Kinetics of photooxidation of papaverine hydrochloride and its major photooxidation products.

HPCE methodology was modified to be used in kinetic experiments on photooxidation reactions of papaverine hydrochloride 1 and its oxidation products (papaverinol 2 and papaveraldine hydrochloride 3) chloroform solutions exposed to UV254 light in aerobic conditions. On photooxygenation of the above compounds is formed the final degradation product, a brown compound X 4, 2,3,9,10-tetramethoxy-12-oxo-12H-indolo[2,1-a]isoquinolinylim chloride. The rate of 4 formation from the above compounds can be given as 2>3>1>1 HCl. The photooxidation reactions of 1 and 2 proceed with pseudo first-order kinetics and that reaction for 3 follows zero-order kinetics. The most labile compound is 2 whose half-life time is 2.4 times shorter than that one of 1 HCL. The most stable product is 3 whose half-life time is 31-fold longer than of 2. The specific quantum yields are equal 0.28, 0.30 and 0.10 for 1 HCl, 2 and 3, respectively which confirm the above stability pattern of the compounds concerned.

Photoprotection of 1,4-dihydropyridine derivatives by dyes.

The possibility of increasing the photochemical stability of nisoldipine by using indigotine and azorubine as photoprotectors has been studied. The course of the photodegradation was monitored by means of UV-vis spectrophotometry and HPLC. Quantitative assessments of the nisoldipine photodegradation included evaluation of the quantum yields and kinetic parameters. In order to establish the light intensity absorbed by a system, Reinecke salt was used as a chemical actinometer. The values of the quantum yields (phi) of photodegradation decreased with increasing dye concentration and were 0.24-0.15 and 0.27-0.25 for indigotine and azorubine, respectively. Furthermore, our attention was focused on determination the role of the selected dyes during the photodegradation of nisoldipine and the calculations were made to eliminate an inner filter effect. The values obtained were used to construct a Stern-Volmer plot and calculate the Stern-Volmer constants (KSV). On the basis of the KSV and the values of the quenching constant (kq) the exited state lifetime (tauo) of nisoldipine in the presence of indigotine and azorubine were estimated. The calculated values of tauo for NS were 6.66x10(-6)s--in the presence of indigotine and 2.76x10(-6)s--in the presence of azorubine, indicating that the photodegradation of nisoldipine occurs from the lowest triplet excited state.

Photooxidation of papaverine, papaverinol and papaveraldine in their chloroform solutions.

Papaverine hydrochloride, papaverinol, and papaveraldine chloroform solutions were exposed to UV light of 254 nm in atmospheric, aerobic and anaerobic (helium) conditions. The same degradation products appear (TLC) in the above papaverine hydrochloride chloroform solutions. However, the rate of papaverine hydrochloride degradation processes is enhanced as a function of oxygen pressure. Papaverinol and papaveraldine photooxidation products are essentially not different from those observed in the above papaverine hydrochloride solutions. However, the amount of an unknown brown degradation product (X) is the greatest in the papaverinol chloroform solution degraded. That brown compound was previously observed in papaverine either hydrochloride or sulfate injection solutions on their storage even when protected from daylight. The preliminary X product structure development was undertaken (TLC, molecular weight, elemental analysis, UV/VIS, IR and 13C MAS NMR spectroscopy).