Structural stabilities and transformation mechanism of rhynchophylline and isorhynchophylline in aqueous and methanol solution based on high-performance liquid chromatography and density functional theory

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Uncaria rhynchophylla, also known as Gou-teng, is one of the most popular Chinese herbs. Its active components are indole alkaloids, mainly rhynchophylline (RIN) and isorhynchophylline (IRN). IRN and RIN convert to each other faster in aqueous solution than in methanol solution, and the former is more stable in methanol solution than the latter by HPLC. The ratio of IRN and RIN is close to 57:39 in aqueous solution and 69:25 in methanol solution. The open loop energy barrier of IRN is higher than that of RIN, and the gaps are 7.16, 10.95, 4.59, and 3.75 kJ/mol in methanol solution and 0.85, 0.44, 2.37, and 0.59 kJ/mol in aqueous solution using the M062X/6-311+g(d,p), M062X/6-31g(d), B3lyp/6-31+g(d,p), and B3lyp/6-31g(d) methods, respectively. The method of theoretical calculation about M062X/6-31g(d) is the closest to the experimental results. The gaps of RIN and IRN in methanol solution are 25.47 and 24.81 kJ/mol lower than those in aqueous solution, respectively. RIN and IRN have a lower solubility in aqueous solution using the M05-2X method. Moreover, the gap of highest occupied molecular orbital–lowest unoccupied molecular orbital has a certain positive correlation with the molecular open loop barrier, which verifies the correctness of the calculated results. In the RIN and IRN molecules, the highest surface electrostatic potential (ESP) is around N11, and the values are 53.54 and 53.22 kcal/mol, respectively. The lowest ESP of RIN is near N17, whereas that of IRN is near O14 with values of -69.95 and -56.22 kcal/mol, respectively.