KINETICS AND MECHANISM OF THE COMPLEXATION OF TRANS-DIAMMINEDIAQUAPLATINUM(II) WITH THE 6-OXOPURINE NUCLEOSIDES INOSINE AND 1-METHYLINOSINEIN AQUEOUS-SOLUTION AS A FUNCTION OF THE PH
M. Mikola et J. Arpalahti, KINETICS AND MECHANISM OF THE COMPLEXATION OF TRANS-DIAMMINEDIAQUAPLATINUM(II) WITH THE 6-OXOPURINE NUCLEOSIDES INOSINE AND 1-METHYLINOSINEIN AQUEOUS-SOLUTION AS A FUNCTION OF THE PH, Inorganic chemistry, 35(26), 1996, pp. 7556-7562
Kinetics of complexation of trans-[Pt(NH3)(2)(H2O)(2)](2+) with the mo
del nucleobases inosine and 1-methylinosine has been studied in aqueou
s solution at 298.2 K in the pH range 2.8-9.6 (I = 0.1 M) by using HPL
C as an analytical tool. The complexation of aquated trans-Pt-II(NH3)(
2) with the nucleobases employed can be quantitatively explained by re
placement of the aqua ligand with the nucleobase, while the hydroxo gr
oup bound to Pt(LI) is inert to substitution reaction relative to the
coordinated water molecule. The stepwise acidity constants of trans-[P
t(NH3)(2)(H2O)(2)](2+) Obtained from kinetic measurements were in exce
llent agreement with those found potentiometrically (pK(a1) = 4.48 +/-
0.02, pK(a2) = 7.20 +/- 0.05). The reactivity of the Pt(II) dication
is 7-8 times higher than that of the monocation despite the moderate t
rans effect OH- > H2O. In excess of the nucleobase, stepwise formation
of 1:1 and 1:2 complexes is observed. 1-Methylinosine forms only N7-b
ound species, as does inosine when pH < 5.5. Although deprotonation of
inosine NIH offers an additional binding site for Pt(II), the ability
of the N7 site to accommodate Pt(II) is increased by the loss of the
N1H proton, as well. Consequently, the N7 binding mode predominates in
the formation of 1:1 complexes throughout the pH range studied. In th
e case of 1-methylinosine the second complexation step is mechanistica
lly straightforward. With inosine, instead, proton transfer formally f
rom N1H to the deprotonated OH group bound to Pt(II) gives substitutio
n labile aqua ligand, the reactivity of which is comparable to that of
the dicationic species. Even at high pH the N7 site is preferred over
the N1 site also in 1:2 complexes. At pH 9.5, the inosine N1/N7 bindi
ng ratio is about 0.6 in 1:1 complexes, whereas the approximate percen
tage of different 1:2 complexes formed is as follows (Pt(II) binding s
ites in parentheses): (N7,N7) 44%, (N1,N7) 44%, and (N1,N1) 12%. In tr
ans-[Pt(NH3)(2)(InoH-N7)(H2O)](2+) the inosine NTH proton is acidified
by 1.7 log units, whereas in trans-[Pt(OH)(NH3)(2)(InoH-N7)](+) the i
nfluence of the Pt(II) unit on the N1H acidity is 1.4 log units.