Spin inversion transfer (SIT) NMR experiments are reported probing the thermodynamics and kinetics of interconversion of two folded forms of a GCN4- like leucine zipper near room temperature. The peptide is 13Cα-labeled at position V9(a) and results are compared with prior findings for position L13(e). The SIT data are interpreted via a Bayesian analysis, yielding local values of T(la), T(1b), k(ab), k(ba), and K(eq) as functions of temperature for the transition F(v9)(a) ⇆ F(v9)(b) between locally folded dimeric forms. Equilibrium constants, determined from relative spin counts at spin equilibrium, agree well with the ratios k(ab)k(ba) from the dynamic SIT experiments. Thermodynamic and kinetic parameters are similar for V9(a) and L13(e), but not the same, confirming that the molecular conformational population is not two-state. The energetic parameters determined for both sites are examined, yielding conclusions that apply to both and are robust to uncertainties in the preexponential factor (kT/h) of the Eyring equation. These conclusions are 1) the activation free energy is substantial, requiring a sparsely populated transition state; 2) the transition state's enthalpy far exceeds that of either F(a) or F(b); 3) the transition state's entropy far exceeds that of F(a), but is comparable to that of F(b); 4) 'Arrhenius kinetics' characterize the temperature dependence of both k(ab) and k(ba), indicating that the temperatures of slow interconversion are not below that of the glass transition. Any postulated free energy surface for these coiled coils must satisfy these constraints.
Bibliographical noteFunding Information:
Mass spectrometry was provided by the Washington University Mass Spectrometry Resource, a National Institutes of Health Research Resource (Grant P41RR0954). Synthesis of the peptide was carried out by Dr. Eva Lovett. Development of the Bayesian techniques employed here was supported by National Institutes of Health Grant NS-35912 and by a license agreement with Varian Associates. One of us (A.H.) acknowledges informative discussions on Eyring theory with Prof. Robert Yaris and the continuing aid of the Luftmensch Society.