Biological macromolecules are essentially homochiral. For example, proteins mostly consist of l-amino acids. What happens when a chiral molecule meets itself in a mirror? For expanded polyvaline, zigzag-helix transformation occurs. In this study, expanded polyvalines containing bis(pyridine)silver(I) moieties were synthesized and isolated as single crystals. The molecular structures were determined by X-ray analysis, which revealed that chiral expanded poly(l-valine) and poly(d-valine) form zigzag chains. However, racemic mixture of these molecules form left- and right-handed 41 helices that retain the original sequences. These secondary structures can be transformed by only flipping the C-terminal amide plane for each unit, which is reminiscent of the relationship between an α-helix and a β-strand. Such expanded polypeptides can be built up into expanded protein, forming a tailor-made three-dimensional structure, which will lead to new functions.
Keywords: amino acids; helical structures; peptides; silver; supramolecular chemistry.
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