Study on the molecular structure and thermal stability of purine nucleoside analogs

• The thermal decompositions of eight purine nucleoside analogs were studied with thermal analytical methods.
• The residues of thermal decompositions were studied with the IR spectra and HPLC methods.
• The molecular bond orders were calculated with an ab initio method of quantum chemistry.
• The mechanisms of the thermal decomposition of purine nucleoside analogs have been proposed.
• The relationship between the molecular structure and the thermal stability has been discussed.

The thermal decomposition processes of eight purine nucleoside analogs were measured with thermogravimetry (TG) and differential scanning calorimetry (DSC). The IR spectra and high-performance liquid chromatography (HPLC) of purine nucleoside analogs and their residues from thermal decomposition at various temperatures were determined. The molecular bond orders were calculated with an ab initio method from the GAMESS program. The mechanisms of thermal decomposition for purine nucleoside analogs were discussed. The results indicate that the main decomposition pathway of purine nucleoside analogs is the fracture of the N-glycosidic bond first, and the residues of the initial stage of decomposition are mainly corresponding purines. A part of purine nucleoside analogs decomposes directly to small molecules and insoluble substances at the first stage. When the N-glycosidic bond order is close to that of the weakest bond within purine ring, most of purine nucleoside analogs will decompose directly. The amino group at purine ring is easy to be oxidized, some oxidation products can be found in the residues of some purine nucleoside analogs. There is a positive correlation between the calculated weakest bond orders and the decomposition temperatures of purine nucleoside analogs. The stronger the weakest bond order, the higher the decomposition temperature. The molecular bond orders can serve as the basis to judge molecular thermal stability for analog compounds with similar molecular structure, size, and energy. The substituent group affects the thermal stability of the purine nucleoside analogs. Increasing the number of electron-donating groups on the purine ring and furanose ring will enhance the N-glycosidic bond, will increase the temperature of thermal decomposition, and be likely to change the mechanism of thermal decomposition.