咖啡因、茶碱和氨茶碱的低温热容和热分析

咖啡因、茶碱和氨茶碱是临床应用中广泛使用的三种重要的甲基化的黄嘌呤.采用绝热量热、热重分析(TG)和差示扫描量热法(DSC)研究了这三种药物的热力学性质.采用绝热量热法测定了β型-咖啡因、茶碱和氨茶碱在80-370K温度范围内的摩尔热容值,结果显示氨茶碱的摩尔热容值最大,茶碱的摩尔热容值最小.采用最小二乘法对这三种药物热容的测量值和温度进行了拟合,得到了热容与折合温度的多项式,计算了咖啡因、茶碱和氨茶碱在298.15K时的热容分别为226.49、218.13和554.78J·K-1·mol-1;并计算了它们相对于298.15K时的焓和熵.采用热综合分析仪对这三种药物的热稳定性进行了评价,结果表明它们的热稳定顺序为氨茶碱咖啡因茶碱.通过DSC分析,得到了咖啡因和茶碱的相转变温度、相转变焓和熵.基于密度泛函理论的第一性原理,计算了咖啡因和茶碱分子的结构稳定性,结果显示咖啡因分子的稳定性低于茶碱,与实验结果吻合.

Low Temperature Heat Capacity and Thermal Analysis of Caffeine,Theophylline and Aminophylline

Caffeine, theophylline, and aminophylline are important methyl-substituted xanthines and are widely used in clinics. In this work, the thermodynamic characteristics of the three drugs were studied by adiabatic calorimetry, thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). The low temperature molar heat capacities of caffeine (in the β crystal form), theophylline and aminophylline were measured by heating the system from 80 to 370 K using an adiabatic calorimeter. The results indicate that the molar heat capacity of aminophylline is the largest while that of theophylline is the smallest. The experimental molar heat capacities of the three drugs were fitted to a polynomial of C_{p, m} vs the reduced temperature (t) by means of the least fitting square method from 80 to 370 K. Their molar heat capacities at 298.15 K were calculated to be 226.49 J·K^-1·mol^-1 (for caffeine), 218.13 J·K^-1·mol^-1 (for theophylline), and 554.78 J·K^-1·mol^-1 (for aminophylline) using the polynomial C_{p, m}-t. Thermodynamic parameters (such as enthalpies and entropies relative to 298.15 K) were calculated for these drugs based on the polynomial C_{p, m}-t. The results of thermal analysis show that the order of thermal stability for these drugs is aminophylline＜caffeine＜theophylline. The temperatures, enthalpies and entropies of the phase transitions for caffeine and theophylline were obtained by DSC. The stabilities of the molecular structures for caffeine and theophylline were calculated by a first-principles calculation based on density functional theory. The results imply that the stability of the caffeine molecule is lower than that of theophylline and this is in good agreement with the experimental results.