Showing papers in "Journal of Computational Chemistry in 1989"

Optimization of parameters for semiempirical methods I. Method

Abstract: A new method for obtaining optimized parameters for semiempirical methods has been developed and applied to the modified neglect of diatomic overlap (MNDO) method. The method uses derivatives of calculated values for properties with respect to adjustable parameters to obtain the optimized values of parameters. The large increase in speed is a result of using a simple series expression for calculated values of properties rather than employing full semiempirical calculations. With this optimization procedure, the rate-determining step for parameterizing elements changes from the mechanics of parameterization to the assembling of experimental reference data.

Optimization of parameters for semiempirical methods II. Applications

Abstract: MNDO/AM1‐type parameters for twelve elements have been optimized using a newly developed method for optimizing parameters for semiempirical methods. With the new method, MNDO‐PM3, the average difference between the predicted heats of formation and experimental values for 657 compounds is 7.8 kcal/mol, and for 106 hypervalent compounds, 13.6 kcal/mol. For MNDO the equivalent differences are 13.9 and 75.8 kcal/mol, while those for AM1, in which MNDO parameters are used for aluminum, phosphorus, and sulfur, are 12.7 and 83.1 kcal/mol, respectively. Average errors for ionization potentials, bond angles, and dipole moments are intermediate between those for MNDO and AM1, while errors in bond lengths are slightly reduced.

Validation of the general purpose Tripos 5.2 force field

Abstract: A molecular mechanics force field implemented in the Sybyl program is described along with a statistical evaluation of its efficiency on a variety of compounds by analysis of internal coordinates and thermodynamic barriers. The goal of the force field is to provide good quality geometries and relative energies for a large variety of organic molecules by energy minimization. Performance in protein modeling was tested by minimizations starting from crystallographic coordinates for three cyclic hexapeptides in the crystal lattice with rms movements of 0.019 angstroms, 2.06 degrees, and 6.82 degrees for bond lengths, angles, and torsions, respectively, and an rms movement of 0.16 angstroms for heavy atoms. Isolated crambin was also analyzed with rms movements of 0.025 angstroms, 2.97 degrees, and 13.0 degrees for bond lengths, angles, and torsions respectively, and an rms movement of 0.42 angstroms for heavy atoms. Accuracy in calculating thermodynamic barriers was tested for 17 energy differences between conformers, 12 stereoisomers, and 15 torsional barriers. The rms errors were 0.8, 1.7, and 1.13 kcal/mol, respectively, for the three tests. Performance in general purpose applications was assessed by minimizing 76 diverse complex organic crystal structures, with and without randomization by coordinate truncation, with rms movements of 0.025 angstroms, 2.50 degrees, and 9.54 degrees for bond lengths, angles and torsions respectively, and an average rms movement of 0.192 angstroms for heavy atoms.

Improvements on the direct SCF method

Abstract: Three improvements on the direct self‐consistent field method are proposed and tested which together increase CPU‐efficiency by about 50%: (i) selective storage of costly integral batches; (ii) improved integral bond for prescreening; (iii) decomposition of the current density matrix into a linear combination of previous density matrices—for which the two‐electron contributions to the Fock matrix are available—and a remainder ΔD, which is minimized; construction of the current Fock matrix only requires processing of the small ΔD which enhances prescreening.

Evaluation of the dispersion contribution to the solvation energy. A simple computational model in the continuum approximation

Abstract: We present a simple computational method for the evaluation of solute-solvent dispersion energy contributions in dilute isotropic solutions, supplementing the method with an analysis of its sensitivity with respect to several parameters (or features of the solvation model) which are left free in the general formulation. The method is a natural complement of the electrostatic solvation procedure described in preceding articles.

Locally dense basis sets for chemical shift calculations

Abstract: Calculations of chemical shifts have been carried out using “locally dense” basis sets for the resonant atom of interest, and smaller, attenuated sets on other atoms in the molecule. For carbon, calculations involving a 6-311G(d) triply split valence set with polarization on the resonant atom and 3-21G atomic bases on other heavy atoms result in good agreement with experiment, and are virtually identical to those found employing the larger basis on all atoms. For species such as nitrogen, oxygen, and fluorine where standard balanced basis sets do not agree well with experiment, use of attenuated sets fail as well. The use of locally dense basis sets permits calculations previously impractical, and the successful application to carbon suggests that the chemical shift is most dependent on the local basis set, and less so on whether or not a balanced or unbalanced calculation is being carried out.

Solving the finite difference linearized Poisson‐Boltzmann equation: A comparison of relaxation and conjugate gradient methods

Abstract: Comparisons have been made between relaxation methods and certain preconditioned conjugate gradient techniques for solving the system of linear equations arising from the finite-difference form of the linearized Poisson-Boltzmann equation. The incomplete Cholesky conjugate gradient (ICCG) method of Meijerink and van der Vorst has been found to be superior to relaxation methods, with at least a factor of two improvement in speed, and only a 50% increase in storage.

Basis set quality versus size II. Approximate GTO wave functions for second row transition metal atoms

Abstract: Basis sets ranging in size from (16, 10, 7) to (20, 14, 11) have been derived for the atoms Y–Cd. Separate sets represent the energy optimized wave functions for each of the s2dn, s1dn+1, and s0dn+2 configurations. The energies from the largest sets are within 3 mhartrees of the values obtained in numerical Hartree–Fock calculations. Reasonable Hartree–Fock s2dn– s1dn+1 and s2dn– s0dn+2 excitation energies may be obtained either using the largest basis sets, or using d-orbitals optimized for the s0dn+2 configurations. The basis sets are slightly unbalanced in favor of the s-functions and in disfavor of the d-functions, but various alternative basis sets may be derived by combining parts of the five parent sets. The convergence of radial expectation values is discussed.

Stochastic search for the conformations of bicyclic hydrocarbons

Abstract: The stochastic search method was used to seek all of the conformations for 32 bicyclic hydrocarbons. Since the size of the random kick used is sufficient to invert the configurations of carbon atoms, the out, out, in, out, and in, in conformations were all found in a single stochastic run for each hydrocarbon. The lowest energy conformer obtained in each category is reported. A modification of the program to prevent inversion of configuration was developed. It was found, in some cases, that in, in and out, out isomers for some bicyclic hydrocarbons were interconverted by this modified program, presumably through the “homeomorphic isomerization” process described by Park and Simmons. A statistical formula for estimating the approximate chances of finding (or missing) any conformer as a function of the number of random kicks given is derived and presented.

Molecular mechanics (MM2) calculations on peptides and on the protein crambin using the CYBER 205

Abstract: The MM2 potential functions for amides and peptides have been further extended by examining the experimental crystal structures for cyclo-(-Ala-Ala-Gly-Gly-Ala-Gly-), I, and cyclo-(-Ala-Ala-Gly-Ala-Gly-Gly-), II. The force field obtained was then applied to a study of the structure of the hydrophobic protein Crambin, for which a high resolution crystal structure is available. The energy minimization was carried out using a version of MM2 adapted to the CYBER 205.

The calculation of molar polarizabilities by the CNDO/2 method: Correlation with the hydrophobic parameter, log P

Abstract: Results of molecular orbital (MO) calculations by the complete neglect of differential overlap (CNDO/2) method on 50 small molecules are reported. The summation of calculated atomic polarizabilities are equated with molecular polarizabilities, and these are compared with experimentally determined values. It is found that there is very good agreement between calculated and experimental molecular polarizability. This provides a reliable method for the determination of molecular polarizabilities for compounds for which experimental values are not known. The relationship between log P and polarizability is discussed and analyzed in terms of contributions from electronic components to the partitioning energy.

Pattern recognition in the prediction of protein structure. I. Tripeptide conformational probabilities calculated from the amino acid sequence

Abstract: A procedure that uses pattern recognition techniques to compute tripeptide conformational probabilities is described. The procedure differs in several respects from the many “secondary structure” prediction algorithms that have been published over the last 20 years. First, the procedure classifies tripeptides into 64 different conformational types, rather than just α, β and coil, as is commonly done. Thus, the procedure can attempt to predict regions of irregular structure. Second, the procedure uses the methods of pattern recognition, which are powerful but conceptually simple. In this approach, amino acid properties are used to map peptide sequences into a multivariate property space. Particular tripeptide conformations tend to map to particular regions of the property space. These regions are represented by multivariate gaussian distributions, where the parameters of the distributions are determined from tripeptides in the protein X-ray data bank. Finally, rather than making simple predictions, the procedure computes probabilities. Tripeptide conformational probabilities are calculated in the multivariate property space using the gaussian distributions. In a prediction, the procedure might find that a particular tripeptide in a protein has a 36% chance of being in the ααα conformation, a 17% chance of being ααϵ, a 14% chance of being ααα*, etc. The α-helical conformation is thus the most probable, but, in predicting the structure of the protein, a search algorithm should also consider some of the other possibilities. The values of the probability provide a rational basis for selecting from among the possible conformations. The second article of this series describes a procedure that uses the probabilities to direct a search through the conformational space of a protein. The third article of the series describes a procedure that generates actual three-dimensional structures, and minimizes their energies. The three articles together describe a complete procedure, termed “pattern recognition-based importance-sampling minimization” (PRISM), for predicting protein structure from amino acid sequence.

Correlation analysis of substituent effects on the acidity of benzoic acids by the AM1 method

Abstract: Theoretical analysis of the electronic effect of aromatic substituents was done with the use of the AM1 computational procedure. The gas-phase acidity of substituted benzoic acids was linear with the difference in the heat of formation between corresponding benzoic acids and benzoate anions, the energy of the highest occupied molecular orbital, and the net charge on the acidic oxygen atoms of the corresponding benzoate anions. The Hammett σ constant was linearly correlated with the net charge on the atoms of the acid moiety of substituted benzoic acids. The AM1 computational procedure satisfactorily reproduced the electronic properties of a wide variety of substituents.

The tautomerism of 1,2,3‐triazole, 3(5)‐methylpyrazole and their cations

Abstract: The annular tautomerism of 1,2,3‐triazole and 3(5)‐methylpyrazole is discussed by means of a combination of theoretical calculations and experimental (ICR) gas‐phase basicities and acidities. In the gas phase 1,2,3‐triazole exists as the 2H‐tautomer, whereas both tautomers of 3(5)‐methylpyrazole are of similar energy. The solvent effects on these prototropic equilibria are discussed taking into account solvent properties as polarity/polarizability, acidity, and basicity. In nonhydrogen bonding solvents, the difference in dipole moments between both tautomers plays a role that has usually been underestimated.

Continuum radial dielectric functions for ion and dipole solution systems

Abstract: Radial dielectric constant (permittivity) functions for ionic solute, polar solvent systems of the type obtainable from the Lorentz-Debye continuum field formulations are reexamined. Major interest is focused on the assumptions underlying these formulations and their expression in limiting field behavior. The analysis is extended to dipolar solutes and the importance of two types of corrections are evaluated. The first draws connections with the concept of the reaction field as employed by Onsager. This correction is shown to be significant as regards range of predicted saturation effects and for dipole moment self-consistency, for the same type molecule serving as solute and solvent. The second type correction involves the phenomenon of electrostriction whose effects appear much more limited both in range and on the intensity of the fields necessary for its observation. Application of the permittivity functions developed to compute modified Born model hydration energies for a variety of ions is illustrated. Excellent asymptotic approximations for all radial permittivity equations of interest are also presented which should enhance their future utility.

Pattern recognition in the prediction of protein structure. II. Chain conformation from a probability-directed search procedure

Abstract: A procedure that finds the most probable conformational states of a protein chain is described. Single-residue conformations are represented in terms of four conformational states, α, ϵ, α*, and ϵ*. The conformation of the entire chain is represented by a sequence of single-residue conformational states; the distinct conformations in this representation are called “chain-states.” The first article in this series described a procedure that computes tripeptide conformational probabilities from the amino acid sequence using pattern recognition techniques. The procedure described in this article uses the tripeptide probabilities to estimate the probabilities of the chain-states. The chain-state probability estimator is a product of conditional and marginal probabilities (obtained from the tripeptide probabilities), with a penalty factor to eliminate conformations containing α-helices and ϵ-strands of excessive length. The probability estimator considers short-range conformational information, medium-range sequence information and some simple long-range information (through the restrictions on helix and strand lengths). Energy minimization calculations can be carried out in the region of conformational space corresponding to a particular chain-state. By selecting the most probable chain-states, the search can be focused on the most probable, or “important,” regions of the conformational space. These energy calculations are described in the third article of the series. The complete procedure described by the three articles is called PRISM, for pattern recognition-based importance sampling minimization.

Algorithm for rapid calculation of excluded volume of large molecules

Abstract: An algorithm is described for rapid calculation of excluded volume of large molecules. The excluded volume is defined based on coordinates of constituent atoms as the volume of overlapping spheres, each standing for a space around an atom inaccessible for a solvent molecule. A computer program based on the algorithm has been tested on a protein, ovomucoid. The accuracy of the numerical calculation is discussed.

The effect of electron correlation on the topological and atomic properties of the electron density distributions of molecules

Abstract: The topological properties of the electron density and the properties of an atom in a molecule are calculated by means of second-order Moller-Plesset perturbation theory (MP2) and compared with the results of configuration interaction calculations (C12) which include all single and double substitutions from the Hartree-Fock reference configuration. A software package for analyzing the effects of electron correlation on the topological properties of the electron density of molecules is described. H2CO is used to provide a numerical example and to indicate that the number of bond critical points is unaffected by the inclusion of electron correlation. Correlation leads to only a small shift in the positions of bond critical points and a small change in the electron density at bond critical points. It is further shown that the energy of an atom in a molecule can be calculated to an accuracy of 1 kcal/mol and the electron population of an atom to about 0.001e. A statistical method is used to show that the deviation of the MP2 correlation correction relative to the CI2 correlation correction for a variety of atomic properties is about 25%.

An ab initio distributed multipole study of the electrostatic potential around an undecapeptide cyclosporin derivative and a comparison with point charge electrostatic models

Abstract: An SCF calculation has been performed on C63H113N11O12, a derivative of the immuno-suppressive drug cyclosporin, using a 3-21G basis set and a Direct SCF method. A distributed multipole analysis has been performed on the resulting charge density to give a set of multipoles at each atomic site, which are used to calculate the electrostatic potential around the molecule. The potential maxima and minima on the accessible surface of the molecule are compared with those predicted using the corresponding Mulliken charges, and also using a potential-derived point-charge model based on the force-field of Kollman et al. The Mulliken charges give a misleading picture of the electrostatic potential around this peptide. The potential-derived charges give results which are in far better agreement with the ab initio distributed multipole model, despite being derived from calculations on smaller molecules with different basis sets and geometries. The limitations of point-charge models for describing the electrostatic interactions of polypeptides are discussed.

Guanidinium‐Type resonance stabilization and its biological implications. I. the guanidine and extended‐guanidine series

Abstract: An unusual type of π-electron delocalization in Y-shaped molecules related to guanidine and its protonated form, the guanidinium ion, has been studied by ab initio methods at the STO-3G and 3-21G levels. Results are reported for tautomeric, rotameric, and protonated forms of the oxygen-substituted guanidine series (urea, carbamic, and carbonic acids); “extended-guanidine” (aminomethylene guanidine) including pseudocyclic forms; and simple ring systems in which the extended-guanidine group is incorporated (3-amino-1,2,4-triazole, 2,4-diaminopyrimidine). Both the guanidine and guanidinium type stabilizations have been characterized in terms of a number of structural and energetic parameters: degree of single/double bond character from bond lengths and π-bond orders, electron distributions, and protonation energies. The major finding is that the structural and energetic properties of the isolated extended-guanidinium group resemble those of the group when incorporated within 6-membered heterocyclic or heterobicyclic rings, although the details vary with the nature of the ring and possibility of reinforcement or interference with the substructure resonance from overall ring delocalization. The implications for stabilization of the protonated forms of some biologically important pteridines is discussed.

Theoretical study of the proton affinities of 2-, 3-, and 4-monosubstituted pyridines in the gas phase by means of MINDO/3, MNDO, and AM1: Proton Affinities of Pyridines

Abstract: Proton affinities (PAs) of 2‐, 3‐, and 4‐monosubstituted pyridines in the gas phase are calculated using the MINDO/3, MNDO, and AM1 methods. The following substituents are considered: F, Cl, CN, CH3, CF3, CHO, NO2, NH2, N(CH3)2, OCH3, and SCH3. The results are compared with experimental values. It is found that all MINDO/3 PAs are ca. 6% too high (mean value) compared to the experimental results; on the other hand, the MNDO values are ca. 7% too low (mean value). However, a much better agreement has been observed for the AM1 method where the theoretical values are only ca. 2.4% too low (mean value). Correlations between the calculated proton affinities on one hand and the charges on the acid H atom and Hammett constants on the other hand are studied. Particularly good linear relationships are found for the 4‐monosubstituted compounds within the AM1 formalism.

Topological electron density analysis of phosphines, phosphaalkenes and phosphaalkynes

Abstract: A general survey of the topological properties of various phospines, phosphaalkenes, and phosphaalkynes is presented. Fifteen compounds containing carbon-phosphorus single, aromatic, double, and triple bonds were optimized at the Hartree-Fock-self-consistent field (HF-SCF) level using the 3-21G, 3-21G(*) and 6-31G* basis sets. Inclusion of d-orbitals was necessary to obtain reasonable structures. The electron densities of these compounds were analyzed using the topological method of Bader, revealing a number of trends. The value of the electron density at the PC bond critical point correlates strongly with the bond distance and bond order. Integrated electron populations correlate with coordination number. The integrated charge indicates a strongly polarized CP bond in all compounds. Comparisons with five CN compounds are made.

Pattern recognition in the prediction of protein structure. III. An importance-sampling minimization procedure

Abstract: A procedure that generates random conformations of a protein chain, and then applies energy minimization to find the structure of lowest energy, is described. Single-residue conformations are represented in terms of four conformational states, α, ϵ, α*, and ϵ*. Each state corresponds to a rectangular region in the ϕ, ψ map. The conformation of an entire chain is then represented by a sequence of single-residue conformational states. The distinct “chain-states” in this representation correspond to multidimensional rectangular regions in the conformational space of the whole protein. A set of highly-probable chain-states can be predicted from the amino acid sequence using the pattern recognition procedure developed in the first two articles of this series. The importance-sampling minimization procedure of the present article is then used to explore the regions of conformational space corresponding to each of these chain-states. The importance-sampling procedure generates a number of random conformations within a particular multidimensional rectangular region, sampling most densely from the most probable, or “important,” sections of the ϕ, ψ map. All values of ϕ and ψ are allowed, but the less-probable values are sampled less often. To achieve this, the random values of ϕ and Φ are generated from bivariate gaussian distributions that are determined from known X-ray structures. Separate gaussian distributions are used for proline residues in the α and ϵ states, for glycine residues in the α, ϵ, α*, and ϵ* states, and for ordinary residues involved in 29 different tripeptide conformations. Energy minimization is then applied to the randomly-generated structures to optimize interactions and to improve packing. The final energy values are used to select the best structures. The importance-sampling minimization procedure is tested on the avian pancreatic polypeptide, using chain-states predicted from the amino acid sequence. The conformation having the lowest energy is very similar to the X-ray conformation.

An ab initio molecular orbital study of the structures and energies of neutral and charged bimolecular complexes of NH3 with the hydrides AHn (A = N, O, F, P, S, and Cl)

Abstract: Hartree-Fock 6-31G(d) structures for the neutral, positive ion, and negative ion bimolecular complexes of NH3 with the first- and second-row hydrides AHn (AHn = NH3, OH2, FH, PH3, SH2, and ClH) have been determined. All of the stable neutral complexes except (NH3)2, the positive ion complexes with NH3 as the proton acceptor, and the negative ion complexes containing first-row anions exhibit conventional hydrogen bonded structures with essentially linear hydrogen bonds and directed lone pairs of electrons. The positive ion complex NH4+ … OH2 has the dipole moment vector of H2O instead of a lone pair directed along the intermolecular line, while the complexes of NH4+ with SH2, FH, and ClH have structures intermediate between the lone-pair directed and dipole directed forms. The negative ion complexes containing second-row anions have nonlinear hydrogen bonds. The addition of diffuse functions on nonhydrogen atoms to the valence double-split plus polarization 6-31G(d,p) basis set usually decreases the computed stabilization energies of these complexes. Splitting d polarization functions usually destabilizes these complexes, whereas splitting p polarization functions either has no effect or leads to stabilization. The overall effect of augmenting the 6-31G(d,p) basis set with diffuse functions on nonhydrogen atoms and two sets of polarization functions is to lower computed stabilization energies. Electron correlation stabilizes all of these complexes. The second-order Moller–Plesset correlation term is the largest term and always has a stabilizing effect, whereas the third and fourth-order terms are smaller and often of opposite sign. The recommended level of theory for computing the stabilization energies of these complexes is MP2/6-31+G(2d,2p), although MP2/6-31+G(d,p) is appropriate for the negative ion complexes.

Calculations of a list of neighbors in molecular dynamics simulations

Abstract: A new algorithm is proposed for the evaluation of nonbonded interactions in Molecular Dynamics simulations. The algorithm is based on a grid search and on partitioning of the atoms into boxes rather than on calculations of distances. The effort associated with the generation of the box list grows only linearly with the number of atoms. The algorithm is particularly advantageous for solvated systems. Test calculations show significant savings in CPU time and storage compared to commonly used algorithms for systems containing in excess of ca. 600 atoms.

Probing the anomeric effect. The diaminomethylene group: Calculations of NCN-containing molecular systems1

Abstract: A study of systems containing the title moiety is described, with special reference to the anomeric effect. We have calculated ab initio, using Gaussian-80 with the 3-21G basis set, all basic conformations of methylene-diamine (H2NCH2NH2) and its N-methyl derivative with full geometry optimization of energy minima and barriers. The structural data thus obtained, were then employed to parameterize Allinger's MM2-80 force field in a procedure similar to that described for oxygen derivatives, including hydrogen-bonding effects and CN bond shortening in tertiary amines. This modification, termed MM2-AE was then used to calculate larger molecules, including N,N′-di- and N,N,N′,N′-tetramethyl-methylenediamine, various 1,3-diazane systems, and 1,4,5,8-tetraazadecalin derivatives of established (x-ray) structures. The results are discussed in light of their verificative and predictive power and appear to validate MM2-AE as a useful computational procedure.

Theoretical studies in molecular recognition: Enantioselectivity in chiral chromatography

Abstract: The enantioselective binding of optical analytes on chiral stationary phases used in column chromatography is investigated with molecular modeling techniques. By rolling the analytes over the van der Waals surface of the phase, configurations are sampled and free energies of transient diastereomeric complexes are computed. These free energies allow us to compute chromatographic separability factors and a linear relationship between computed and observed values is found. The intermolecular potential energy surfaces of these diastereomeric complexes are flat with gentle rolling hills and multiple minima. The binding sites are ill-defined and the analytes are found to freely slide over the chiral stationary phases. An energy partitioning algorithm is used to determine how much of the total binding energy is attributable to a given molecular fragment on the phase. It is found that the fragments of the phase bearing the stereogenic carbons are the least cognizant of differences between optical antipodes.

Contraction of the well-tempered Gaussian basis sets: the first-row diatomic molecules

Abstract: The well-tempered Gaussian basis sets (14s 10p) for atoms from lithium to neon were contracted and used in restricted Hartree–Fock calculations on 13 systems: Li2(Σ), B2(Σ), C2(Σ), N2(Σ), O2(Σ), F2(Σ), Ne2(Σ), LiF(Σ), BeO(Σ), BF(Σ), CN−(Σ), CO(Σ), and NO+(Σ). Spectroscopic constants (Re, ωe, ωexe, Be, αe, and ke) and one-electron properties (dipole, quadrupole, and octupole moments at the center of mass and electric field, electric field gradient, potential, and electron density at the nuclei) were evaluated and compared with the Hartree–Fock results. The largest contracted basis set (7s6p3d) gives results very close to the Hartree–Fock values; the remaining differences are attributed to the absence of the f functions in the present basis sets. For Ne2, the interaction energy was calculated; the magnitude of the basis-set superposition error was found to be very small (less than 3 μEh at 2.8 a0 and less than 2 μEh at 5.0 a0).

Molecular mechanics conformational analysis of cyclononane using the RIPS method and comparison with quantum-mechanical calculations

Abstract: The recently reported Random Incremental Pulse Search (RIPS) technique has been used to probe the conformational energy surface of cyclononane. The stochastic method permits searching of the potential energy surface for all minimum‐energy conformations. The search located all previously reported structures together with three additional conformations that were not found by earlier, primitive searching techniques. Two of these structures are high‐nergy skew forms, and the third is a low‐energy conformer that should contribute significantly to the overall equilibrium set of cyclononane conformations. The global minimum has been found to be the D3 symmetrical twist chair‐boat (TBC) form in accordance with previous studies. The newly discovered low‐energy structure, which lies only 2.2 kcal/mol above the global minimum, has been designated twist chair‐twist chair (TCTC). The two higher energy conformers are skewed chair‐chair (SCC) and skewed boat‐boat (SBB) forms that are 5.7 kcal/mol and 10.4 kcal/mol above the global minimum, respectively. The seven reported conformations were reanalyzed quantum mechanically (AM1), and a comparison between MM2 and AM1 results is presented.

Atomic charge models for polypeptides derived from abinitio calculations

Abstract: Ab Initio charge distributions for amino acid dipeptides are derived utilizing two medium-sized basis sets. Peptide charges differ in two ways from those of existing force fields: the magnitude of the peptide dipole and the dependency on the residue type. The merging of charge distributions of side chain and backbone fragments within a semiclassical model including polarization is investigated. Polarization plays a small, but distinct role in improving the correspondence with ab initio data derived for the complete dipeptide. A description in terms of partly overlapping, interacting fragments correlates well with the ab initio data. The method can be used to derive the electrostatic properties of biological macromolecules by combining accurate descriptions of short range interactions (using good quality basis sets on not too small fragments) with good classical models of long range interactions (using multicenter multipole expansions and atomic polarizability tensors). Factors limiting the accuracy of the present representations are discussed.