Intramolecular Ring-Opening Decomposition of Aryl Azetidines.

The ring strain present in azetidines can lead to undesired stability issues. Herein, we described a series of N-substituted azetidines which undergo an acid-mediated intramolecular ring-opening decomposition via nucleophilic attack of a pendant amide group. Studies were conducted to understand the decomposition mechanism enabling the design of stable analogues.

Conserved Mechanism of Conformational Stability and Dynamics in G-Protein-Coupled Receptors.

G-protein-coupled receptors (GPCRs) are transmembrane receptors involved in diverse biological functions. Despite the diversity in their amino acid sequences, class A GPCRs exhibit a conserved structural topology and possibly a common mechanism of receptor activation. To understand how this high sequence diversity translates to a conserved functional mechanism, we have compared the dynamic behavior of eight class A GPCRs comprised of six biogenic amine receptors, adenosine A2A, and the peptide receptor protease-activated receptor 1. Starting from the crystal structures of the inactive state of these receptors bound to inverse agonists or antagonists, we have performed multiple all-atom MD simulations adding up to several microseconds of simulation. We elucidated the similarities and differences in the dynamic behavior and the conformational ensembles sampled by these eight class A GPCRs. Among the six biogenic amine receptors studied here, ß2-adrenergic receptor shows the highest level of fluctuation in the sixth and seventh transmembrane helices, possibly explaining its high basal activity. In contrast, the muscarinic acetylcholine receptors show the lowest fluctuations as well as tight packing and low hydration of the transmembrane domain. All eight GPCRs show several conserved allosteric communication pipelines from the residues in the agonist binding site with the G-protein interface. Positions of the residues along these pipelines that serve as major hubs of allosteric communication are conserved in their respective structures. These findings have important implications in understanding the dynamics and allosteric mechanism of communication in class A GPCRs and hence are useful for designing conformation-specific drugs.

Discovery of the Potent and Selective M1 PAM-Agonist N-[(3R,4S)-3-Hydroxytetrahydro-2H-pyran-4-yl]-5-methyl-4-[4-(1,3-thiazol-4-yl)benzyl]pyridine-2-carboxamide (PF-06767832): Evaluation of Efficacy and Cholinergic Side Effects.

It is hypothesized that selective muscarinic M1 subtype activation could be a strategy to provide cognitive benefits to schizophrenia and Alzheimer's disease patients while minimizing the cholinergic side effects observed with nonselective muscarinic orthosteric agonists. Selective activation of M1 with a positive allosteric modulator (PAM) has emerged as a new approach to achieve selective M1 activation. This manuscript describes the development of a series of M1-selective pyridone and pyridine amides and their key pharmacophores. Compound 38 (PF-06767832) is a high quality M1 selective PAM that has well-aligned physicochemical properties, good brain penetration and pharmacokinetic properties. Extensive safety profiling suggested that despite being devoid of mAChR M2/M3 subtype activity, compound 38 still carries gastrointestinal and cardiovascular side effects. These data provide strong evidence that M1 activation contributes to the cholinergic liabilities that were previously attributed to activation of the M2 and M3 receptors.

Solvation thermodynamic mapping of molecular surfaces in AmberTools: GIST.

The expulsion of water from surfaces upon molecular recognition and nonspecific association makes a major contribution to the free energy changes of these processes. In order to facilitate the characterization of water structure and thermodynamics on surfaces, we have incorporated Grid Inhomogeneous Solvation Theory (GIST) into the CPPTRAJ toolset of AmberTools. GIST is a grid-based implementation of Inhomogeneous Fluid Solvation Theory, which analyzes the output from molecular dynamics simulations to map out solvation thermodynamic and structural properties on a high-resolution, three-dimensional grid. The CPPTRAJ implementation, called GIST-cpptraj, has a simple, easy-to-use command line interface, and is open source and freely distributed. We have also developed a set of open-source tools, called GISTPP, which facilitate the analysis of GIST output grids. Tutorials for both GIST-cpptraj and GISTPP can be found at ambermd.org. © 2016 Wiley Periodicals, Inc.

Overcoming potential energy distortions in constrained internal coordinate molecular dynamics simulations.

The Internal Coordinate Molecular Dynamics (ICMD) method is an attractive molecular dynamics (MD) method for studying the dynamics of bonded systems such as proteins and polymers. It offers a simple venue for coarsening the dynamics model of a system at multiple hierarchical levels. For example, large scale protein dynamics can be studied using torsional dynamics, where large domains or helical structures can be treated as rigid bodies and the loops connecting them as flexible torsions. ICMD with such a dynamic model of the protein, combined with enhanced conformational sampling method such as temperature replica exchange, allows the sampling of large scale domain motion involving high energy barrier transitions. Once these large scale conformational transitions are sampled, all-torsion, or even all-atom, MD simulations can be carried out for the low energy conformations sampled via coarse grained ICMD to calculate the energetics of distinct conformations. Such hierarchical MD simulations can be carried out with standard all-atom forcefields without the need for compromising on the accuracy of the forces. Using constraints to treat bond lengths and bond angles as rigid can, however, distort the potential energy landscape of the system and reduce the number of dihedral transitions as well as conformational sampling. We present here a two-part solution to overcome such distortions of the potential energy landscape with ICMD models. To alleviate the intrinsic distortion that stems from the reduced phase space in torsional MD, we use the Fixman compensating potential. To additionally alleviate the extrinsic distortion that arises from the coupling between the dihedral angles and bond angles within a force field, we propose a hybrid ICMD method that allows the selective relaxing of bond angles. This hybrid ICMD method bridges the gap between all-atom MD and torsional MD. We demonstrate with examples that these methods together offer a solution to eliminate the potential energy distortions encountered in constrained ICMD simulations of peptide molecules.

Design and optimization of selective azaindole amide M1 positive allosteric modulators.

Selective activation of the M1 receptor via a positive allosteric modulator (PAM) is a new approach for the treatment of the cognitive impairments associated with schizophrenia and Alzheimer's disease. A novel series of azaindole amides and their key pharmacophore elements are described. The nitrogen of the azaindole core is a key design element as it forms an intramolecular hydrogen bond with the amide N-H thus reinforcing the bioactive conformation predicted by published SAR and our homology model. Representative compound 25 is a potent and selective M1 PAM that has well aligned physicochemical properties, adequate brain penetration and pharmacokinetic (PK) properties, and is active in vivo. These favorable properties indicate that this series possesses suitable qualities for further development and studies.

GneimoSim: a modular internal coordinates molecular dynamics simulation package.

The generalized Newton-Euler inverse mass operator (GNEIMO) method is an advanced method for internal coordinates molecular dynamics (ICMD). GNEIMO includes several theoretical and algorithmic advancements that address longstanding challenges with ICMD simulations. In this article, we describe the GneimoSim ICMD software package that implements the GNEIMO method. We believe that GneimoSim is the first software package to include advanced features such as the equipartition principle derived for internal coordinates, and a method for including the Fixman potential to eliminate systematic statistical biases introduced by the use of hard constraints. Moreover, by design, GneimoSim is extensible and can be easily interfaced with third party force field packages for ICMD simulations. Currently, GneimoSim includes interfaces to LAMMPS, OpenMM, and Rosetta force field calculation packages. The availability of a comprehensive Python interface to the underlying C++ classes and their methods provides a powerful and versatile mechanism for users to develop simulation scripts to configure the simulation and control the simulation flow. GneimoSim has been used extensively for studying the dynamics of protein structures, refinement of protein homology models, and for simulating large scale protein conformational changes with enhanced sampling methods. GneimoSim is not limited to proteins and can also be used for the simulation of polymeric materials.

Routine Microsecond Molecular Dynamics Simulations with AMBER on GPUs. 2. Explicit Solvent Particle Mesh Ewald.

We present an implementation of explicit solvent all atom classical molecular dynamics (MD) within the AMBER program package that runs entirely on CUDA-enabled GPUs. First released publicly in April 2010 as part of version 11 of the AMBER MD package and further improved and optimized over the last two years, this implementation supports the three most widely used statistical mechanical ensembles (NVE, NVT, and NPT), uses particle mesh Ewald (PME) for the long-range electrostatics, and runs entirely on CUDA-enabled NVIDIA graphics processing units (GPUs), providing results that are statistically indistinguishable from the traditional CPU version of the software and with performance that exceeds that achievable by the CPU version of AMBER software running on all conventional CPU-based clusters and supercomputers. We briefly discuss three different precision models developed specifically for this work (SPDP, SPFP, and DPDP) and highlight the technical details of the approach as it extends beyond previously reported work [Götz et al., J. Chem. Theory Comput. 2012, DOI: 10.1021/ct200909j; Le Grand et al., Comp. Phys. Comm. 2013, DOI: 10.1016/j.cpc.2012.09.022].We highlight the substantial improvements in performance that are seen over traditional CPU-only machines and provide validation of our implementation and precision models. We also provide evidence supporting our decision to deprecate the previously described fully single precision (SPSP) model from the latest release of the AMBER software package.

Routine Access to Millisecond Time Scale Events with Accelerated Molecular Dynamics.

In this work, we critically assess the ability of the all-atom enhanced sampling method accelerated molecular dynamics (aMD) to investigate conformational changes in proteins that typically occur on the millisecond time scale. We combine aMD with the inherent power of graphics processor units (GPUs) and apply the implementation to the bovine pancreatic trypsin inhibitor (BPTI). A 500 ns aMD simulation is compared to a previous millisecond unbiased brute force MD simulation carried out on BPTI, showing that the same conformational space is sampled by both approaches. To our knowledge, this represents the first implementation of aMD on GPUs and also the longest aMD simulation of a biomolecule run to date. Our implementation is available to the community in the latest release of the Amber software suite (v12), providing routine access to millisecond events sampled from dynamics simulations using off the shelf hardware.

Dynamics and dissipation in enzyme catalysis.

We use quantized molecular dynamics simulations to characterize the role of enzyme vibrations in facilitating dihydrofolate reductase hydride transfer. By sampling the full ensemble of reactive trajectories, we are able to quantify and distinguish between statistical and dynamical correlations in the enzyme motion. We demonstrate the existence of nonequilibrium dynamical coupling between protein residues and the hydride tunneling reaction, and we characterize the spatial and temporal extent of these dynamical effects. Unlike statistical correlations, which give rise to nanometer-scale coupling between distal protein residues and the intrinsic reaction, dynamical correlations vanish at distances beyond 4-6 Å from the transferring hydride. This work finds a minimal role for nonlocal vibrational dynamics in enzyme catalysis, and it supports a model in which nanometer-scale protein fluctuations statistically modulate--or gate--the barrier for the intrinsic reaction.

Graphene layer growth chemistry: five- and six-member ring flip reaction.

Reaction pathways are presented for hydrogen-mediated isomerization of a five- and six-member carbon ring complex on the zigzag edge of a graphene layer. A new reaction sequence that reverses the orientation of the ring complex, or "flips" it, was identified. Competition between the flip reaction and the "ring separation" was examined. Ring separation is the reverse of the five- and six-member ring complex formation reaction, previously reported as "ring collision". The elementary steps of the pathways were analyzed using density functional theory (DFT). Rate coefficients were obtained by solution of the energy master equation and classical transition-state-theory utilizing the DFT energies, frequencies, and geometries. The results indicate that the flip reaction pathway dominates the separation reaction and should be competitive with other pathways important to the graphene zigzag edge growth in high-temperature environments.

Reagents for electrophilic amination: a quantum Monte Carlo study.

Electrophilic amination is an appealing synthetic strategy to construct carbon-nitrogen bonds. The authors explore the use of the quantum Monte Carlo method and a proposed variant of the electron pair localization function--the electron pair localization function density--as a measure of the nucleophilicity of nitrogen lone pairs as a possible screening procedure for electrophilic reagents.

Quantum Monte Carlo study of first-row atoms using transcorrelated variational Monte Carlo trial functions.

The effect of using the transcorrelated variational Monte Carlo (TC-VMC) approach to construct a trial function for fixed node diffusion Monte Carlo (DMC) energy calculations has been investigated for the first-row atoms, Li to Ne. The computed energies are compared with fixed node DMC energies obtained using trial functions constructed from Hartree-Fock and density functional levels of theory. Despite major VMC energy improvement with TC-VMC trial functions, no improvement in DMC energy was observed using these trial functions for the first-row atoms studied. The implications of these results on the nodes of the trial wave functions are discussed.

Zori 1.0: a parallel quantum Monte Carlo electronic structure package.

The Zori 1.0 package for electronic structure computations is described. Zori performs variational and diffusion Monte Carlo computations as well as correlated wave function optimization. This article presents an overview of the implemented methods and code capabilities.

A sparse algorithm for the evaluation of the local energy in quantum Monte Carlo.

A new algorithm is presented for the sparse representation and evaluation of Slater determinants in the quantum Monte Carlo (QMC) method. The approach, combined with the use of localized orbitals in a Slater-type orbital basis set, significantly extends the size molecule that can be treated with the QMC method. Application of the algorithm to systems containing up to 390 electrons confirms that the cost of evaluating the Slater determinant scales linearly with system size.