Effect of pH and $\beta$-cyclodextrin on the photophysical properties of lamotrigine

Özet

Öz Photophysical properties of lamotrigine (LMT) were investigated at different pH values. Lamotrigine exhibited a broad absorption band at 308 nm that was shifted to 268 nm at pH 2.0 due to protonation (HLMT$+)$. Emission spectra of LMT at different pH values showed a single band at 483 nm except for at pH 2.0, where a second band at 502 nm was observed due to the deprotonation from the singlet excited state. LMT pH profile was used to calculate p$K$a of LMT in excited and ground states (p$K$a* $=$ 4.50 and p$K$a $=$ 5.90). Absorption spectra of LMT at different pH values for LMT in the presence of $\beta$-CD showed an isosbestic point at 290 nm. A significant enhancement in the emission intensity of LMT upon increasing $\beta$-CD (0-7 mmol L$^{-1})$ was reported. Benesi--Hildebrand analysis showed that LMT/$\beta$-CD and HLMT$+$/$\beta$-CD complexes have 1:1 stoichiometry, with K11 values ranging from 122 at pH 2.0 to 50 at pH 8.0. Thermodynamic parameters showed that complexation process is enthalpy driven ($\Delta $H$^{\circ} = -$23.1 kJ.mol$^{-1}$ at pH 2.0 and -24.1 kJ.mol$^{-1}$ at pH 8.0). Molecular mechanical calculations by MM$+$ force field indicated that LMT is preferentially included within the $\beta$-CD cavity through its diamino-1,2,4-triazine moiety.