1.D.174. The Synthetic Self-assembled 2-Hydroxy-N1,N3-diarylIsophthalamide (SSHI) Family
The isophthalamide core has been used to develop small-molecule ion transporters. The isopthalamide core was modified by functionalizing position 2 with a hydroxyl group to give 2-hydroxy-N1,N3-diarylisophthalamide connected to two identical aromatic side arms to vary the lipophilicity for better permeation and pKa values of the amide NHs (Malla et al. 2021). It was expected that the intramolecular C=O⋅⋅⋅H−O, and C−O⋅⋅⋅H−N interactions would provide a preorganized geometry of individual monomers. The intermolecular π-π stacking interactions among aromatic rings and hydrogen bonding interactions among amide moieties should lead to the aggregation of individual monomers in the membrane to form an active channel. Recognition sites for cations can be provided by either C−F or C=O moieties via cation-dipole interactions. Such systems are selective for M+/Cl− ion transport. Channel formation was supported by planar bilayer conductance studies, and molecular dynamics simulation studies supported the dimeric self-assembly of individual monomers and M+/Cl− selective nature of the channel (Malla et al. 2021).
Zheng et al. 2019 unveiled a new design for anion-selective channels, the T-quartet (see figure below). The triazolium cation in 58H+ is well suited for anion binding, using synergistic H-bonding, electrostatic and anion–π interactions, and its alkyl chains can pack to shield those anion binding sites, while interacting favourably with the bilayer. The authors presented X-ray structures, complemented by solution NMR data, illustrating how anions template self-assembled channels from 4 molecules of 58H+ (Davis et al. 2020). Interestingly, these T-quartets bind pairs of anions, via hydrogen bonds and electrostatic interactions. Anion–π interactions between the anions and protonated triazoles in neighbouring T-quartets generate a discrete channel of 3–4 Å diameter, featuring a “π-slide” along which anions might flow along their concentration gradient.
The authors found evidence that this self-assembled system forms ion channels. Voltage-clamp experiments in planar bilayers showed that application of 58 supported conduction at voltages of ±100–200 mV.
The protonated triazoles were, by themselves, relatively sluggish transporters in EYPC liposomes. However, if the K+ carrier valinomycin was added along with 58, then anion transport was much enhanced. In the presence of valinomycin, 58 had its lowest EC50 value of 4.47% molar relative to lipid for transport of Br−. The observed selectivity of Br− > Cl− > I− > NO3− suggests that factors other than just ease of anion dehydration control anion transport mediated by 58.
Further study of these T-quartet assemblies, by modifying the triazole
ring and lipid chains, could well lead to improved anion-selective
transporters. Importantly, Barboiu has now introduced an anion
transporter analogous to the G-quartet, a motif used for building
synthetic cation channels (Kaucher et al. 2006; Debnath et al. 2020).
tested 2-hydroxy-N1,N3-diarylisophthalamides as anion transporters (Fig. 40). Compound 59, with 3,5-trifluoromethyl chains, was the best transporter with EC50 = 0.48 μM. HPTS assays indicated that 59 facilitated co-transport of K+ and Cl−, with Hill coefficient of n = 2, consistent with a dimer being the basic unit for self-assembly. Voltage-clamp experiments showed that 59 formed single channels of 100 ± 2 pS, indicating a channel with d = 5 Å. These experiments showed the channels had a permeability ratio of PCl−/PK+ = 8.29.