1. Department of Biochemistry, University of Cambridge, CB2 1GA, UK
2. Latvian Institute of Organic Synthesis, Aizkraukles 21, Riga, LV-1006, Latvia
3. Division of Cancer Biology, The Institute of Cancer Research, London SW3 6JB, UK
4. Yusuf Hamied Department of Chemistry, University of Cambridge, CB2 1EW, UK