title: A thermodynamic study of colchicine and colcemid dimerization.
abstract: The dimerization of colchicine is demonstrated using the technique of concentration difference spectra. The difference spectra are characterized by an isosbestic point at 372 nm, a positive peak at 387 nm, and two negative peaks at 360 and 330 nm. The study of the concentration dependence and the effect of temperature allowed the determination of the molar extinction change and the dimerization equilibrium constant at four temperatures. The van't Hoff plot is linear, and the following thermodynamic parameters are calculated: standard enthalpy change, delta Ho/kJ . mol-1 = -31.0; and standard entropy change, delta So/J . mol-1 . K-1 = -69.5. For colcemid, similar results are obtained. The difference spectra show an isosbestic point at 385 nm and a positive peak at 397 nm. The thermodynamic parameters obtained are delta Ho/kJ . mol-1 = -26.8, and delta So/J . mol-1 . K-1 = -61.1. These thermodynamic parameters are comparable to the values obtained for the dimerization of polar dyes and the stacking of nucleotide bases.
mesh_terms: D055598:Chemical Phenomena; D002621:Chemistry; D003078:Colchicine; D004396:Coloring Agents; D003703:Demecolcine; D013329:Structure-Activity Relationship; D013816:Thermodynamics
chemical_list: D004396:Coloring Agents; D003078:Colchicine; D003703:Demecolcine