Anthracycline proton complexation

Base Proton Complexation Shifts The complexation shifts of certain nucleic acid base resonances of poly(dA-dT) on formation of the daunomycin neighbor exclusion complex reflect the shielding contribution due to the anthracycline ring less the contribution from one neighboring base pair which is displaced following intercalation. Thus, the adenosine H-2 resonance remains unperturbed (Figure 27) while the thymidine exchangeable H-3 proton... 

Figure 25. The 360-MHz correlation proton NMR spectra of the daunomycin poly(dA-dT) complex in /M NaCl, lOmNi cacodylate, lOmM EDTA, 80% HaO— 20% 2H 20. Spectrum A corresponds to the Nuc/D = 11.8 complex, pH 6.0 at 67°C and Spectrum B corresponds to the Nuc/D = 5.9 complex, pH 6.05 at 57°C. The strong resonance corresponds to thymidine H-3 proton of the nucleic acid while the weaker resonances (designated hy asterisks) corresponds to hydroxyl protons at Positions 9 and 11 on Ring B of the anthracycline ring of daunomycin...

Figure 30. The temperature dependence of the daunomycin anthracycline Ring D aromatic protons (7.3 to 7.7 ppm), the anomeric sugar H-V proton (5.1 to 5.5 ppm), and the anomeric CH.,-5 proton (1.2 to 1.3 ppm) of the daunomycin poly-(dA-dT) complex, Nuc/D = 25, 9 and 5 in 1M NaCl, lOmM cacodvlate, ImM EDTA, 2HjO. The poly(dA-dT) concentration was fixed at 19.3mM in phosphates and the daunomycin concentration was varied to make the different Nuc/D ratio...

Overlap Geometry at the Intercalation Site We shall attempt to utilize the nucleic acid base and anthracycline ring proton com-plexation shifts to deduce which anthracycline aromatic ring(s) overlap with nearest neighbor base pairs in the daunomycin poly-(dA-dT) intercalation complex. It should be noted that the nonplanarity of ring A in the antibiotic requires that the aromatic portion of the anthracycline chromophore cannot intercalate with its long axis colinear to the direction of the Watson-Crick hydrogen bonds at the intercalation site as was demonstrated for proflavine-nucleic acid complexes. 

A large number of protonation, namely HAdH2, AdH2, "iJAdH, AdH, Ad etc (Ad adriamycin core without two ionisable H) are possible with pK values like 8.22, 9.01, 9.36, 10.1 and 13.2 [65]. Closeness of these pK values makes study with anthracyclines a very complex proposition. In the above formulation, fully protonated adriamycin was represented as HAdH2 where the first H refers to the amino-sugar NH2 and the other two to those on the hydroquinone. 

Metallation of anthracyclines releases protons, and using a of 10.0 for the first phenolic group, Martin calculated log values (for M + H2L M(HL) + H", where H2L refers to the neutral ligand) of 11.0 and 7.3 for Fe " and Cu ", respectively [81]. Metal ion binding must take into account possible formation of hydroxo and polymeric metal complexes at basic pH, and such events make analysis difficult, e.g. a polymeric 1 1 complex CuL forms at high pH [84]. With variation of pH and molar ratio, various complexes are formed between the ligand and these metals. The 2 1 Cu(HL)2 adduct predominates at 5 < pH < 8 [84, 86], with a reported log P = 16.66 [84], which seems high in comparison to the value of 7.3 reported [81]. Resonance Raman spectroscopy has been particularly useful in analysis of these systems and studies on the Cu(II)— adriamycin—DNA adduct indicated that an intercalated adduct could be formed [87]. 

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