Uncovering Bioactive Compounds in Propolis Extracts Prior to Isolation Through NMR Chemometric Analysis

Propolis is a resinous bee product with a long history of medicinal use, valued for its antimicrobial, antioxidant and anti-inflammatory properties. Identification of its key bioactive constituents would enable chemical standardization and quality control of propolis-based products, once their activity is confirmed. In the current study we investigated a Greek propolis sample (PR09) belonging to a phenolic-rich type, dominated by flavonoids. After extraction and fractionation with Fast Centrifugal Chromatography (FCPC), the fractions were evaluated for their DPPH and collagenase inhibitory activity while their NMR metabolic profiles were recorded. NMR HeteroCovariance Approach (NMR-HetCA) analysis of PR09 propolis methanolic extract revealed the presence of 26 secondary metabolites: seven diterpenes, 13 flavonoids and six caffeic acid esters. All compounds were identified from NMR-HetCA and Statistical Total Correlation Spectroscopy (STOCSY) plots prior to their isolation. NMR-HetCA analysis indicated that caffeic acid derivatives were the most potent inhibitors of the DPPH free radical and collagenase. Additionally, galangin (11) and 3-O-methyl galangin (24) appeared to contribute considerably to the antioxidant activity, while together with pinocembrin (12), they all contributed to the extract's collagenase inhibitory activity. In contrast, metabolites such as isocupressic acid (8), 13-epi-cupressic acid (18), pinostrobin (17) and chrysin (7) appeared not to contribute to the observed activities. Bioassays of selected metabolites confirmed the NMR-HetCA's predictions, with caffeic acid phenethyl ether (1) exhibiting very high inhibition (92.54 ± 0.16%), and notable collagenase inhibition close to 50% (at 100 μg/mL). Overall, the findings demonstrate that NMR-HetCA enables rapid identification of bioactive compounds in propolis extracts and is proposed as a tool in accelerating the evaluation of propolis samples prior to laborious isolation procedures.