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Quantitative Structure-Activity Relationships (QSAR) Studies of Bioactive Compounds from Croatian Propolis

Propolis (bee glue) is the generic name for the resinous substance collected by honeybees (Apis mellifera, L.) from various plant sources. History of using propolis in folk medicine dates back to the ancient Egyptians, Greeks, and Romans. Nowadays, many in vitro studies confirm its antibacterial, antifungal, antiviral, hepatoprotective, chemopreventive, anti-inflammatory, and antioxidative activities. Its chemical composition is very complex and varies with geographic origin. Although propolis is considered to be a complex mixture of around 200 compounds, all listed activities and potential toxic effects are assumed to originate from various plant phenolics. Quantitative Structure-Activity Relationship (QSAR) and Quantitative Structure-Property Relationship (QSPR) modelling are used increasingly in rational design of bioactive molecules. The most frequently employed approach to obtain QSAR/QSPR models is to perform regression analysis using molecular descriptors. Propolis extracts are currently used for the treatment of oronasal infection. In our work, we analysed 38 propolis extracts from different Croatian regions using liquid chromatography, and investigated their ability to inhibit the growth of Staphylococcus aureus and Streptococcus pyogenes. These activities were correlated with their total phenolic and flavonoid content. In our previous study, we postulated two QSPR models based on topological indices, describing molecular weight and van der Waals volume for group of flavonoids and phenolic acids usually present in Croatian propolis. Topological indices used for molecular modelling are Wiener index and valence connectivity index. The aim of our further study was to check the validity of postulated models. We investigated linear relationship between molecular weight and van der Waals volume data and values obtained using QSPR models. Statistical analysis was performed using STATISTICA v7.0. In our study, we used propolis extracts prepared in 80% ethanol. The analysis of composition was performed using liquid chromatiography, while susceptibility of the gram-positive bacteria to the propolis samples was determined using the disc diffusion method (results are expressed as zone of inhibition – Z.I.). Linear regression analysis was found suitable to describe the predictive power of postulated QSPR models. From 38 samples, we chose 12 which expressed the best antimicrobial activities and correlated the content of phenolic acids and flavonoids with the obtained zones of inhibition. Only in the case of inhibition of Staphylococcus aureus growth, we could observe satisfactory correlation between the content of flavonoids and antimicrobial activity of tested propolis samples. In all other cases, we could not observe any significant relationship. According to our findings, for the antimicrobial activity of propolis samples (considering the growth inhibition of Staphylococcus aureus), more important is the content of flavonoids than phenolic acids.

QSAR; propolis

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