Fluorescent detection of amidinium–carboxylate and amidinium formation using anthracene-based diamidine: an application for the analysis of dicarboxylic acid binding

Solvation-Enhanced Salt Bridges

Salt bridges formed by amidines and carboxylic acids represent an important class of noncovalent interaction in biomolecular and supramolecular systems. Isothermal titration calorimetry was used to study the relationships between the strength of the interaction, the chemical structures of the components, and the nature of the solvent. The stability of the 1:1 complex formed in chloroform changed by 2 orders of magnitude depending on the basicity of the amidine and the acidity of the acid, which is consistent with proton transfer in the complex. Polar solvents reduce the stabilities of salt bridges formed with N,N'-dialkylamidines by up to 3 orders of magnitude, but this dependence on solvent polarity can be eliminated if the alkyl groups are replaced by protons in the parent amidine. The enhanced stability of the complex formed by benzamidine is due to solvation of the NH sites not directly involved in salt bridge formation, which become significantly more polar when proton transfer takes place, leading to more favorable interactions with polar solvents in the bound state. Calculation of H-bond parameters using density functional theory was used to predict solvent effects on the stabilities of salt bridges to within 1 kJ mol-1. While H-bonding interactions are strong in nonpolar solvents, and solvophobic interactions are strong in polar protic solvents, these interactions are weak in polar aprotic solvents. In contrast, amidinium-carboxylate salt bridges are stable in both polar and nonpolar aprotic solvents, which is attractive for the design of supramolecular systems that operate in different solvent environments.

A Modular and Convergent Synthetic Route to Supramolecular Cyclic Dimers Based on Amidinium-Carboxylate Interactions

We describe herein the optimized design and modular synthetic approach towards supramolecularly programmed monomers that can form discrete macrocyclic species of controllable size and shape through amidinium-carboxylate interactions in apolar and polar media.

Amidinium–carboxylate salt bridge mediated proton-coupled electron transfer in a donor–acceptor supramolecular system

A supramolecular polymer that allows for intrapolymer proton-coupled photoinduced electron transfer was constructed by means of amidinium-carboxylate salt bridges.

Polyamide-Polyamine Cryptand as Dicarboxylate Receptor: Dianion Binding Studies in the Solid State, in Solution, and in the Gas Phase

Polyamide-polyamine hybrid macrobicycle L is explored with respect to its ability to bind α,ω-dicarboxylate anions. Potentiometric studies of protonated L with the series of dianions from succinate (suc^2-) through glutarate (glu^2-), α-ketoglutarate (kglu^2-), adipate (adi^2-), pimelate (pim^2-), suberate (sub^2-), to azelate (aze^2-) have shown adipate preference with association constant value of K = 4900 M^-1 in a H₂O/DMSO (50:50 v/v) binary solvent mixture. The binding constant increases from glu^2- to adi^2- and then continuously decreases with the length of the anion chain. Further, potentiometric studies suggest that hydrogen bonding between the guest anions and the amide/ammonium protons of the receptor also contributes to the stability of the associations along with electrostatic interactions. Negative-mode electrospray ionization of aqueous solutions of host-guest complexes shows clear evidence for the selective formation of 1:1 complexes. Single-crystal X-ray structures of complexes of the receptor with glutaric acid, α-ketoglutaric acid, adipic acid, pimelic acid, suberic acid, and azelaic acid assist to understand the observed binding preferences. The solid-state structures reveal a size/shape complementarity between the host and the dicarboxylate anions, which is nicely reflected in the solution state binding studies.

Selective fluorescence sensing of salicylic acids using a simple pyrenesulfonamide receptor

The highest change in relative intensity of probe 3 among the probes 2–5 with 3,5-DNSA demonstrated the significance of sulfonamide N–H and imidazole C2–H with the highest association constant and complete quenching.

Four-component assembly in the crystalline state driven by amidinium-carboxylate salt bridge formation from an aqueous solution

A series of diamidine dihydrochlorides was prepared utilizing a spacer unit to control the distance between the two amidinium groups. The introduction of two amidinium groups to the 1,8-position of each spacer unit (i.e., 9,10-dihydroanthracene, anthracene, biphenylene) can control the direction of formation of a self-assembled structure. The fine-tuning of the distances between the two amidinium groups in the spacer units can help control the stabilizing interactions of two carboxylic acid units (intermolecular attraction) after the four-component assembly (see ). Based on this concept, we succeeded in the formation of a four-component box-like assembled structure using amidinium-carboxylate salt bridge formation in the crystalline state from aqueous solutions.