UV–Visible properties of oxicams in solution: A TD‐DFT and experimental study

Abstract

Microspecies involved in prototropic equilibria of isoxicam, piroxicam, meloxicam, tenoxicam, and lornoxicam are characterized by time-dependent density functional theory (TD-DFT) and UV-vis spectrophotometry, their relative Gibbs free energies in solution and Maxwell-Boltzmann populations are theoretically calculated. TD-DFT calculations of the most intense transition energies for the microspecies involved in equilibria are compared with those obtained in solvents of different dielectric constants (1,4-dioxane, ethanol, water, and formamide) and at various pH conditions. Our TD-DFT results allow establishing the predominant oxicam microspecies at the experimental conditions. Geometry optimizations are performed at level of theory PCM-PBE0/ 6-31þG(d, p), and transition energies are computed using the protocol PCM-PBE0/6-311þþG(d, p).

SERS of the anti‐inflammatory drug piroxicam adsorbed on the surface of silver or gold colloids as nanocarrier model

“…Our results compare quite well with the obtained using UV-Vis data [27] where zwitterionic form of PX is reported the predominant form in formamide solution (similar to our DMSO) while the enol closed-PX is predominant in 1,4-dioxane and concentrated ethanol.…”

“…. While in 1,4‐dioxan, there is a single band ( λ max = 326 nm), which corresponds to the enol closed‐form of PX, a prominent shoulder around 378 nm appears in the DMSO solution which, although with lower intensity, is also noticed in the alcohols and corresponds to the theoretical prediction for the zwitterionic form of PX . Having the possibility of using laser excitation at 442 nm, we tried to acquire its (pre)resonance Raman spectra, which are given in Fig.…”

“…PX exhibits low water solubility that rules out the Raman study of PX in water. Besides, the extreme sensitivity of PX to its microenvironments [17] recommends to use solvents with different proticity and/or polarity similarly to studies of PX in solution employing UV-Vis spectroscopy, [27] aiming also to compare the results obtained with the two molecular spectroscopic techniques. In this sense and being DMSO, a well-known polar and non-protic 'practically universal' solvent, we studied PX in pure DMSO (1 × 10 À1 M), and in X:DMSO mixtures (X = 1,4-dioxane (non-protic), ethanol and 1-propanol, both alcohols as weak polar; 1 × 10 À2 M), being in all mixtures concentration of DMSO less than or equal to 10% v/v (see experimental section).…”

“…Previous studies of PX in solution have employed ultraviolet‐visible (UV‐Vis), fluorescence, or nuclear magnetic resonance spectroscopies, while Raman spectroscopy proved to be extremely useful for characterizing the different polymorphs of PX in solid state . Considering the large applicability of Raman and SERS spectroscopies in the pharmaceutical and biomedical fields, we have carried out the molecular characterization (Raman) of PX in ‘concentrated’ (10 −2 m ) solutions in different solvents and using different laser excitation, as the first part of the present paper.…”

“…[24][25][26] In this paper, we will present and discuss the Raman spectra of PX in different organic solvents and the SERS results obtained for PX in silver and gold (aqueous) colloids and at different pH conditions. Previous studies of PX in solution have employed ultraviolet-visible (UV-Vis), [17,27] fluorescence, [22,28] or nuclear magnetic resonance [29] spectroscopies, while Raman spectroscopy proved to be extremely useful for characterizing the different polymorphs of PX in solid state. [30,31] Considering the large applicability of Raman and SERS spectroscopies in the pharmaceutical and biomedical fields, [32][33][34][35][36][37] we have carried out the molecular characterization (Raman) of PX in 'concentrated' (10 À2 M) solutions in different solvents and using different laser excitation, as the first part of the present paper.…”