C7790 Introduction to Molecular Modelling -1Lesson 9 Model C7790 Introduction to Molecular Modelling TSM Modelling Molecular Structures Petr Kulhánek kulhanek@chemi.muni.cz National Centre for Biomolecular Research, Faculty of Science Masaryk University, Kamenice 5, CZ-62500 Brno PS/2020 Distant Form of Teaching: Rev1 C7790 Introduction to Molecular Modelling -2Reality vs Simulation Problem Model Result approximations Result Result experiment approximation prediction Why? ⚫ incomplete theory ⚫ insufficient performance of current and future (?) computers Solution ... ⚫ use approximation for solution of problems using the available computing capacity Is it possible to accurately simulate the reality around us? Unfortunately, no :-( Model chemistry (Calculation method) C7790 Introduction to Molecular Modelling -3Do we need a model? )()(ˆ rr kkk EH  = In theory, no model is needed because it is outcome of SR equation solution. The only input: nuclei and electrons and their description of interactions and motions Solutions: energy and wavefunction describing QM states (microstates) In practice, we need to employ the BO approximation, which then requires a model (R). C7790 Introduction to Molecular Modelling -4Born-Oppenheimer Approximation ),(),(ˆ RrRr  EH = ෡𝐻𝑒Ψ(𝐫, 𝐑) = 𝐸𝑒(𝐑)Ψ(𝐫, 𝐑) ෡𝐻 𝑅 𝜒(𝐑) = 𝐸 𝑉𝑅𝑇 𝜒(𝐑) electronic properties of the molecule vibrational, rotational, translational motions of a molecule )(),(),( RRrRr  = Born-Oppenheimer approximation ✓ We need a structure (model) to calculate energies. C7790 Introduction to Molecular Modelling -5What is a model? A model is smallest representation of studied system, which can describe studied phenomena by chosen computational method (model chemistry). molecule (molecules) in vacuum molecule (molecules) in implicit environment molecule (molecules) in explicit environment is explicitly modelled by atoms/molecules environment (typically solvent, membrane, etc.) is implicitly modelled as a mean field environment representation (e.g., polarizable dielectric) Main model types: C7790 Introduction to Molecular Modelling -6Is environment important? In molecular modelling, it is not good idea to neglect environment even when qualitative outcome is required. ! Neglecting environment can lead to wrong conclusions ! Modelling of molecules in vacuum (only) must be carefully justified. bambus[6]uril/anion interaction vacuum binding affinities: F- > Cl- > Br- > Isolvent (MeOH/CH3Cl) binding affinities: I- > Br- > Cl- > Fthe order is changed due to anion desolvation energies SLAVÍK, Jan. Počítačové modelování glykolurilových struktur, Bachelor's thesis. Masaryk University, Brno, 2010 C7790 Introduction to Molecular Modelling -7Is environment important? cont. molecule (molecules) in implicit environment molecule (molecules) in explicit environment ➢ Mainly used in QM calculations ▪ PCM (Polarizable continuum model) ▪ COSMO (Conductor like screening model) ▪ … ➢ but also in MM • PB (Poisson–Boltzmann solvent model) • GB (Generalized Born solvent model) • 3D-RISM (3D reference interaction site model) • … ➢ Too complex for QM calculations (rarely used). ➢ Typically used in MM and MD • TIP3P (water model) • SPC/E (water model) • … Homework: How accurately different solvent types (implicit/explicit) can describe interactions at solute/solvent interface? C7790 Introduction to Molecular Modelling -8Model is a compromise Problem Model Result approximations Result Result experiment approximation prediction Model chemistry (Calculation method) too big too complex We need to choose between accuracy (computational feasibility) and model reliability (reasonable representation of studied system). C7790 Introduction to Molecular Modelling -9Model sizes and time scales Hsu, C.C., Buehler, M.J. & Tarakanova, A. The Order-Disorder Continuum: Linking Predictions of Protein Structure and Disorder through Molecular Simulation. Sci Rep 10, 2068 (2020). https://doi.org/10.1038/s41598-020-58868-w C7790 Introduction to Molecular Modelling -10- Example ➢ DNA (15-nt long) ➢ 948 atoms ➢ c(DNA)=7 mM ➢ explicit ions ➢ n(Na+)=35, c(Na+)=244 mM ➢ n(Cl-)=7, c(Cl-)=49 mM ➢ effective c(NaCl)=154 mM* ➢ explicit water (TIP3P model) ➢ n(H20)=7592 ➢ 22776 atoms ➢ PBC with truncated octahedral box ➢ largest subscribed sphere Rin=29 Å *Machado,M.R. and Pantano,S. (2020) Split the Charge Difference in Two! A Rule of Thumb for Adding Proper Amounts of Ions in MD Simulations. J. Chem. Theory Comput., 16, 1367–1372 C7790 Introduction to Molecular Modelling -11Where to get a model? ➢ In silico modelling ➢ small molecules ➢ 2D -> 3D conversions (high-throughput modelling, virtual screening) ➢ ab initio prediction of biomolecular structures ➢ Modelling based on experimental structures ➢ small molecules ➢ large molecules (proteins, DNA, biomolecular complexes, …) ➢ Experimentally guided modelling ➢ NMR (NOE contacts, …) ➢ cryoEM, SAXS (electron density, shape, …) ➢ Similarity modelling ➢ in silico modification of experimental structures ➢ homology modelling C7790 Introduction to Molecular Modelling -12In silico modelling Avogadro Nemesis Other software (commercial): ➢ Spartan, Hyperchem ➢ SCM (ADF) ➢ … free drawing of molecular structures piecewise assembly of molecular structures For modelling, we need 3D structures. )(RE https://en.wikipedia.org/wiki/Comparison_of_software_for_molecular_mechanics_modeling Overview of software: C7790 Introduction to Molecular Modelling -13- 2D vs 3D structure 2D structure contains information about the atoms and bonds. This information describes the constitution (topology) of the system. 3D structure contains information on the spatial distribution of atoms in space. Other information (e.g., bonds) is computable. 3D2D benzoic acid ✓ suitable for modelling C7790 Introduction to Molecular Modelling -14- 3D <-> 2D conversions conversion is easy 3D2D benzoic acid conversion is difficult or impossible C7790 Introduction to Molecular Modelling -15- 3D/2D conversions, complications cyclohexane chair conformation twist boat conformation For small molecules, 2D->3D conversion is possible. Usually, the most stable conformer is modelled. ✓ C7790 Introduction to Molecular Modelling -16Most common formats: ➢ SMILES (Simplified Molecular-Input Line-Entry System) ➢ InChI (IUPAC International Chemical Identifier) ➢ InChIKey (IUPAC International Chemical Identifier Key) 2D structure usage Representation of molecules in 2D formats is employed mainly for: • storing information in databases • searching in such databases (InChiKey and other variants) • predicting the chemical properties of molecules using chemoinformatic approaches (machine learning) • automatic structure generation, generating libraries of molecules (computer aided combinatorial chemistry) - virtual screening benzoic acid C(=O)(O)c1ccccc1 InChI=1S/C7H6O2/c8-7(9)6-4-2-1-3-5-6/h1-5H,(H,8,9) WPYMKLBDIGXBTP-UHFFFAOYSA-N hash constant length, possible collisions C7790 Introduction to Molecular Modelling -17Virtual screening (motivation) Bacteria Early inhibition of bacterial lectin surface hinder bacterial adhesion to host cells. Potent inhibitor (glycomimetics) can be used in treatment of bacterial infections. (development of new antibiotics) Cell Bacteria Cell saturation with potent inhibitor protection C7790 Introduction to Molecular Modelling -18Virtual screening Docking • method that tries to find geometry of ligand/receptor complex receptor ligands (ligand library) Virtual screening • identification of compounds with highest affinity towards receptor • plus special properties … Which of them is the best? C7790 Introduction to Molecular Modelling -19Screening library How to obtain the library? Potential ligand sources: • in silico modelling (2D -> 3D conversion) • precalculated/experimental structure libraries • PubChem (https://pubchem.ncbi.nlm.nih.gov/) • ZINC (https://zinc.docking.org/) C7790 Introduction to Molecular Modelling -20- 3D/2D conversions, biomolecules closed form of the enzyme open form of the enzyme The same primary structure (amino acid sequence). different conformations In molecular modelling, ab initio modelling (prediction) of biomolecular structures is challenging task. C7790 Introduction to Molecular Modelling -21Experimental structures It contains about half a million structures of small molecules determined by Xray and neutron diffraction. Software for working with data:Mercury http: // www.ccdc.cam.ac.uk/Solutions/CSDSystem/Pages/Mercury.aspx Cambridge Structural Database (CSD) http://www.ccdc.cam.ac.uk/Solutions/CSDSystem/Pages/CSD.aspx It contains about 94 thousand structures biomolecular systems determined mainly by X-ray structural analysis. Protein Data Bank (PDB) http: //www.pdb.org Experimental method Proteins (P) Nucleic acids (NA) P / NA complexes Other Overall X ray 77445 1481 4069 3 82998 NMR 8851 1046 193 7 10097 electron microscopy 469 45 129 0 643 status in September 2013 C7790 Introduction to Molecular Modelling -22Experimental structures, cont. ➢ Experimental structures are usually sources for models of biomolecular structures or complicated small molecules. ➢ Due to low resolution and molecular flexibility, some parts might be unresolved. ➢ Missing parts need to be modelled in silico ➢ hydrogen atoms (assignment can be sensitive to pH, PROPKA, https://github.com/jensengroup/propka) ➢ flexible protein loops (Modeller, https://salilab.org/modeller/) ➢ Structures can be influenced by the crystal packing. ➢ It is advisable to check source electron density, especially for low-resolution structures. ➢ Check B-factors to evaluate structure quality. C7790 Introduction to Molecular Modelling -23- B-factors The Debye–Waller factor (DWF, B-factor, temperature factor) is used in condensed matter physics to describe the attenuation of X-ray scattering or coherent neutron scattering caused by thermal motion. For protein structures: The B-factors (B) can be taken as indicating the relative vibrational motion of different parts of the structure. ORTEP diagram drawn with 40% ellipsoid probability for non-H atoms ➢ Atoms with low B-factors belong to a part of the structure that is well ordered. ➢ Atoms with large B-factors generally belong to part of the structure that is very flexible. 𝐵 = 8𝜋2 3 < 𝑢2 > u - displacement of scattering center (atoms) <> - time or thermal average C7790 Introduction to Molecular Modelling -24Similarity modelling http://www.unil.ch/pmf/en/home/menuinst/technologies/homology-modeling.html Homology modelingModifying existing experimental structures Available experimental structure(s) is (are) modified • structure substitution • assembly of complexes • … C7790 Introduction to Molecular Modelling -25- Summary ➢ Proper model is a key element for molecular modelling. ➢ Any error in the model propagates to calculated results. ➢ Therefore, it is worth to spent some time to check validity of the model (especially for in silico modelled parts). ➢ It is also advisable to put some effort in cleaning/improving the model (atom names, etc.) as it can save a lot of time in later analyses.