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Comparison of the acid-base properties of purine derivatives in aqueous solution : determination of intrinsic proton affinities of various basic sites
Journal
Journal of the Chemical Society. Perkin transactions 2
Volume
2002, H. 7
Number
7
Pages / Article-Number
1320-1327
Abstract
The acidity constants of protonated 7,9-dimethylguanine, 7-methylguanosine, 7,9-dimethylhypoxanthine, 7-methylinosine, 9-methyladenine, 1,9-dimethyladenine, 7,9-dimethyladenine and 1-methyladenosine were determined in aqueous solution at 25 degreesC and I = 0.1 M (NaNO3). In those instances where pK(a) > 2 potentiometric pH titrations were used for the determinations; when pK(a) < 2, UV spectrophotometric and H-1-NMR shift measurements were employed (25 &DEG;C). In these latter instances, where I is often larger than 0.1 M, the H-0 scale was applied to de ne the H+ activity of the strong acid (HClO4; HNO3). A combination of the present results with values taken from our earlier work allowed us to quantify the intrinsic acidic properties in aqueous solution of the (N1)H-0 (or) (+) and (N7)H+ sites via micro acidity constant schemes for seven purine derivatives and to calculate the tautomeric ratios regarding the monoprotonated species, that is N7-N1&BULL;H versus H&BULL;N7-N1 meaning that in one isomer H+ is at the N1 site and in the other at N7. A plot of the micro acidity constants pk(H&BULL;N7-N1)(N7-N1), which quantify the acidity of the (N7)H+ site, versus the macro acidity constants pK(a/(N1)H), which largely refer to the release of the proton from the (N1)H unit, results in a straight line for the guanine and hypoxanthine derivatives. This fact allows estimation of the micro acidity constant for any related derivative provided a value for pK(a/(N1)H) is known. The presented results are also meaningful for nucleic acids because they quantify the acid-base properties of their individual sites.
