Schellman motif

A second example that illustrates the power of exhaustive conformational searching is foimd in the recent analysis of glycine terminated helices (Gly at C) (2). Specifically, two recurrent motifs are observed in a-heHces terminated by a glycine (Figure 1). In each, the glycine residue adopts a left-handed conformation (i.g. ( )>0). In one case, termed the Schellman motif, a distinctive, doubly hydrogen bonded pattern between backbone partners, consisting of 6 1,5 2... 

For the Schellman motif, modeling was used to address three questions. For each question, the model peptide consisted of a helical gment followed by a Schellman sequence. In detail, all conformations of the peptide CHaCO-Leu-Ala-Ala-Ala-Gly-Ile-NHCHa were generated, subject to the constraint that backbone torsions of the subsequence. ..-Leu-Ala-Ala-... were maintained in helical conformation (< i=-64°, In hehx nomenclature, this fi agment... 

Summarizing the modeling for the Schellman motif, all three of the questions posed can be answered affirmatively. A notable and automatic consequence of the motif is to position the C glycine in a left handed turn conformation. For the motif, glycine is the only sterically allowed residue at the C position. This conclusion from modeling is confirmed experimentally. In every instance of an aL motif... 

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