figure from a paper with a different research question but the same graph(Figure 2), albeit more detailed. (The same graph can be found in Brandon and Tooze (fig.2.8).) In the paper from which this is taken “A loop structure was defined as any region between two regular secondary structures that was at least three residues in length.”