Observation of a Silk-Like Crystal Structure in a Genetically Engineered Periodic Polypeptide

A genetically engineered periodic polypeptide consisting of 36 repeats of the octapeptide sequence -(AlaGly)_3GluGly- has been crystallized from solution in aqueous lithium bromide. Analysis by solid-state nuclear magnetic resonance spectroscopy, vibrational spectroscopy, and wide-angle x-ray diffraction reveals a crystal structure analogous to that obtained when silk solutions are allowed to dry under quiescent conditions, i.e., the silk I structure. Previous analyses of the same polymer have demonstrated an antiparallel β-sheet architecture (silk II structure) in samples crystallized from formic acid. These results illustrate the potential for controlling the solid-state structures and properties of genetically engineered materials through selection of appropriate processing and crystallization conditions.