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  • 1
    Electronic Resource
    Electronic Resource
    The reaction of n-butyllithium with benzoic acid: is nucleophilic addition competitive with deprotonation?Dedicated to Frederick G. Bordwell for his many and continuing contributions to physical organic chemistry (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 537-541 
    Details
    ISSN: 0894-3230
    Keywords: n-Butyllithium ; benzoic acid ; nucleophilic addition ; deprotonation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---An evaluation of a branching vs sequential mechanism for the reaction of benzoic acid with n-butyllithium favors the latter. © 1997 John Wiley & Sons, Ltd.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Free energy perturbation calculations on models of active sites: Applications to adenosine deaminase inhibitors (1990)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 11 (1990), S. 994-1002 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Free energy perturbation calculations were performed to determine the free energy of binding associated with the presence of perhaps an unusual hydroxyl group in the transition state analog of nebularine, an inhibitor of the enzyme adenosine deaminase. The presence of a single hydroxyl group in this inhibitor has been found to contribute -9.8 kcal/mol to the free energy of binding, with a 108-fold increase in the binding affinity by the enzyme. In this work, we calculate the difference in solvation free energy for the 1,6-dihydropurine complex versus that of the 6-hydroxyl-1,6-dihydropurine complex to determine if this marked increase in binding affinity is attributed to an unusually hydrophobic hydroxyl group. The calculated ΔG associated for the solvation free energy is -11.8 kcal/mol. This large change in the solvation free energy suggests that this hydroxyl is instead unusually hydrophilic and that the difference in free energy of interaction for the two inhibitors to the enzyme must be at least ca. 20 kcal/mol. Although the crystal structure for adenosine deaminase is currently not known, we attempt to mimic the nature of the active site by constructing models which simulate the enzyme-inhibitor complex. We present a first attempt at determining the change in free energy of binding for a system in which structural data for the enzyme is incomplete. To do this, we construct what we believe is a minimal model of the binding between adenosine deaminase and an inhibitor. The active site is simulated as a single charged carboxyl group which can form a hydrogen bond with the hydroxyl group of the analog. Two different carboxyl anion models are used. In the first model, the association is modeled between an acetic acid anion and the modified inhibitor. The second model consists of a hydrophobic amino acid pocket with an interior Glu residue in the active site. From these models we calculate the change in free energy of association and the overall change in free energy of binding. We calculate the free energies of interaction both in the absence and presence of water. We conclude from this that the presence of a single suitably placed-CO-2 group probably cannot explain the binding effect of the-OH group and that additional interactions will be found in the adenosine deaminase active site.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540110811
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  • 3
    Electronic Resource
    Electronic Resource
    Comparison of ab initio, semiempirical, and molecular mechanics calculations for the conformational analysis of ring systems (1992)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 13 (1992), S. 525-532 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The potential energy surfaces of four cyclic alkanes have been examined using molecular mechanics, semiempirical, and ab initio methods to determine if they produce mutually consistent results and investigate the source of any errors between the methods. The C5 — C8 cyclic alkanes were chosen since these structures present a finite set of conformations and transition-state geometries and are still within the computational time and memory limits of the quantum mechanical approaches. We also examined several conformations of 1,2-dideoxyribose to determine the effect of heteroatoms on the results for the 5-membered ring. The molecular mechanics and ab initio calculations are consistent in the relative energies and geometries determined for the conformers of all ring systems. While the semiempirical calculations yielded geometries consistent with the other methods (except for 5-membered rings), the relative energies often deviated substantially. A decomposition analysis of the semiempirical and molecular mechanics energies revealed that the disparities are mainly due to errors in the 1-center energies of the semiempirical calculations. The 2-center bonding and nonbonding energies followed reasonable trends for the conformers. The core-repulsion function, however, is suspected of producing anomalies. A minimum in the attractive Gaussian of this term at 2.1 Å for H—H interactions partly explains the propensity of the 5-membered rings to optimize to near planarity (decreasing 1,2-diaxial hydrogen distances to 2.3 Å) and the underestimation of the relative energy of the boat structure of cyclohexane.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540130413
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  • 4
    Electronic Resource
    Electronic Resource
    AMBERCUBE MD, parallelization of Amber's molecular dynamics module for distributed-memory hypercube computers (1993)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 14 (1993), S. 312-329 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: A fully functional parallel version of the molecular dynamics (MD) module of AMBER3a has been implemented. Procedures parallelized include the calculation of the long-range nonbonded Coulomb and Lennard-Jones interactions, generation of the pairlist, intramolecular bond, angle, dihedral, 1-4 nonbonded interaction terms, coordinate restraints, and the SHAKE bond constraint algorithm. As far as we can determine, this is the first published description where a distributed-memory MIMD parallel implementation of the SHAKE algorithm has been designed to treat not only hydrogen-containing bonds but also all heavy-atom bonds, and where “shaken” crosslinks are supported as well. We discuss the subtasking and partitioning of an MD time-step, load balancing the nonbonded evaluations, describe in algorithmic detail how parallelization of SHAKE was accomplished, and present speedup, efficiency, and benchmarking results achieved when this hypercube adaptation of the MD module AMBER was applied to several variant molecular systems. Results are presented for speedup and efficiency obtained on the nCUBE machine, using up to 128 processors, as well as benchmarks for performance comparisons with the CRAY YMP and FPS522 vector machines. © 1993 John Wiley & Sons, Inc.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540140307
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  • 5
    Electronic Resource
    Electronic Resource
    Free energy perturbation calculations on parallel computers: Demonstrations of scalable linear speedup (1994)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 15 (1994), S. 351-373 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: A coarse-grain parallel implementation of the free energy perturbation (FEP) module of the AMBER molecular dynamics program is described and then demonstrated using five different molecular systems. The difference in the free energy of (aqueous) solvation is calculated for two monovalent cations ΔΔGaq(Li+ Δ Cs+), and for the zero-sum ethane-to-ethane′ perturbation ΔΔGaq(CH3—methyl—X → X—methyl—CH3), where X is a ghost methyl. The difference in binding free energy for a docked HIV-1 protease inhibitor into its ethylene mimetic is examined by mutating its fifth peptide bond, ΔG(CO—NH → CH=CH). A potassium ion (K+) is driven outward from the center of mass of ionophore salinomycin (SAL-) in a potential of mean force calculation ΔGMeOH(SAL- · K+) carried out in methanol solvent. Parallel speedup obtained is linearly proportional to the number of parallel processors applied. Finally, the difference in free energy of solvation of phenol versus benzene, ΔΔGoct(phenol → benzene), is determined in water-saturated octanol and then expressed in terms of relative partition coefficients, Δ log(Po/w). Because no interprocessor communication is required, this approach is scalable and applicable in general for any parallel architecture or network of machines. FEP calculations run on the nCUBE/2 using 50 or 100 parallel processors were completed in clock times equivalent to or twice as fast as a Cray Y-MP. The difficulty of ensuring adequate system equilibrium when agradual configurational reorientation follows the mutation of the Hamiltonian is discussed and analyzed. The results of a successful protocol for overcoming this equilibration problem are presented. The types of molecular perturbations for which this method is expected to perform most efficiently are described. © 1994 by John Wiley & Sons, Inc.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540150310
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  • 6
    Electronic Resource
    Electronic Resource
    Free energy perturbation calculations involving potential function changes (1992)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 13 (1992), S. 362-370 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The solvation free energies of thymine and adenine were calculated using free energy methods to examine the effect of applying Lennard-Jones 6-12 and 10-12 perturbations to the hydrogen-bonding groups. The calculations were performed using a new free energy algorithm developed for the AMBER 4.0 program package that allows an interaction described by a Lennard-Jones 6-12 potential to be changed into one described by a hydrogen bond 10-12 potential. The algorithm applied allows this change to occur smoothly without the generation of more extrema on the potential surface. Results using this algorithm have been compared with those determined using the standard AMBER 3.0 Revision A program package, which provides for 6-12 to 6-12 parameter perturbations only. We have also developed a procedure to perform pyrimidine to purine nucleoside mutations to calculate the relative free energies of solvation directly. The theoretical results are compared to experimental energies derived from solvation and vaporization data taken from the literature. The free energies calculated using the new algorithm show good agreement with the derived experimental values. This is also true for the calculations that employ the 6-12 function only, but with 6-12 parameters modified to reflect the correct hydrogen-bonding interactions. However, perturbation of the “standard” 6-12 parameters without changing the functional form proves to be less effective in determining solvation free energies correctly, and demonstrates the importance of accurate hydrogen bond descriptions in free energy simulations.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540130309
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  • 7
    Electronic Resource
    Electronic Resource
    Calculation of molecular geometries, relative conformational energies, dipole moments, and molecular electrostatic potential fitted charges of small organic molecules of biochemical interest by density functional theory (1995)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 16 (1995), S. 1483-1506 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Density functional theory is tested on a large ensemble of model compounds containing a wide variety of functional groups to understand better its ability to reproduce experimental molecular geometries, relative conformational energies, and dipole moments. We find that gradient-corrected density functional methods with triple-ζ plus polarization basis sets reproduce geometries well. Most bonds tend to be approximately 0.015 Å longer than the experimental results. Bond angles are very well reproduced and most often fall within a degree of experiment. Torsions are, on average, within 4 degrees of the experimental values. For relative conformational energies, comparisons with Hartree-Fock calculations and correlated conventional ab initio methods indicate that gradient-corrected density functionals easily surpass the Hartree-Fock approximation and give results which are nearly as accurate as MP2 calculations. For the 35 comparisons of conformational energies for which experimental data was available, the root mean square (rms) deviation for gradient-corrected functionals was approximately 0.5 kcal mol-1. Without gradient corrections, the rms deviation is 0.8 kcal mol-1, which is even less accurate than the Hartree-Fock calculations. Calculations with extended basis sets and with gradient corrections incorporated into the self-consistent procedure generate dipole moments with an rms deviation of 5%. Dipole moments from local density functional calculations, with more modest basis sets, can be scaled down to achieve roughly the same accuracy. In this study, all density functional geometries were generated by local density functional self-consistent calculations with gradient corrections added in a perturbative fashion. Such an approach generates results that are almost identical to the self-consistent gradient-corrected calculations, which require significantly more computer time. Timings on scalar and vector architectures indicate that, for moderately sized systems, our density functional implementation requires only slightly less computer resources than established Hartree-Fock programs. However, our density functional calculations scale much better and are significantly faster than their MP2 counterparts, whose results they approach. © 1995 John Wiley & Sons, Inc.
    Additional Material: 8 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540161206
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  • 8
    Electronic Resource
    Electronic Resource
    AMBER: Assisted model building with energy refinement. A general program for modeling molecules and their interactions (1981)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 2 (1981), S. 287-303 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: We describe a computer program we have been developing to build models of molecules and calculate their interactions using empirical energy approaches. The program is sufficiently flexible and general to allow modeling of small molecules, as well as polymers. As an illustration, we present applications of the program to study the conformation of actinomycin D. In particular, we study the rotational isomerism about the D-Val-, L-Pro, and L-Pro-Sar amide bonds as well as comparing the energy and structure of the Sobell model and the x-ray structure of actinomycin D.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540020311
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  • 9
    Electronic Resource
    Electronic Resource
    An approach to computing electrostatic charges for molecules (1984)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 5 (1984), S. 129-145 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: We present an approach for deriving net atomic charges from ab initio quantum mechanical calculations using a least squares fit of the quantum mechanically calculated electrostatic potential to that of the partial charge model. Our computational approach is similar to those presented by Momany [J. Phys. Chem., 82, 592 (1978)], Smit, Derissen, and van Duijneveldt [Mol. Phys., 37, 521 (1979)], and Cox and Williams [J. Comput. Chem., 2, 304 (1981)], but differs in the approach to choosing the positions for evaluating the potential. In this article, we present applications to the molecules H2O, CH3OH, (CH3)2O, H2CO, NH3, (CH3O)2PO2-, deoxyribose, ribose, adenine, 9-CH3 adenine, thymine, 1-CH3 thymine, guanine, 9-CH3 guanine, cytosine, 1-CH3 cytosine, uracil, and 1-CH3 uracil. We also address the question of inclusion of “lone pairs,” their location and charge.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540050204
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  • 10
    Electronic Resource
    Electronic Resource
    An all atom force field for simulations of proteins and nucleic acids (1986)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 7 (1986), S. 230-252 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: We present an all atom potential energy function for the simulation of proteins and nucleic acids. This work is an extension of the CH united atom function recently presented by S.J. Weiner et al. J. Amer. Chem. Soc., 106, 765 (1984). The parameters of our function are based on calculations on ethane, propane, n-butane, dimethyl ether, methyl ethyl ether, tetrahydrofuran, imidazole, indole, deoxyadenosine, base paired dinucleoside phosphates, adenine, guanine, uracil, cytosine, thymine, insulin, and myoglobin. We have also used these parameters to carry out the first general vibrational analysis of all five nucleic acid bases with a molecular mechanics potential approach.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540070216
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