A porphyrin-based nanopore was constructed through a bilayer lipid membrane (BLM). The macrocyclic porphyrin (the inner diameter = ca. 2.1 nm) having six carboxylic acid groups directed up and down was synthesized by self-assembly of three trisporphyrins, metathesis linking, and subsequent hydrolysis. The porphyrin was incorporated into soybean lecithin based BLM, and the ion currents through the nanopore were observed under 500 mM KCl, LiCl, CaCl2, and tetraalkylammonium chloride solutions, indicating formation of a large pore through BLM. Blocking of the ion current was achieved by addition of the polycationic fourth-generation PAMAM dendrimer.

Multivalency is important in molecular self-assembly, although it remains a challenge to correlate the single-molecule results with the multivalent interaction. As the first example, we have combined a well-defined self-assembling system with AFM-based single-molecule force spectroscopy (SMFS) to investigate the intermolecular interactions and multivalency between C(60) and porphyrin derivatives. Compared with the interaction between C(60) and a single porphyrin (29 pN), SMFS has revealed that porphyrin tweezers can provide an enhanced binding interaction with C(60), resulting in a more than 2-fold higher unbinding force (75 pN, at the same loading rate). In addition, a much lower dissociation rate and a shorter effective distance between the bound state and transition state of the interaction are indicated by dynamic force spectroscopy. These results provide new quantitative information on the divalency effect in the unbinding process of C(60) and porphyrin tweezers at the single-molecule level, which is of significance in understanding the strength of the multivalency in the molecular assembly.

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