Total synthesis of bleomycin A2 and related agents. 1. Synthesis and DNA binding properties of the extended C-terminus: Tripeptide S, tetrapeptide S, pentapeptide S, and related agents

Article abstract of DOI:10.1021/ja00092a011

Full details of concise, diastereocontrolled syntheses of 2-5 and their incorporation into tri-, tetra-, and pentapeptide S, the C-terminus of bleomycin A₂, are described. The extension of the studies to the synthesis of a complete set of tri- and tetrapeptide S structural analogs 29a,b and 43b-j is detailed, and their DNA binding constants (apparent K(B), calf thymus DNA) and apparent binding site sizes were determined. Consistent with past observations, the studies highlight the fact that the majority of the DNA binding affinity for bleomycin A₂ (1.0 X 10⁵ M^-1) and deglycobleomycin A₂ (1.1 x 10⁵ M^-1) is embodied within N-BOC-tripeptide S (0.26 x 10⁵ M^-1). The additional comparisons of 29a (0.18 x 10⁵ M^-1), N-BOC-tetrapeptide S (0.21 x 10⁵ M^-1), 43h (0.20 x 10⁵ M^-1), and N-BOC pentapeptide S (0.23 x 10⁵ M^-1) versus N-BOC-dipeptide S (0.10 x 10⁵ M^-1) indicate productive stabilizing binding interactions for the tripeptide S L-threonine subunit and substituent, illustrate that the entire pentanoic acid subunit of tetrapeptide S and its substituents do not significantly contribute to DNA binding affinity, and indicate that the entire β-hydroxy-L-histidine subunit of pentapeptides does not contribute to DNA binding affinity. With the exception of the L-threonine side chain substituent, the observations suggest that the tri- and tetrapeptide S substituent effects on the bleomycin A₂ DNA cleavage reaction are not due to substantial stabilizing binding interactions with duplex DNA. In addition, the measured apparent binding site sizes for bleomycin A₂ (3.8 base pairs), deglycobleomycin A₂ (3.9 base pairs), N-BOC-tripeptide S (3.6 base pairs), N-BOC-tetrapeptide S (3.7 base pairs), 43h (3.5 base pairs), and N-BOC-pentapeptides (4.2 base pairs) versus N-BOC-dipeptide S (2.2 base pairs) and 29a (2.7 base pairs) suggest that it is the tripeptide S subunit of bleomycin A₂ that is fully bound to duplex DNA, that the tripeptide S L-threonine hydroxyethyl substituent detectably affects the agent interaction with duplex DNA, but that the presence or absence of the other tetrapeptide S and pentapeptide S backbone substituents do not substantially alter the binding site size or tripeptide S binding mode.