Chem. Pharm. Bull. 53(8) 1034—1036 (2005)

Ghana, is chewed together with the kola nut by the local people before each meal to promote digestion, and is therefore called the false kola nut. It is believed to improve the flavor of anything eaten after it and even to render putrid water palatable. The beneficial effects of chewing this nut are generally thought to include the mechanical cleansing effect and anti-microbial substances in the seeds. During the initial screening of 10 common Nigerian chewing sticks for antibacterial activity, we noted the methanol extract of Garcinia kola displayed good activity. Previous studies indicated that water-soluble polyphenolic compounds may be responsible for the anti-bacterial activity of G. kola. However, we reported that the active components could be fractionated into the ether fraction, suggesting that a different class of compounds may be responsible for the activity. Many phytochemical studies have revealed that biflavonoids are the major constituents of G. kola, and GB1 is one of the major biflavonoids. In this study, a biflavanoid was isolated as the major component of the active fraction, which showed inhibitory effects against methicillin-resistant Staph. aureus (MRSA), and vancomycin-resistant enterococci (VRE). However, it is difficult to identify the chemical structure of this biflavonoid due to the lack of sufficient reference data. This compound exhibits two sets of signals in its nuclear magnetic resonance (NMR) spectra at room temperature, which commonly leads to the conclusion of mixed compounds. As a single compound, it presented one spot or one peak in various TLC and HPLC tests. The biflavonoids always present substantial spectral complexity at the dimeric level due to hindered rotation between the flavanone and the flavanonol moieties around the C-3/C-8 axis. The confusing signals due to its rotameric behavior (atropisomerism) collapse with increasing rotation around 3/8 bond at higher temperature like 100 °C to a single set of signals. This phenomenon indicated a characteristic of this kind of 3,8 -linked biflavanoids. We compared its NMR data obtained at different temperatures including 50 °C, 30 °C, 21 °C, 70 °C, and 90 °C, and in different solvents such as acetone, pyridine, and dimethyl sulfoxide (DMSO). The results support its complicated conformations due to hindered rotation. From these analyses, we have identified this biflavonoid to be GB1 on the basis of its NMR data which was assigned ambiguously by two dimensional (2D) NMR spectra obtained at 90 °C in DMSO-d6. This is the first report of its complete assignment of its NMR data. In this paper, we wish to report the NMR analysis of its interesting rotameric behavior, the complete data assignment, and the related bioassays.