An experimental and theoretical study of the structure of Lamotrigine in its neutral and protonated forms: evidence of Lamotrigine enantiomers

Theoretical and Spectroscopic Characterization of API-Related Azoles in Solution and in Solid State

Azoles are a family of five-membered azacyclic compounds with relevant biological and pharmacological activity. Different subclasses of azoles are defined depending on the atomic arrangement and the number of nitrogen atoms present in the ring: pyrazoles, indazoles, imidazoles, benzimidazoles, triazoles, benzotriazoles, tetrazoles and pentazoles. The complete characterization of their structure and the knowledge about their crystal packing and physical and chemical properties are of vital importance for the advancement in the design of new azole-containing drugs. In this review, we report the latest recent contributions to azole chemistry, in particular, those in which theoretical studies have been performed.

Structure and electronic spectra of neutral and protonated forms of anticonvulsant drug lamotrigine

In this work, the geometry, acid-base properties, pK_a, electronic spectra, and fluorescence spectrum of anticonvulsant drug lamotrigine (LTG) are investigated with the DFT/TD-DFT method and PCM solvent model. Calculated transition with the B3LYP functional at 295 nm corresponds to experimental absorption transition at 306 nm in water. In acidic conditions, the computed maximum transition occurs at 249 nm, comparing with experimental one at 270 nm. The dependence of calculated transitions on density functional used and different solvents in PCM model was studied. The computed transition of fluorescence is at 435 nm, while experimental occurs at 370 nm. Maps of electrostatic potential (MEPs) for S₀ and S₁ reveal that the ground state of LTG is more polar than the first excited state. Structurally, in the excited state of LTG, the triazine ring is noticeably distorted. Graphical Abstract Molecular elecrostatic potentials for S₀ and S₁ states of the lamotrigine molecule.

A vibrational circular dichroism (VCD) methodology for the measurement of enantiomeric excess in chiral compounds in the solid phase and for the complementary use of NMR and VCD techniques in solution: the camphor case

For the first time, the success of a methodology for the determination of enantiomeric excess (% ee) in chiral solid samples by vibrational circular dichroism (VCD) spectroscopy is reported. We have used camphor to determine the % ee in a blind sample constituted by a mixture of its two enantiomers as a test for the validity of our approach. IR and VCD spectra of different enantiomeric mixtures of R/S-camphor in Nujol mulls were recorded and linear regressions of VCD intensities (ΔAbs.) vs. % ee for selected bands were found. Finally, the VCD intensities of a blind sample were interpolated in these linear regressions, obtaining its % ee with a rms of 2.4. These results in the solid phase were complemented with the determination of % ee in the liquid phase by VCD and NMR techniques, which are proved to be complementary techniques to carry out this kind of analysis. In the same way as in the VCD solid phase, linear regressions of ΔAbs. vs. % ee for selected bands were established, obtaining a rms of 1.1 in the % ee determination of a blind sample. ¹H NMR experiments at 600 MHz using the chiral solvating agent, (S,S)-ABTE, allow the determination of the proportions of enantiomers in CD₂Cl₂ solution with great accuracy. ¹³C CPMAS NMR spectra prove that this technique cannot be used for conglomerates and/or solid solutions.

A theoretical NMR study of selected benzazoles: Comparison of GIPAW and GIAO-PCM (DMSO) calculations

This paper compares the absolute shieldings obtained by gauge-including-projected-augmented-wave (GIPAW) to those obtained by gauge-invariant atomic orbital/Becke, 3-parameter, Lee-Yang-Parr (GIAO/B3LYP)/6-311++G(d,p)-polarizable continuum model (PCM, dimethyl sulfoxide) for nine benzazoles (benzimidazoles, indazoles, and benzotriazoles) recorded in the solid-state. Three nuclei were explored, ¹³ C, ¹⁵ N, and ¹⁹ F, and the gauge-including-projected-augmented-wave approach only proved better for ¹⁵ N MAS NMR.

Abiotic amidine and guanidine hydrolysis of lamotrigine-N2-glucuronide and related compounds in wastewater: The role of pH and N2-substitution on reaction kinetics

The stability of lamotrigine (LMG) and its principal human metabolite, lamotrigine N2-glucuronide (LMG-N2-G), was studied as a function of pH (4-9). While LMG was stable across the entire pH range, under neutral-basic conditions, LMG-N2-G was converted to three transformation products (TPs) which were identified using high resolution mass spectrometry (HRMS). The MS fragmentation studies indicated that two TPs were the result of the hydrolysis of the amidine and guanidine moieties. The third TP detected was an intermediate in the guanidine hydrolysis reaction. In order to evaluate the transformation kinetics of the LMG-N2-G degradation, another set of pH-dependent experiments was carried out in hospital effluent, wastewater influent and effluent spiked at 20 and 200 nM after pH adjustment (pH 6.5, 7, 8, 8.5 and 9), demonstrating that, at higher pH, LMG-N2-G is degraded at higher rate. Later, the pH-dependent stability of related compounds with different nitrogen N2-substituents (N2-R) on the 1,2,4-triazine ring was studied. This revealed that because of different imino tautomer equilibrium LMG (N2-H) and LMG-N2-oxide ((+)N2-O(-)) were stable at all pHs but N2-methyl-LMG (N2-CH3) as well as LMG-N2-G were susceptible to amidine and guanidine hydrolysis at basic pH. Finally, hospital effluent samples collected over the course of one week were monitored for their presence. LMG, LMG-N2-G and two of its TPs were detected with concentrations ranging between 0.01 and 1 μgL(-1).

A theoretical and experimental study of the NMR spectra of 4,5,6,7-tetrafluorobenzazoles with special stress on PCM calculations of chemical shifts

The chemical shifts and several (19)F-(19)F, (13)C-(19) F and (1)H-(19)F spin-spin coupling constants (SSCSs) of eight 4,5,6,7-tetraflurobenzazoles (three benzimidazoles, three benzimidazolinones and two indazoles) have been determined. The chemical shifts were discussed using gauge including atomic orbital-density functional theory calculations taking into account solvent effects (polarizable continuum model) and, for the solid state, hydrogen bonds (clusters up to three molecules).