ISSN:
0947-6539
Keywords:
acid-base equilibria
;
adenine palladium
;
platinum
;
structure elucidation
;
Chemistry
;
General Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Steric blockage of the N1 and N7 sites of 9-methyladenine for metal complexation is achieved by twofold methylation of the exocyclic amino group. With 6′,6′,9-trimethyladenine (TrimeA), binding of MII(dien) (M = Pt, Pd) as well as of trans-[Pt(NH3)2Cl]+ therefore takes place through N 3. X-ray crystal structure determinations and NMR spectroscopic studies of three compounds are reported, and the effects of PtII and PdII on the geometry and the acid-base properties of the TrimeA ligand have been measured and compared with those of the free base. TrimeA has a very pronounced self-stacking tendency in water (K=85M-1 according to the isodesmic model of indefinite noncooperative self-association). Acidity constants derived from potentiometric pH titrations, spectrophotometry, and NMR shift experiments display excellent agreement with each other. Twofold protonated TrimeA, that is H2(TrimeA)2+, carries protons at the N7 and N1 sites; the acidity constants are pKHH2(TrimeA) = - 0.75 and pKHH(TrimeA) = 4.15. These values compare well with those of other adenine residues. Protonation of [Pt(dien)-(TrimeA-N3)]2+ occurs at the N7 position, as shown by spectrophotometry and NMR spectroscopy. The acidity constant of the H+ (N7) site in this complex is low, that is pKHH[Pt(dien)(TrimeA)] = 0.3 (as determined by spectrophotometry), but it is not as low as that for the same site when a proton resides at N1 of unmetalated TrimeA. The pKa of the doubly protonated complex, pKHH2[Pt(dien)(TrimeA)], in which the second acidic proton is situated at N1, is about-1.2 ± 0.3. These findings indicate that upon metalation of N3, the sequence of adenine protonation is reversed. While the N7 site still displays basic properties, the N1 site has undergone a dramatic loss in basicity.
Additional Material:
10 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/chem.19970030310
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