Erythromycin B: conformational analysis and antibacterial activity

Erythromycin B, an acid-stable co-metabolite of the important antibiotic erythromycin A, differs from erythromycin A only in the absence of a hydroxyl group at C12, yet it has never been licensed for clinical use. We describe an NMR-based analysis of the conformation of erythromycin B, both free in aqueous solution and when weakly bound to bacterial ribosomes and show that it is conformationally similar to erythromycin A. The antibacterial activity of erythromycin B is shown to be similar to that of erythromycin A, but after acid-treatment, resembling exposure to the stomach, erythromycin B substantially retains antibacterial activity, whereas erythromycin A does not.