Naturally ocurring polyphenols – Relationship of Structure to Antimicrobial Activity

Among the flavonoids that exhibit antimicrobial activity, isoflavones are potent candidates for combating infections caused by methicillin-resistant Staphylococcus aureus (MRSA) (Mukne et al. 13-14). Such antibacterial activity of isoflavones is associated with the presence of functional groups such as prenyl and phenolic OH groups at specific positions. A C6 prenyl group for instance enhances the antimicrobial activity of isoflavones, with isoflavones such as Genistein that lack the prenyl substituent, having low antimicrobial activity (Mukne et al. 14). Reduced activity also occurs when the prenyl substituent is moved to C-8. For instance, Isoangustone, which possesses a prenyl [(Ch3)2CH=CHCh2]- group at C6, exhibits higher antimicrobial activity against MRSA compared to Glycyrrhisoflavone (which lacks the prenyl substituent) and FS1 (which has a C8 prenyl substituent) (Mukne, et al. 14).

Hydroxyl groups on ring A also contribute to the microbial activity of isoflavones. Hydroxyl groups at position 7 and/or position 5 are a necessity for such activity with substitution at such positions with a carbohydrate moiety abolishing the activity (Mukne et al. 14-15). Although methylation of OH groups in ring A decreases anti-S. aureus activity, it increases anti MRSA activity (Mukne et al. 14). However, when such methylation is accompanied by loss of the prenyl substituent, the antimicrobial activities for both S. aureus and MRSA decline (Mukne et al. 14 – 15). Marginal decrease in activity also arises following cyclization between OH and prenyl groups, but presence of a free prenyl substituent on ring B at position 3′ retains activity in some forms (C6 prenyl, C7 OH cyclization) but not in others (C-8 prenyl, C-7 OH cyclization) (Mukne et al. 14-15). Go to part six here.

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