Sesquiterpene lactones isolated from a medicinal plant of the Venezuelan Andes

Abstract

Most of nucleoside 5'-monophosphates have been extensively investigated as regards their structure for long, but potassium salts of them have never been studied by the use of X-ray crystallography. The results of structural investigations on five mono-and dipotassium salts of two purine [K2(AMP) . 6H2O, K(IMP) . 3H2O] and two pyrimidine [K2(CMP) . 6.25H2O, K(UMP), K2(UMP) . 5H2O] ribonucleotides will be presented. The K + salts crystallize with Z' = 1-4 for the respective crystal. In all the mono-/dianions, the orientation of the base with respect to the sugar ring is anti, which is typical for nucleotides. The ribose rings in AMP and UMP salts exist in ideal envelope conformations: 2 E in two dianions in K2(AMP) . 6H2O and two dianions in K2(UMP) . 5H2O, and 3 E in the monoanion in K(UMP). The sugar rings in the remaining crystals are slightly deformed from the ideal pucker, with the nearest descriptors being: 3 T2 for the IMP monoanion, 2 T1 for two CMP dianions and 3 T4, 2 E for two remaining CMP dianions. The orientation of the phosphate relative to the ribose ring is the same in all the anions: gauche-gauche, which is characteristic of the 5'-nucleotides. The arrangement of the terminal phosphate atoms relative to the ester C5' is antiperiplanar (ap) for one of O atoms and +/-synclinal (+/-sc) for the others in most of the presented nucleotides. However the phosphate group in the IMP monoanion is disordered and exists in two orientations: typical ap/sc and quite uncommon sp/ac.