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Short Peptides Directing 1D Helical Arrays of Polyoxometalates with Controllable Pitches.

A series of cationic peptides with alternating hydrophilic and hydrophobic residues were elaborately designed and synthesized. These kinds of short peptides with protonated lysine groups can interact with anionic polyoxometalate nanoclusters through multivalent ionic bonds and hydrogen bonds, resulting in the formation of helical polyoxometalate arrays in aqueous solution. Fourier transform infrared (FTIR) spectroscopy, circular dichroism (CD), transmission electron microscopy (TEM), and dynamic light scattering (DLS) were utilized to characterize the self-assembled structures. TEM revealed that the polyoxometalate clusters form periodic arrays within the helical nanofibers. This work reports that the handedness of the helical fibers was attributed to the precise chirality expression of peptides. The l-type peptide directed the formation of left-handed polyoxometalate arrays, whereas right-handed polyoxometalate arrays were observed when the peptide was constituted by d-amino acids. It was also found that the pitch of the helical nanofibers is inversely proportional to the hydrophobicity of peptides with less hydrophobicity giving a larger helical pitch.