Influence of Alternating L-/D-Amino Acid Chiralities and Disulfide Bond Geometry on the Capacity of Cysteine-Containing Reversible Cyclic Peptides to Disperse Carbon Nanotubes

Although single-walled carbon nanotubes (SWNTs) have exciting properties and potential applications, their hydrophobic nature makes them difficult to purify and manipulate. To fully realize the potential of SWNTs, strategies for the effective dispersion, separation, and organization of these materials must be devised. In this article, work involving the recent design and characterization of reversible cyclic peptides (RCPs) and RCP/SWNT composites will be described. The peptides in this work contain alternating L- and D-amino acid sequences, as well as N- and C-terminal cysteine residues (RCP-Cys) that allow for their covalent, closure around the circumference of individual SWNTs. When RCPs are oxidized in the presence of SWNTs, dispersions are produced that are stable against dilution by dialysis without the formation of aggregates. The reported studies using Raman spectroscopy and UV/Vis/NIR were focused on answering the questions (1) does the chirality of the disulfide bond impact the capacity of the RCP-Cys to disperse SWNTs, and (2) is the alternating chirality of the amino acids in the RCP-Cys peptides important for SWNT dispersion. It was found that though Cys-containing RCPs are indeed able to disperse SWNTs, the chirality of the Cys residues on the N- and C- termini does not have a significant influence on the dispersed SWNT population. However, there is a large decrease in the dispersability by RCP-Cys when the alternating L/D-chiral pattern of amino acids is replaced with all L-amino acids. (C) 2009 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 92: 212-221, 2009.