Revision of AMBER Torsional Parameters for RNA Improves Free Energy Predictions for Tetramer Duplexes with GC and iGiC Base Pairs

All-atom force fields are important for predicting thermodynamic, structural, and dynamic properties of RNA. In this paper, results are reported for thermodynamic integration calculations of free energy differences of duplex formation when CG pairs in the RNA duplexes r(CCGG)(2), r(GGCC)(2), r(GCGC)(2), and r(CGCG)(2) are replaced by isocytidine isoguanosine (iCiG) pairs. Agreement with experiment was improved when epsilon/zeta, alpha/beta, chi, and x torsional parameters in the AMBER99 force field were revised on the basis of quantum mechanical calculations. The revised force field, AMBER99TOR, brings free energy difference predictions to within 1.3, 1.4, 2.3, and 2.6 kcal/mol at 300 K, respectively, compared to experimental results for the thermodynamic cycles of CCGG -> iCiCiGiG, GGCC -> iGiGiCiC, GCGC -> iGiCiGiC, and CGCG -> iCiGiCiG. In contrast, unmodified AMBER99 predictions for GGCC -> iGiGiCiC and GCGC -> iGiCiGiC differ from experiment by 11.7 and 12.6 kcal/mol, respectively. In order to test the dynamic stability of the above duplexes with AMBER99TOR, four individual SO ns molecular dynamics (MD) simulations in explicit solvent were run. All except r(CCGG)(2) retained A-form conformation for >= 82% of the time. This is consistent with NMR spectra of r(iGiGiCiC)(2), which reveal an A-form conformation. In MD simulations, r(CCGG)(2) retained A-form conformation 52% of the time, suggesting that its terminal base pairs may fray. The results indicate that revised backbone parameters improve predictions of RNA properties and that comparisons to measured sequence dependent thermodynamics provide useful benchmarks for testing force fields and computational methods.