Acid-base properties of terbutaline in terms of protonation macro- and microconstants

Biorelevant physicochemical profiling of (E)- and (Z)-resveratrol determined from isomeric mixtures

Biorelevant, isomer-specific physicochemical parameters of resveratrol, a multifunctional component in red wines, with cardioprotective, anti-Alzheimer and several other pharmacologic activities were determined. The parameters include site-specific basicities, lipophilicities, solubilities and diffusion constants for the two geometric isomers. The protonation equilibria of (E)- and (Z)-resveratrol were monitored by ¹H NMR-pH titrations. Five closely related auxiliary compounds ((E)-pinostilbene, (Z)-pinostilbene, (E)-pterostilbene, (Z)-pterostilbene and resorcinol) were also studied. Combining the datasets, the group-specific protonation constants of resveratrol isomers were determined. The results show that (Z)-resveratrol is more basic at every protonation site than the (E)-isomer. Lipophilicities are quantified in terms of logP values and were determined by octanol/water partition experiments and quantitative NMR spectroscopy: (E)-resveratrol was found to be more lipophilic. Since the molecular geometries of the isomers differ, diffusion ordered NMR spectroscopy (DOSY) experiments were also carried out to quantify the diffusion capabilities of the isomers: (Z)-resveratrol of bent shape has a slightly higher diffusion coefficient than its extended (E) counterpart. A striking 10-fold difference of water solubility was found in favor of the (Z) isomer, due obviously to the reduced water-repellent character in the more compact molecule. This is so far the greatest recorded solubility difference between geometric isomers of any compounds.

α-Bulnesene, a PAF inhibitor isolated from the essential oil of Pogostemon cablin

Steam distilled essential oil from the aerial parts of Pogostemon cablin was analyzed by gas chromatography-mass spectrometry (GC-MS). Forty one compounds were identified of which alpha-guaiene (20.62%) and alpha-bulnesene (16.18%) were major constituents. Furthermore, fractionation of the essential oil from P. cablin guided by inhibitory activity against PAF-induced platelet aggregation led to the isolation of the sesquiterpene, alpha-bulnesene.

Multiwavelength spectrophotometric determination of acid dissociation constants part V: microconstants and tautomeric ratios of diprotic amphoteric drugs

The acid-base equilibria of several diprotic amphoteric drugs, namely, niflumic acid, norfloxacin, piroxicam, pyridoxine and 2-methyl-4-oxo-3H-quinazoline-3-acetic acid have been characterized in terms of microconstants and tautomeric ratios. A multiwavelength spectrophotometric (WApH) titration method for determination of acid dissociation constants (pKa values) of ionizable compounds developed previously was applied for this purpose. Microspeciation was investigated by three approaches: (1) selective monitoring of ionizable group by spectrophotometry, (2) deductive method and (3) k(z) method for determination of tautomeric ratio from co-solvent mixtures. The formulation for (3) has been derived and found to invoke fewer assumptions than a reported procedure (K. Takács-Novák, A. Avdeef, K.J Box, B. Podányi, G. Szász, J. Pharm. Biomed. Anal., 12 (1994) 1369-1377). It has been shown that the WApH technique, for such types of ampholytes, is able to deduce the microconstants and tautomeric ratios which are in good agreement with literature data.

Multiwavelength spectrophotometric determination of acid dissociation constants of ionizable drugs

A multiwavelength spectrophotometric approach has been developed to determine acid dissociation constants (pKa values) of sparingly soluble drug compounds. UV absorption spectra of the drug solution are acquired using a versatile device based on a fiber optics dip probe, a light source and a photodiode array (PDA) detector while the PH and the ionic strength of the chemical system is manipulated precisely by means of a commercially available titrator. Target factor analysis (TFA) has been applied to deduce the pKa values from the multiwavelength UV absorption data recorded at different pH values. We have called this multiwavelength approach the WApH technique because the pKa results are determined from changes in Wavelength and Absorbance as a function of pH (WApH). The WApH technique is exemplified by using several pure drugs, namely, niflumic acid, nitrazepam, pyridoxine, quinine and terbutaline. The pKa values obtained agree well with those derived from pH-metric titrations. It has been demonstrated that the WApH technique is able to deduce pKa values with high accuracy even if the absorption spectra of the reacting species are very similar.