1. Department of Chemistry; Rhodes University; Grahamstown 6140 South Africa
2. Department of Chemistry; Tshwane University of Technology; Pretoria 0001 South Africa
3. Centre for Chemico- and Biomedicinal Research; Rhodes University; Grahamstown 6140 South Africa
4. Department of Biochemistry and Microbiology; Rhodes University; Grahamstown 6140 South Africa
5. Division of Clinical Pharmacology, Department of Medicine; University of Cape Town; Observatory Cape Town 7925 South Africa