Abstract
Prenylated (iso)flavonoids, -flavans and pterocarpans from taxa in Erythrina are repeatedly flagged as potent antimicrobial compounds. In the current study, bark from E. lysistemon was extracted and seven isoflavone derivatives were purified: erybraedin A (1), phaseollidin (2), abyssinone V-4′ methyl ether (3), eryzerin C (4), alpumisoflavone (5), cristacarpin (6) and lysisteisoflavone (7). Minimum inhibition concentration (MIC) values were determined against a range of species of bacteria (skin pathogens), then values for another 67 derivatives from Erythrina, only against Staphylococcus aureus, were mined from the literature. Of the seven isolates, MIC values widely ranged from 1–600 μg/mL, with no obvious pattern of selectivity for Gram-types. Nevertheless, using the mined and experimentally determined values against S. aureus, Klekota-Roth fragments (Structure Activity Relationship: SAR) were determined then used as molecular descriptors to make a ‘decision tree’ based on structural characters inspired by the classes of antimicrobial potency (classes A-D). Furthermore, to make quantitative predictions of MIC values (Quantitative SAR: QSAR) ‘pace regression’ was utilized and validated (R² = 0.778, Q² = 0.727 and P² = 0.555). Evidently, the position and degree of prenylation is important; however, the presence of hydroxyl groups at positions 5 and 7 in ring A and 4′ in ring B is associated with lower MIC values. While antimicrobial results continue to validate the traditional use of E. lysistemon extracts (or Erythrina generally) in therapeutic applications consistent with anti-infection, it is surprising that this class of compound is not being utilized more often in general industry applications, such as food or cosmetic preservation, or in topical antimicrobial creams. Prenylated (iso)flavonoids are derived from several other Genera, such as Dorstenia (Moraceae), Ficus (Moraceae), Glycyrrhiza (Fabaceae), Paulownia (Lamiales) or Pomifera (Moraceae).
Highlights
Erythrina is a common feature of the materia medica of many of the world’s cultures, across Africa, the America’s, India, south-east Asia, Australia and the Polynesian Islands
The popularity of E. lysistemon in traditional medicine in southern Africa is reflected in the numerous vernacular names that have been recorded for this species
The results of the current study demonstrate that the prenylatedflavonoid derivatives are important metabolites in achieving the antimicrobial outcomes from extracts of Erythrina, in traditional use systems
Summary
Erythrina is a common feature of the materia medica of many of the world’s cultures, across Africa, the America’s, India, south-east Asia, Australia and the Polynesian Islands. 120 known species, 38 occur in Africa and Madagascar, 70 in the neotropics (mostly South America) and 12 in Asia and Oceania [1]. Salient examples of medicinally important species include E. variegata. In south-east Asian practice [4,5] and E. lysistemon Hutch. A vernacular name commonly associated with the species of Erythrina is ‘coral’, such as ‘coral tree’. The etymology of such a name is clearly related to the appearance of the mostly bright red flowers, which can cover leafless branches in the spring and confer the appearance of coral or barnacles
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