Open AccessThermodynamics of Stacking and of Self‐Association of the Dinucleoside Monophosphate m 6 2 A‐U from Proton NMR Chemical Shifts:
Author(s) -
HARTEL Arnold J.,
LANKHORST Peter P.,
ALTONA Cornelis
Publication year - 1982
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1982.tb07057.x
Subject(s) - stacking , intramolecular force , intermolecular force , chemistry , chemical shift , molecule , proton , thermodynamics , crystallography , computational chemistry , stereochemistry , physics , organic chemistry , quantum mechanics
Chemical shifts of base and sugar protons of the modified ribodinucleoside monophosphate N 6 ‐dimethyl‐adenylyl(3′‐5′)uridine (m 6 2 A‐U) were measured at 100, 360 and 400 MHz in aqueous solution. Seven different samples were used with concentrations ranging from 0.28 mM to 32.7 mM. The temperature was varied from –5 °C to 105 °C. An internal temperature calibration was used. The effects of intermolecular self‐association and of intramolecular stacking on the chemical shifts were quantitatively separated by means of a new approach: differential concentration/temperature profiles (DCTP). Several computational models were tested and the analysis allowed deeper insight into the behaviour of m 6 2 A‐U at the molecular level. The simple two‐state approach for both self‐association and stacking already afforded a significant improvement over models in which the association is entirely neglected. A computer least‐squares analysis of the chemical shift behaviour of each individual proton yielded thermodynamic parameters for self‐association and stacking. However, the two‐state model did not suffice to reproduce accurately all of the observations. A satisfactory fit required two additional assumptions: (a) the aromatic protons experience different association shifts in stacked and in unstacked molecules; (b) a temperature‐dependent conformational equilibrium exists between sets of unstacked microstates. The stacked state is taken to represent a single conformational species. The implementation of this extended model in the leastsquares optimization allowed the reproduction of over one thousand chemical shift observations within experimental error. Thermodynamic equilibrium parameters deduced for intramolecular stacking are: Δ H o x =–28.8 kJ mol −1 , Δ S o x =–93 J mol −1 K −1 . These numbers agree well with those obtained earlier by us from circular dichroism spectra. The equilibrium enthalpy and entropy values deduced for the association process are: Δ H o A =–35 kJ mol −1 and Δ S o A =–95 J mol −1 K −1 .