Molecular mechanics CHARMM

Very recently, we have developed and incorporated into the CHARMM molecular mechanics program a version of LN that uses direct-force evaluation, rather than linearization, for the fast-force components [91]. The scheme can be used in combination with SHAKE (e.g., for freezing bond lengths) and with periodic boundary conditions. Results for solvated protein and nucleic-... 

Their results are indicative of a low-density region inside the dendrimer with a considerable end-group interaction in the shell. Similar results were obtained from CHARMm molecular mechanics calculations on the dendritic box (Figure 16.12b), which indicated a globular architecture in which the dendritic interior is... 

CHARMM (chemistry at Harvard macromolecular mechanics) a molecular mechanics force field... 

This term is essential to obtain the correct geometry, because there is no Pauli repulsion between quantum and classical atoms. The molecular mechanics energy tenn, E , is calculated with the standard potential energy term from CHARMM [48], AMBER [49], or GROMOS [50], for example. 

The energy functions for folding simulations include atom-based potentials from molecular mechanics packages [164] such as CHARMM [81], AMBER [165], and ECEPP... 

CHARMM (Chemistry at HARvard Molecular Mechanics) General-purpose... 

The GEMM software on the ST-100 is not a stand-alone package, and it requires a front-end simulation software package that runs on the host to provide data and to send command requests. It was designed and written with CHARMM (Chemistry at HARvard Macro-molecular Mechanics) (14) as the primary front-end, but additional software packages, such as AMBER (15), have subsequently been modified to drive GEMM. 

In an attempt to aid interpretation of the IR spectrum of MbCO we decided to model the full protein by use of a hybrid quantum mechanics/molecular mechanics approach (QM/MM), to evaluate changes in the CO stretching frequency for different protein conformations. The QM/MM method used [44] combines a first-principles description of the active center with a force-field treatment (using the CHARMM force field) of the rest of the protein. The QM-MM boundary is modeled by use of link atoms (four in the heme vinyl and propionate substituents and one on the His64 residue). Our QM region will include the CO ligand, the porphyrin, and the axial imidazole (Fig. 3.13). The vinyl and propionate porphyrin substituents were not included, because we had previously found they did not affect the properties of the Fe-ligand bonds (Section 3.3.1). It was, on the other hand, crucial to include the imidazole of the proximal His (directly bonded to the... 

Hypercube, Inc. at http //www.hyper.com offers molecular modeling packages under the HyperChem name. HyperChem s newest version, Hyper-Chem Release 7.5, is a full 32-bit application, developed for the Windows 95, 98, NT, ME, 2000, and XP operating systems. Density Functional Theory (DFT) has been added as a basic computational engine to complement Molecular Mechanics, Semiempirical Quantum Mechanics and ab initio Quantum Mechanics. The DFT engine includes four combination or hybrid functions, such as the popular B3-LYP or Becke-97 methods. The Bio+ force field in HyperChem represents a version of the Chemistry at HARvard using Molecular Mechanics (CHARMM) force field. Release 7.5 of HyperChem updates... 

The molecular mechanics method is extremely parameter dependent. A force field equation that has been empirically parameterized for calculating peptides must be used for peptides it cannot be applied to nucleic acids without being re-parameterized for that particular class of molecules. Thankfully, most small organic molecules, with molecular weights less than 800, share similar properties. Therefore, a force field that has been parameterized for one class of drug molecules can usually be transferred to another class of drug molecules. In medicinal chemistry and quantum pharmacology, a number of force fields currently enjoy widespread use. The MM2/MM3/MMX force fields are currently widely used for small molecules, while AMBER and CHARMM are used for macromolecules such as peptides and nucleic acids. 

HyperChem is the PC-based molecular modeling and simulation software package marketed by Hypercube Inc. (http //www.hyper.com). The program provides molecular mechanics (with MM +, AMBER, BIO+ (CHARMm), and OPLS force... 

MD simulations provide a detailed insight in the behavior of molecular systems in both space and time, with ranges of up to nanometers and nanoseconds attainable for a system of the size of a CYP enzyme in solution. However, MD simulations are based on empirical molecular mechanics (MM) force field descriptions of interactions in the system, and therefore depend directly on the quality of the force field parameters (92). Commonly used MD programs for CYPs are AMBER (93), CHARMM (94), GROMOS (95), and GROMACS (96), and results seem to be comparable between methods (also listed in Table 2). For validation, direct comparisons between measured parameters and parameters calculated from MD simulations are possible, e.g., for fluorescence (97) and NMR (cross-relaxation) (98,99). In many applications where previously only energy minimization would be applied, it is now common to perform one or several MD simulations, as Ludemann et al. and Winn et al. (100-102) performed in studies of substrate entrance and product exit. 

MMGK (Molecular mechanics with Gillespie-Kepert terms) [193] is designed for application to coordination compounds. It is based on CHARMM, but an additional term describing repulsion of some effective interaction centers placed on the coordination bonds is added. 

Force-field-based scoring functions use arbitrary empirical estimates of interaction energies obtained by molecular mechanics energy functions. This simple approximation, which takes into account only enthalpic contribution often correlates well with the experiment. Solvent effects are described by atom-based solvation parameters, which are computed for the surface of both ligand and receptor which is buried upon complexation. DOCK-chemical27 and CHARMm scoring functions represent this class. 

Tel. 800-424-9737, fax 415-491-8311 (U.S.A.), tel. 41-38-337633 (U.K.) Model building, display, charge density, electrostatic potential, and molecular orbital plots. Stick, sphere, and dot surface display. 2D to 3D conversion. Protein and DNA fragment libraries. MM+, BIO+ (implementations of MM2 and CHARMM, respectively), OPLS, and AMBER molecular mechanics and dynamics. Solvent box. Semiempirical calculations by Extended Hiickel, CNDO, INDO, MINDO/3, MNDO, AMI, and PM3. Originated at Hypercube, Inc. (Dr. N. Ostlund et al.), of Ontario, Canada. Runs under Windows on a 386 or 486 PC and under Motif on a Silicon Graphics workstation. 

A wide variety of energy functions have been used as part of the various GA-based protein structure prediction protocols. These range from the hydrophobic potential in the simple HP lattice model [19] to energy models such as CHARMM, based on full fledged, detailed molecular mechanics [9]. Apparently, the ease by which various energy functions can be incorporated within the framework of GAs as fitness functions encouraged researchers to modify the energy function in very creative ways to include terms that are not used with the traditional methods for protein structure prediction. 

Molecular mechanics calculations with the CHARMm program of the DAB-tfen[Pg.59]

No experimental results are available for the nucleic acids, with or without methyl substitution, to test the theories, but we can compare the results for thymine to three theoretical estimates based on the linearized Poisson-Boltzmann equation. The AM1-SM2 and PM3-SM3 values are —16.5 and -20.1 kcal/mol, respectively. Using charges and force field parameters from the AMBER,347 CHARMM, and OPLS molecular mechanics force fields and a solute dielectric constant of 1, Mohan et al.i calculated solvation energies of -19.1, -10.4, and -8.4 kcal/mol. The wide variation is disconcerting. In light of such wide variations with off-the-shelf parameters, the SMx approach based on parameters specifically adjusted to solvation energies appears to be more reliable. 

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