* Corresponding authors

^a School of Biology, Biomedical Sciences Research Complex, University of St Andrews, St Andrews KY16 9ST, Scotland, UK

^b Drug Discovery and Development Centre (H3D) and South African Medical Research Council Drug Discovery and Development Research Unit, Department of Chemistry and Institute of Infectious Disease and Molecular Medicine, University of Cape Town, Rondebosch 7701, South Africa

^c School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife, UK

^d Marine Biodiscovery Centre, Department of Chemistry, University of Aberdeen, Old Aberdeen, Scotland, UK

^e Retired Chief, Natural Products Branch, NIH/NCI, Frederick National Laboratory, MD, USA

^f Dept. of Biochemistry and Microbiology, Rhodes University, Makhanda (Grahamstown), South Africa
E-mail: r.dorrington@ru.ac.za