CHARMM (Chemistry at HARvard Molecular Mechanics) is a highly versatile and widely used molecular simulation program. It has been developed over the last three decades with a primary focus on molecules of biological interest, including proteins, peptides, lipids, nucleic acids, carbohydrates and small molecule ligands, as they occur in solution, crystals, and membrane environments. For the study of such systems, the program provides a large suite of computational tools that include numerous conformational and path sampling methods, free energy estimators, molecular minimization, dynamics, and analysis techniques, and model-building capabilities. In addition, the CHARMM program is applicable to problems involving a much broader class of many-particle systems. Calculations with CHARMM can be performed using a number of different energy functions and models, from mixed quantum mechanical-molecular mechanical force fields, to all-atom classical potential energy functions with explicit solvent and various boundary conditions, to implicit solvent and membrane models. The program has been ported to numerous platforms in both serial and parallel architectures. This paper provides an overview of the program as it exists today with an emphasis on developments since the publication of the original CHARMM paper in 1983.
Molecular dynamics simulations of five water models, the TIP3P (original and modified), SPC (original and refined), and SPC/E (original), were performed using the CHARMM molecular mechanics program. All simulations were carried out in the microcanonical NVE ensemble, using 901 water molecules in a cubic simulation cell furnished with periodic boundary conditions at 298 K. The SHAKE algorithm was used to keep water molecules rigid. Nanosecond trajectories were calculated with all water models for high statistical accuracy. The characteristic self-diffusion coefficients D and radial distribution functions, g OO, g OH, and g HH for all five water models were determined and compared to experimental data. The effects of velocity rescaling on the self-diffusion coefficient D were examined. All these empirical water models used in this study are similar by having three interaction sites, but the small differences in their pair potentials composed of Lennard-Jones (LJ) and Coulombic terms give significant differences in the calculated self-diffusion coefficients, and in the height of the second peak of the radial distribution function g OO.
The B-form of DNA can populate two different backbone conformations: BI and BII, defined by the difference between the torsion angles ε and ζ (BI = ε-ζ < 0 and BII = ε-ζ > 0). BI is the most populated state, but the population of the BII state, which is sequence dependent, is significant and accumulating evidence shows that BII affects the overall structure of DNA, and thus influences protein-DNA recognition. This work presents a reparametrization of the CHARMM27 additive nucleic acid force field to increase the sampling of the BII form in MD simulations of DNA. In addition, minor modifications of sugar puckering were introduced to facilitate sampling of the A form of DNA under the appropriate environmental conditions. Parameter optimization was guided by quantum mechanical data on model compounds, followed by calculations on several DNA duplexes in the condensed phase. The selected optimized parameters were then validated against a number of DNA duplexes, with the most extensive tests performed on the EcoRI dodecamer, including comparative calculations using the Amber Parm99bsc0 force field. The new CHARMM model better reproduces experimentally observed sampling of the BII conformation, including sampling as a function of sequence. In addition, the model reproduces the A form of the 1ZF1 duplex in 75 % ethanol, and yields a stable Z-DNA conformation of duplex (GTACGTAC) in its crystal environment. The resulting model, in combination with a recent reoptimization of the CHARMM27 force field for RNA, will be referred to as CHARMM36.
Magnesium ions have an important role in the structure and folding mechanism of ribonucleic systems. To properly simulate these biophysical processes, the applied molecular models should reproduce, among others, the kinetic properties of the ions in water solution. Here, we have studied the kinetics of the binding of magnesium ions with water molecules and nucleic acids systems using molecular dynamics simulation in detail. We have validated the parameters used in biomolecular force fields, such as AMBER and CHARMM, for Mg2+ ions, and also for the biological relevant ions, Na+, K+ and Ca2+ together with three different water models (TIP3P, SPC/E and TIP5P). The results show that Mg2+ ions have a slower exchange rate than Na+, K+ and Ca2+ in agreement with experimental trend, but the simulated value underestimates the experimentally observed Mg2+-water exchange rate with several orders of magnitudes, irrespective of force field and water model. A new set of parameters for Mg2+ was developed to reproduce the experimental kinetic data. This set also leads to better reproduction of structural data than existing models. We have applied the new parameters set to Mg2+ binding with a mono-phosphate model system and with the purine riboswitch, add A-riboswitch. In line with the Mg2+-water results, the newly developed parameters show a better description of the structure and kinetic of the Mg2+-phosphate 2 binding than all other models. The characterization of the ion binding to the riboswitch system shows that the new parameter set does not affect the global structure of the ribonucleic acid system or the number of ions involved in direct or indirect binding. A slight decrease in the number of water-bridged contacts between A-riboswitch and Mg2+ ion is observed. The results support the ability of the newly developed parameters to improve the kinetic description of the Mg2+ and phosphate ions and their applicability in nucleic acid simulation.
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