Diastereomers of the anticancer peptide CIGB-300 with altered b-turn structures

The next-generation anti-tumor drug peptide CIGB-300, developed by the Center for Genetic Engineering and Biotechnology (CIGB), targets casein kinase 2 (CK2) and its substrates, implicating significant therapeutic potential in cancer treatment. A key focus of this study was to compare CIGB-300 and a primary synthetic byproduct, CIGB-300iso, which shares the amino acid sequence with CIGB-300 but was proposed to differ due to racemization. This study explores the synthesis, characterization, and structural elucidation of CIGB-300 and its isomer CIGB-300iso. A comprehensive NMR analysis of seven synthesized diastereomers including amino acid residues C15, H21, and C25 revealed that CIGB-300iso contains one D enantiomer at position H21. The structures of both isoforms derived from NMR constraints disclosed that the L and D enantiomers have an altered peptide supersecondary structure, with a β-turn type IV ₃ found in CIGB-300 and a type I β-turn in CIGB-300iso, significantly impacting the peptide's conformations, sidechain orientations and, potentially, its biological activity. These findings highlight the importance of enantiomerically pure peptides for the design and synthesis of drug peptides.