Handedness of crossover connections in beta sheets.

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Abstract
In a crossover connection, the polypeptide chain leaves one end of a .beta. sheet, forms a loop of any length and any conformation, and reenters the same .beta. sheet from the opposite end. Of the 85 examples of crossover connections which occur in the known protein structures, 83 are righthanded and only 2 are lefthanded. Consistent handedness, even in long irregular loops, could be produced by the preferred twist direction of extended chain and the righthandedness of .alpha.-helices, provided certain conditions hold during the protein folding process.