Molecules-in-molecules fragment-based method for the calculation of chiroptical spectra of large molecules: Vibrational circular dichroism and Raman optical activity spectra of alanine polypeptides

Classical Trajectory of Molecules in Electromagnetic Field: A Handy Method to Simulate Molecular Vibrational Spectra

Within harmonic approximations, molecular vibrational spectra are simulated in a standard way through force field diagonalization and following transformation of Cartesian to normal-mode tensor derivatives. This may become tedious for large systems of many thousands of atoms and also not necessary because of a limited resolution required to interpret an experiment. We developed an algorithm based on the real-time real-field molecular dynamics, effectively at zero temperature, invoked in a molecule by the electromagnetic field of light. The algorithm is simple to implement and suitable for parallel computing, and it can be potentially extended to more complicated molecular-light interaction modes. It circumvents the diagonalization and is suitable to model vibrational optical activity (vibrational circular dichroism and, to a lesser extent, Raman optical activity). For large molecules, it becomes faster than diagonalization, but it also enables the assignment of vibrational spectral bands to local molecular motions.

α-Synuclein conformations followed by vibrational optical activity. Simulation and understanding of the spectra

For α-synuclein novel structural markers were identified in vibrational optical activity spectra and supported by theoretical modeling.

Analysis of Vibrational Circular Dichroism Spectra of Peptides: A Generalized Coupled Oscillator Approach of a Small Peptide Model Using VCDtools

Vibrational circular dichroism (VCD) is one of the major spectroscopic tools to study peptides. Nevertheless, a full understanding of what determines the signs and intensities of VCD bands of these compounds in the amide I and amide II spectral regions is still far from complete. In the present work, we study the origin of these VCD signals using the general coupled oscillator (GCO) analysis, a novel approach that has recently been developed. We apply this approach to the ForValNHMe model peptide in both α-helix and β-sheet configurations. We show that the intense VCD signals observed in the amide I and amide II spectral regions essentially have the same underlying mechanism, namely, the through-space coupling of electric dipoles. The crucial role played by intramolecular hydrogen bonds in determining VCD intensities is also illustrated. Moreover, we find that the contributions to the rotational strengths, considered to be insignificant in standard VCD models, may have sizable magnitudes and can thus not always be neglected. In addition, the VCD robustness of the amide I and II modes has been investigated by monitoring the variation of the rotational strength and its contributing terms during linear transit scans and by performing calculations with different computational parameters. From these studies-and in particular, the decomposition of the rotational strength made possible by the GCO analysis-it becomes clear that one should be cautious when employing measures of robustness as proposed previously.

Vibrational optical activity for structural characterization of natural products

This review presents the recent progress towards elucidating the structures of chiral natural products and applications using vibrational optical activity (VOA) spectroscopy.

Energy-Screened Many-Body Expansion: A Practical Yet Accurate Fragmentation Method for Quantum Chemistry

We introduce an implementation of the truncated many-body expansion, MBE(n), in which the n-body corrections are screened using the effective fragment potential force field, and only those that exceed a specified energy threshold are computed at a quantum-mechanical level of theory. This energy-screened MBE(n) approach is tested at the n = 3 level for a sequence of water clusters, (H₂O)_N=6-34. A threshold of 0.25 kJ/mol eliminates more than 80% of the subsystem electronic structure calculations and is even more efficacious in that respect than is distance-based screening. Even so, the energy-screened MBE(3) method is faithful to a full-system quantum chemistry calculation to within 1-2 kJ/mol/monomer, even in good quality basis sets such as aug-cc-pVTZ. These errors can be reduced by means of a two-layer approach that involves a Hartree-Fock calculation for the entire cluster. Such a correction proves to be necessary in order to obtain accurate relative energies for conformational isomers of (H₂O)₂₀, but the cost of a full-system Hartree-Fock calculation remains smaller than the cost of three-body subsystem calculations at correlated levels of theory. At the level of second-order Møller-Plesset perturbation theory (MP2), a screened MBE(3) calculation plus a full-system Hartree-Fock calculation is less expensive than a full-system MP2 calculation starting at N = 12 water molecules. This is true even if all MBE(3) subsystem calculations are performed on a single 40-core compute node, i.e., without significant parallelization. Energy-screened MBE(n) thus provides a fragment-based method that is accurate, stable in large basis sets, and low in cost, even when the latter is measured in aggregate computer time.

The effect of protein backbone hydration on the amide vibrations in Raman and Raman optical activity spectra

Raman and specifically Raman optical activity (ROA) spectroscopy are very sensitive to the solution structure and conformation of biomolecules. Because of this strong conformational sensitivity, density functional theory (DFT) calculations are often used to get a better understanding of the experimentally observed spectral patterns. While e.g. for carbohydrate structure the water molecules that surround the solute have been demonstrated to be of vital importance to get accurate modelled ROA spectra, the effect of explicit water molecules on the calculated ROA patterns of peptides and proteins is less well studied. Here, the effect of protein backbone hydration was studied using DFT calculations of HCO-(l-Ala)5-NH2 in specific secondary structure conformations with different treatments of the solvation. The effect of the explicit water molecules on the calculated spectra mainly arises from the formation of hydrogen bonds with the amide C[double bond, length as m-dash]O and N-H groups. Hydrogen bonding of water with the C[double bond, length as m-dash]O group determines the shape and position of the amide I band. The C[double bond, length as m-dash]O bond length increases upon formation of C[double bond, length as m-dash]OH2O hydrogen bonds. The effect of the explicit water molecules on the amide III vibrations arises from hydrogen bonding of the solvent with both the C[double bond, length as m-dash]O and N-H group, but their contributions to this spectral region differ: geometrically, the formation of a C[double bond, length as m-dash]OH2O bond decreases the C-N bond length, while upon forming a N-HH2O hydrogen bond, the N-H bond length increases.

Variational Formulation of the Generalized Many-Body Expansion with Self-Consistent Charge Embedding: Simple and Correct Analytic Energy Gradient for Fragment-Based ab Initio Molecular Dynamics

The many-body expansion (MBE) and its extension to overlapping fragments, the generalized (G)MBE, constitute the theoretical basis for most fragment-based approaches for large-scale quantum chemistry. We reformulate the GMBE for use with embedding charges determined self-consistently from the fragment wave functions, in a manner that preserves the variational nature of the underlying self-consistent field method. As a result, the analytic gradient retains the simple "sum of fragment gradients" form that is often assumed in practice, sometimes incorrectly. This obviates (without approximation) the need to solve coupled-perturbed equations, and we demonstrate stable, fragment-based ab initio molecular dynamics simulations using this technique. Energy conservation fails when charge-response contributions to the Fock matrix are neglected, even while geometry optimizations and vibrational frequency calculations may yet be accurate. Stable simulations can be recovered by means of straightforward modifications introduced here, providing a general paradigm for fragment-based ab initio molecular dynamics.