Calculation of pKa values of carboxylic acids: Application to bilirubin

Reversed-phase dispersive liquid-liquid microextraction based on decomposition of deep eutectic solvent for the determination of lead and cadmium in vegetable oil

In this work, a reversed-phase dispersive liquid-liquid microextraction procedure based on the decomposition of deep eutectic solvent was suggested for the first time. The procedure was utilized for fast and simple separation of lead and cadmium from vegetable oil samples. The procedure assumed mixing of oil sample and DES based on menthol, formic acid and water. Water as component of DES promoted its decomposition in sample matrix resulting menthol dissolution in the sample phase and dispersion of aqueous formic acid solution. In this procedure menthol acted as a dispersive solvent during DES decomposition for dispersion of aqueous formic acid solution. The metals were determined by the square-wave anodic stripping voltammetry. The limits of detection, were 0.01 µg kg^-1 for lead and 0.006 µg kg^-1 for cadmium. The RSD was less then 6% for both analytes. The enrichment factor was 36 and 39 for lead and cadmium, respectively.

Hydrogen-Deuterium Exchange in Basic Near-Critical and Supercritical Media: An Experimental and Theoretical Study

Treatment of homo- and heterocyclic aromatic substrates with basic deuterium oxide under near- or supercritical conditions results in rapid base-catalyzed hydrogen-deuterium exchange (HDE) in aromatic and benzylic positions. It has been postulated that HDE follows a simple deprotonation-reprotonation mechanism, but little evidence has been provided to date. This study correlates experimentally observed proton exchanges in n-butylbenzene with ab initio calculations of the acidities and potential energy (PE) profiles. In addition to providing further support for carbanion intermediacy in HDE, these results offer new insights into substrate acidities in near- and supercritical aqueous media and the optimal conditions required for their isotope exchange.

Computational chemical analysis of unconjugated bilirubin anions and insights into pKa values clarification

The pKa, the negative logarithm of the acid dissociation equilibrium constant, of the carboxylic acid groups of unconjugated bilirubin in water is a discussed issue because there are quite different experimental values reported. Using quantum mechanical calculations we have studied the conformational behavior of unconjugated bilirubin species (in gas phase and in solution modeled implicitly and explicitly) to provide evidence that may clarify pKa values because of its pathophysiological relevance. Our results show that rotation of carboxylate group, which is not restricted, settles it in a suitable place to establish stronger interactions that stabilizes the monoanion and the dianion to be properly solvated, demonstrating that the rationalization used to justify the high pKa values of unconjugated bilirubin is inappropriate. Furthermore, low unconjugated bilirubin (UCB) pKa values were estimated from a linear regression analysis.

First principles pKa calculations on carboxylic acids using the SMD solvation model: effect of thermodynamic cycle, model chemistry, and explicit solvent molecules

Aqueous pK(a) values are calculated from first principles for a set of carboxylic acids using the SMD solvation model with various model chemistries, thermodynamic cycles, and treatments of explicit solvation. In all, 108 unique theoretical protocols are examined. The direct (D) and water proton exchange (PX) cycles are trialled along with a new approach, termed the semidirect (SD) cycle. The SD thermodynamic cycle offers some improvements over the D and PX schemes, as it bypasses the gas-phase heterolytic bond dissociation calculation required in the conventional D approach while also avoiding an aqueous OH(-) calculation required by the PX method when using water as the reference acid. With all three cycles, the recommended model chemistry employs M05-2X/cc-pVTZ Gibbs energies of solvation with a single discrete water molecule and a high-level composite method for the gas-phase reaction energies. With the SD cycle, these calculations result in a mean unsigned error of less than 1 pK(a) units, with respective mean signed error and maximum unsigned error of less than 0.5 and 2 pK(a) units. Similar results are obtained with the D and PX cycles, and further improvement is required in both the gas and aqueous phase ab initio energy calculations before we can truly discriminate between the thermodynamic cycles investigated here.

Intrinsic proton-donating power of zinc-bound water in a carbonic anhydrase active site model estimated by NMR

Using liquid-state NMR spectroscopy we have estimated the proton-donating ability of Zn-bound water in organometallic complexes designed as models for the active site of the metalloenzyme carbonic anhydrase (CA). This ability is important for the understanding of the enzyme reaction mechanism. The desired information was obtained by (1)H and (15)N NMR at 180 K of solutions of [Tp(Ph,Me)ZnOH] [1, Tp(Ph,Me) = tris(2-methyl-4-phenylpyrazolyl)hydroborate] in CD(2)Cl(2), in the absence and presence of the proton donors (C(6)F(5))(3)BOH(2) [aquatris(pentafluorophenyl)boron] and Col-H(+) (2,4,6-trimethylpyridine-H(+)). Col-H(+) forms a strong OHN hydrogen bond with 1, where the proton is located closer to nitrogen than to oxygen. (C(6)F(5))(3)BOH(2), which exhibits a pK(a) value of 1 in water, also forms a strong hydrogen bond with 1, where the proton is shifted slightly across the hydrogen-bond center toward the Zn-bound oxygen. Finally, a complex between Col and (C(6)F(5))(3)BOH(2) was identified, exhibiting a zwitterionic OHN hydrogen bond, where H is entirely shifted to nitrogen. The comparison with complexes of Col with carboxylic acids studied previously suggests that, surprisingly, the Zn-bound water exhibits in an aprotic environment a similar proton-donating ability as a carboxylic acid characterized in water by a pK(a) of 2.2 ± 0.6. This value is much smaller than the value of 9 found for [Zn(OH(2))(6)](2+) in water and those between 5 and 8 reported for different forms of CA. Implications for the biological function of CA are discussed.