1. Department of Chemistry, Rhodes University, P. O. Box 94, Makhanda 6139, South Africa
2. Department of Chemistry, University of Pretoria, 02 Lynwood Road, Hatfield, Pretoria 0002, South Africa
3. Department of Chemistry, Nelson Mandela University, Summerstrand, P. O. Box 77000, Gqeberha 6031, South Africa
4. Faculty of Pharmacy, Rhodes University, P. O. Box 94, Makhanda 6139, South Africa