Zigzag-Helix Transformation of Expanded Polyvaline Induced by Racemization

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 4(1) 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 alpha-helix and a beta-strand. Such expanded polypeptides can be built up into expanded protein, forming a tailor-made three-dimensional structure, which will lead to new functions.