Proton transfer between sulfonic acids and various propylamines by density functional theory calculations.

CONTEXT: Proton transfer in acid-base systems is not well understood. Some acid-base reactions do not proceed to the extent that is expected from the difference in the pKa values between the base and acid in aqueous solutions, yet some do. In that regard, we have computationally studied the process of proton transfer from the acids of varying strength (benzenesulfonic acid (BSu), methansulfonic acid (MsO), and sulfuric acid (SA)) to the amines with different numbers of propyl substituents on the nitrogen atom (propylamine (PrA), dipropylamine (DPrA), and tripropylamine (TPrA)) upon complexation. Density functional theory calculations were used to thoroughly examine the energetic and structural aspects of the molecular complexes and/or ionic pairs resulting from the acid-base interaction. The potential energy curves along the proton transfer coordinate in these acid-amine systems were analyzed. The change in free energies accompanying the molecular complexes and ionic pair formations was calculated, and the relationship between the energy values and the ΔРА parameter (difference in proton affinity of the acid anion and amine) was established. The larger ΔРА values were found to be unfavorable for the formation of ionic pairs. Using structural, energy, QTAIM, and NBO analyses, we determined that the hydrogen bonds in the molecular complexes PrA-MsO and PrA-BSu are stronger than those in their corresponding ionic pairs. The ionic pairs with the TPrA cation possess the strongest hydrogen bonds of all the ionic pairs being studied, regardless of the anion. The results showed that hydrogen bonding interactions in the molecular complexes contribute significantly to the energies of the acid-base interaction, while in the ionic pairs, the most important energy contribution comes from Coulomb interactions, followed by hydrogen bonding and dispersion forces. The ionic pairs with propylammonium, dipropylammonium, and tripropylammonium cations have stronger ion-ion interactions than tetrapropylammonium (TetPrA)-containing ionic pairs with the same anions. This effect rises with the order of the cation: TetPrA → TPrA → DPrA → PrA, and the sequence of anions is SA → BSu → MsO. The results obtained here expand the concept of acid-base interaction and provide an alternative to experimental searches for suitable acids and bases to obtain new types of protic ionic liquids. METHODS: All quantum-chemical calculations were carried out by using the DFT/B3LYP-GD3/6-31++G(d,p) level as implemented in the Gaussian 09 software package. For the resulting structures, the electron density distribution was analyzed by the "atoms in molecules" (QTAIM) and the natural bond orbital (NBO) methods on the wave functions obtained at the same level of theory by AIMAll Version 10.05.04 and Gaussian NBO Version 3.1 programs, respectively.

Discovery of N-Aryloxypropylbenzylamines as Voltage-Gated Sodium Channel NaV 1.2-Subtype-Selective Inhibitors.

We previously reported that a lipophilic N-(4'-hydroxy-3',5'-di-tert-butylbenzyl) derivative (1) of the voltage-gated sodium channel blocker mexiletine, was a more potent sodium channel blocker in vitro and in vivo. We demonstrate that replacing the chiral methylethylene linker between the amine and di-tert-butylphenol with an achiral 1,3-propylene linker (to give (2)) maintains potency in vitro. We synthesized 25 analogues bearing the 1,3-propylene linker and found that minor structural changes resulted in pronounced changes in state dependence of blocking human NaV 1.2 and 1.6 channels by high-throughput patch-clamp analysis. Compared to mexiletine, compounds 1 and 2 are highly selective NaV 1.2 inhibitors and >500 times less potent in inhibiting NaV 1.6 channels. On the other hand, a derivative (compound 4) bearing 2,6-dimethoxy groups in place of the 2,6-dimethyl groups found in mexiletine was found to be the most potent inhibitor, but is nonselective against both channels in the tonic, frequency-dependent and inactivated states. In a kindled mouse model of refractory epilepsy, compound 2 inhibited seizures induced by 6 Hz 44 mA electrical stimulation with an IC50 value of 49.9±1.6 mg kg-1 . As established sodium channel blockers do not suppress seizures in this mouse model, this indicates that 2 could be a promising candidate for treating pharmaco-resistant epilepsy.