Beta-pleated sheet antiparallel

From WAXS and SAED data of both ProNectin F lyophilized powder and sprayed fibrils, the current model indicates that ProNectin F crystallizes into a chain folded pleated sheet of beta strands (Anderson et a/. 1994). The strands are oriented antiparallel. The beta strands are not fully extended, but have a more compressed crankshaft conformation. This conformation agrees with the predicted conformation of unoriented silk fibroin protein, the Silk I structure (Lotz and Keith 1971). The crystal dimension in the c direction (along the peptide backbone) is consistent with a theoretical length of 11.6 nm for nine SEP blocks (54 amino acids) in this conformation. This predicts that the width of the ProNectin F tile is controlled at least in part by the number of amino acids in the silklike block domains. 

The other structures that Pauling discovered by model-building are the j8-structures (pronounced beta), also known as the pleated sheet, from its appearance when many lengths of chain associate in parallel. They are formed by side-to-side association of almost fully extended chains, and the reason that two forms are possible is that adjacent chains may run in parallel or antiparallel (i.e. the same or opposite) directions, as shown here ... 

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