Stacking Interactions of nucleobases: NMR-Investigations I. Selfassociation of N6,N9-Dimethyladenine and N6-Dimethyl-N9-Ethyladenine

Abstract
The selfassociation of N6,N9-dimethyladenine and N6-dimethyl-N9-ethyladenine has been studied by means of NMR technique. The thermodynamic quantities have been calculated using an isodesmic NMR model with three NMR parameters (the monomer shift δM and two complex shifts δ 2 and δ 3). The dependence of the thermodynamic quantities on the NMR parameters is discussed. Special attention is given to the determination of δ M and its temperature dependence. Calculations with δ 3=2·δ 2 and δ M taken independently of temperature result in an average entropy δ S =−17.9±1.8 e.u. for N6,N9-dimethyladenine and δ S =−16.7±1.7 e.u. for N6-dimethyl-N9-ethyladenine and in an average enthalpy δH=−7.2±0.6 kcal· mol−1 for both substances investigated.

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