Effects of Pressure and pH on the Physical Stability of an I-Motif DNA Structure

The thermodynamic stability of a cytosine(C)-rich i-motif tract of DNA, which features pH-sensitive [C..H..C]⁺ moieties, has been studied as function of both pressure (0.1-200 MPa) and pH (3.7-6.2). Careful attention was paid to correcting citrate buffer pH for known variations that stem from changes in pressure. Once pH-corrected, (i) at pH >4.6 the i-motif becomes less stable as pressure is increased (K_D decreases), giving a small negative volume change for dissociation (Δ_D V°) of the i-motif - a conclusion opposite to that which would be drawn if the buffer pH was not corrected for the effects of pressure; (ii) the i-motif's melting temperature increases by more than 30 K between pH 6.5 and 4.5, the consequence of an enthalpy for dissociation (Δ_D H°) of 77(3) and 90(3) kJ (mol H⁺ )^-1 at 0.1 and 200 MPa, respectively; (iii) below pH 4.6 at 0.1 MPa (pH 4.3 at 200 MPa) the melting temperature decreases as a result of double protonation of cytosine pairs, and Δ_D H° and Δ_D V° change signs; and (iv) the combination of Δ_D H° and Δ_D V° lead to the melting temperature at pH 4.3 being 3 K higher at 200 MPa than at 0.1 MPa.