Title: Supramolecular hydrogels based on the epitope of potassium ion channels

Authors: Yi Kuang, Yuan Gao, Junfeng Shi, Hsin-Chieh Lin and Bing Xu

Journal: Chemical Communications

Affiliation: Department of Chemistry, Brandeis University

Supramolecular hydrogels are a growing field of research due to their biocompatibility and the numerous possibilities for applications medicine.  In this paper the researchers take a page from nature’s book and design a hydrogel based on the K⁺ binding epitope of K⁺ channels.  In general, potassium channels consist of four epitopes that selectively allow K⁺ to flow through.  Due to electrostatic interactions the entry of K⁺ is greatly favored over the entry of Na⁺.  The interesting properties of this epitope led to the synthesis of a unique hydrogelator.

The authors found that the pentapeptide (TIGYG) epitope with a fluorene group attached made a good hydrogelator (a compound with the ability to form a hydrogel).  The hydrogelator was found to have a minimum gelation concentration of 0.10 wt% – above this concentration the hydrogelator will self-assemble to form a hydrogel.  In order to gauge the interaction between K⁺ or Na⁺ and the hydrogelator they first observed whether or not various concentrations of K⁺ or Na⁺ cause hydrogels to form below the minimum gelation concentration.  A 0.05 wt% of the hydrogelator did not gel when the ratio of [Na⁺]/[hydrogelator] was increased at intervals from 2.33 to 82.33.  However when the ratio of [K⁺]/[hydrogelator] was increased from 2.33 to 12.33 a hydrogel was formed.

By varying the concentration of K⁺ the properties of the hydrogel could be altered.  When [K⁺]/[hydrogelator] reaches 23.33 the hydrogel becomes softer and softness increases as the ratio is increased to 42.33 and 82.33.  The authors used TEM to investigate this effect.  They find that changing [K⁺] changes both the width and aggregation of the nanofibers that make up the hydrogel.  The authors suggest that the surface of the nanofibers contain epitopes of the correct configuration to bind K⁺ like the K⁺ channels in nature thereby “gluing” the nanofibers together when K⁺ is at a suitable concentration.  The authors attribute the softening of the hydrogel to the decrease in width of the nanofibers which causes a general weakening of the network and to the aligment of the nanofibers which reduces the amount of water that can be retained.

The authors also studied the importance of the peptide sequence to the ability to control hydrogelation via K⁺ concentration.  When the sequence was changed from TIGYG to TGGIY the ability to form a gel at 0.05 wt% with addition of K⁺ was lost.

This paper demonstrates not only our ability to develop compounds that respond to specific stimuli, but also our ability to construct novel materials by emulating nature.