Organic Nanotube with pH Sensitive Interior Based on Benzimidazole Containing Cyclic Peptide

Organic nanotubes have been rapidly gaining attention for use in a wide range of applications related to transport and separations. In particular, the field of membrane separations can benefit greatly from the development of better organic nanotubes, and many early composite, porous membranes have shown promise for water purification and gas separations by incorporating them. Better membranes can be achieved by developing well-tuned nanotubes with uniform channel dimensions around the size of individual molecules and by having the right chemistries to allow for fast but selective transport./nWhile a wide range of nanotubes based on different chemistries exist, one of the greatest hurdles is controlling the properties of the interior lining. Hydrophobic interior surfaces have been achieved by many materials such as carbon nanotubes, yet introducing hydrophilic groups to their interior has remained challenging. Similarly, nanotubes and nanochannels based on crown ethers, pillararenes, surfactants, polymers, and zeolites have been developed and provide hydrophobic or polar interior properties, yet they too lack precise and modular control over the interior. However, being able to precisely control the interior properties is crucial and highly sought after in synthetic nanochannels. The benefits of doing so can be observed in nature: transmembrane proteins including the water-selective aquaporin protein and the proton-selective M2 channel utilize both chemistry and geometry to control transport. Regarding internal chemistry, a select few yet important polar amino acids such as the histidine residue are needed within a mostly hydrophobic core to achieve fast, yet highly selective transport./nCyclic peptides have highly tunable properties and can self-assemble into 1D nanochannels (CPNs). The chemistry, size, and shape of the nanotubes can be precisely controlled by changing the amino acid sequence and the number of amino acids in the backbone. Moreover, they can be blended with block copolymers to form subnanometer porous films with an oriented, dense array of uniformly sized channels, providing an exciting platform for membrane-based separations. While early CPNs were simple and didn’t provide any additional properties on the interior other than their amide backbone, the field has recently started focusing on developing artificial amino acids to insert new properties into the interior. The results have been mixed and there is a push to increase the library of CPNs./nThis dissertation discusses the synthesis and characterization of a new cyclic peptide, Bzim-CP, that incorporates a benzimidazole functionality in the backbone and forms nanotubes. Development began with the synthesis of an artificial amino acid and was followed by its incorporation into a peptide sequence. Through various characterization techniques, many aspects of the CPN were analyzed including structure, self-assembly, and response to different chemical environments. Most excitingly, the new functionality in the CPN chemically and structurally mimics histidine, and the CPN provides opportunities for enhanced transport through pH-responsive, subnanometer wide channels.

Advisor: Xu, Ting