Coordination of {Ru II(NO +)} 3+ and {Ru II(H 2O)} 2+ to bleomycin: most favored [Ru II(L)(BLM-A 2)] structure according to 1H NMR and molecular mechanics methods

Abstract

The {Ru II(NO) +} 3+ derivatives of bleomycin-A 2 and bleomycin-B 2 have been prepared from the reaction of Ru(NO)Cl 3(H 2O) 2 with the commercially available Blenoxane antitumor agent. The coordination identity of the BLM donor groups in [RuNO)(BLM's)] was established using 500 MHz 1H NMR spectral studies as the histidine imidazole, deprotonated histidyl amide, the pyrimidyl nitrogen, the secondary amine and the terminal β-aminoalanine functionalities as proposed previously for Fe II(BLM), ‘active Fe(BLM)’, Co III(BLM), and Cu II(BLM) by various other investigators. Equivalent chemical shifts for the donors and adjacent protons of the metal binding region support the same coordination of both the A 2 and B 2 forms at the metal-binding domain. Molecular mechanics calculations were carried out for the dominant [Ru II(L)(BLM-A 2)] complex, L=NO + and H 2O, using mmff94 software for 15 different structures that include all the possible screw-sense isomers that involve the stereochemical placement of the bithiazole binding tail and the mannose sugar region of BLM. These calculations determined the R, R-sp-1 structure of [Ru II(NO +)(BLM-A 2)] to be of lowest energy, the opposite chirality from that proposed by Stubbe, Kozarich and co-workers for the stereochemistry of [Co III(BLM-A 2)(OOH −)] and [Co III(BLM-A 2)(H 2O)]. The R, R-sp-1 structure places the bithiazole tail near the axial β-aminoalanine amine donor, and the mannose sugar region near the axial NO + chromophore, but not at an H-bonded distance. The R, R-sp-1 structure as indicated for the lowest energy form by mmff94 for [Ru(NO)(BLM-A 2)] is also indicated as the one present at 25 °C in D 2O solution by the 1H NMR spectra. Of the commonly suggested structures for M(BLM's), the mmff94 calculations for [Ru II(NO +)(BLM-A 2)] agree with Zimmer's former findings using Monte Carlo methods for the R, R screw-sense isomer as being lowest in energy for [Co III(BLM-A 2)(OOH −)], and gave a stability order of the various proposed M(BLM) structures for [Ru(NO)(BLM-A 2's)] as R, R-sp-1> R, R-sp basket (after Marzilli et al.)>Stubbe et al.'s R, R> S, S-sp basket> S, S-sp-1>Stubbe et al.'s S, S>OP (after Oppenheimer–Hecht) structures. The presence of a coordinated H 2O ligand instead of NO + changes the strain, raising the energies of the complexes relative to their nitrosyl analogues. The energy-minimized structures show that depending upon the chirality of the species, either the bithiazole or the sugar pendant arm approaches near (within ca. 2.0–2.5 A) of the water ligand, establishing an H-bonded association with the coordinated H 2O. This interaction alters the relative order of structural stabilities, and makes the energies of the several structural forms more nearly comparable. The mmff94 stability order as calculated for [Ru II(H 2O)(BLM-A 2)] is observed to be R, R-sp∼ S, S-sp∼Stubbe S, S∼ R, R-sp-1(all with very similar energies)> S, S-sp-1>Stubbe R, R≫OP. Therefore, having an H-bondable axial ligand such as H 2O or O 2H − makes all structural forms, except for the OP structure, more comparable in energy. Among these, a structure of R, R chirality is modestly favored, although the Stubbe S, S approaches very comparable molecular mechanics energetics to that of R, R-sp or R, R-sp-1. The present work emphasizes the delicate balance in favored M(L)(BLM) structures, and that order of these are a function of the nature of the axial ligand L and the metal center chosen for study. Therefore, conclusions as to the structures of metallo-bleomycins that are drawn from data taken from ‘inert’, non-H-bonding axial ligands such as CO may not be transferable to complexes that possess H-bondable axial ligands.