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Xavier Fradera

Publications and source records attributed to Xavier Fradera.

8 recordsLinked to original sources

Second-order atomic Fukui indices from the electron-pair density in the framework of the atoms in molecules theory.

Atomic Fukui indices, which are obtained from the electron density, have been previously shown to be useful in predicting which atoms in a molecule are most likely to suffer nucleophilic, electrophilic, or radicalary attacks. Here, we present a second-order generalization of these indices based on the electron pair density. We show how second-order atomic Fukui indices can be used to analyze the effects of electron loss or gain in several molecules from an electron pair point of view. Further, these indices also highlight which atoms or pairs of atoms are more likely to suffer nucleophilic, electrophilic, or radical attacks. In conclusion, second-order indices can complement first-order ones by affording relevant information on molecular reactivity from an electron pair perspective.

Journal Article↗

Unsupervised guided docking of covalently bound ligands.

An approach for docking covalently bound ligands in protein enzymes or receptors was implemented in MacDOCK, a similarity-driven docking program based on DOCK 4.0. This approach was tested with a small number of covalent ligand-protein structures, using both native and non-native protein structures. In all cases, MacDOCK was able to generate orientations consistent with the known covalent binding mode of these complexes, with a performance similar to that of other docking programs. This method was also applied to search for known covalent thrombin inhibitors in a medium-sized molecular database (ca. 11,000 compounds). Detection of functional groups suitable for covalent docking was carried out automatically. A significant enrichment in known active molecules in the first 5% of the database was obtained, showing that MacDOCK can be used efficiently for the virtual screening of covalently bound ligands.

Computer Simulation↗

Guided docking approaches to structure-based design and screening.

With the number of protein-ligand complexes available in the Protein Data Bank constantly growing, structure-based approaches to drug design and screening have become increasingly important. Alongside this explosion of structural information, a number of molecular docking methods have been developed over the last years with the aim of maximally exploiting all available structural and chemical information that can be derived from proteins, from ligands, and from protein-ligand complexes. In this respect, the term 'guided docking' is introduced to refer to docking approaches that incorporate some degree of chemical information to actively guide the orientation of the ligand into the binding site. To reflect the focus on the use of chemical information, a classification scheme for guided docking approaches is proposed. In general terms, guided docking approaches can be divided into indirect and direct approaches. Indirect approaches incorporate chemical information implicitly, having an effect on scoring but not on orienting the ligand during sampling. In contrast, direct approaches incorporate chemical information explicitly, thus actively guiding the orientation of the ligand during sampling. Direct approaches can be further divided into protein-based, mapping-based, and ligand-based approaches to reflect the source used to derive the features capturing the chemical information inside the protein cavity. Within each category, a representative list of docking approaches is discussed. In view of the limitations of current scoring functions, it was generally found that making optimal use of chemical information represents an efficient knowledge-based strategy for improving binding affinity estimations, ligand binding-mode predictions, and virtual screening enrichments obtained from protein-ligand docking.

Binding Sites↗

Molecular dynamics study of 2rotaxanes: influence of solvation and cation on co-conformation.

The conformational preference of a [2]rotaxane system has been examined by molecular dynamics simulations. The rotaxane wheel consists of two bridged binding components: a cis-dibenzo-18-crown-6 ether and a 1,3-phenyldicarboxamide, and the penetrating axle consists of a central isophthaloyl unit with phenyltrityl capping groups. The influence of solvation on the co-conformation of the [2]rotaxane was evaluated by comparing the conformational flexibility in two solvents: chloroform and dimethyl sulfoxide. Attention was also paid to the effect of cation binding on the dynamical properties of the [2]rotaxane. The conformational stability of the [2]rotaxane was calculated using a MM/PB-SA strategy, and the occurrence of specific motions was examined by essential dynamics analysis. The changes in the co-conformational properties in the two solvents and upon cation binding are discussed in light of the available NMR data. The results indicate that in chloroform solution the [2]rotaxane system exists as a mixture of co-conformational states including some that have hydrogen bonds between axle C=O and wheel NH groups. Analysis of the simulations allow us to hypothesize that the [2]rotaxane's circumrotation motion can occur as the result of a dynamic process that combines a preliminary axle sliding step that breaks these hydrogen bonds and a conformational change in the ester group more distant from the wheel. In contrast, no hydrogen-bonded co-conformation was found in dimethyl sulfoxide, which appears to be due to the preferential formation of hydrogen bonds between the wheel NH groups with solvent molecules. Moreover, the axle experiences notable changes in anisotropic shielding, which would explain why the NMR signals are broadened in this solvent. Insertion of a sodium cation into the crown ether reduces co-conformational flexibility due to an interaction of the axle with the cation. Overall, the results reveal how both solvent and ionic atmosphere can influence the co-conformational preferences of rotaxanes.

Journal Article↗

An insight into the local aromaticities of polycyclic aromatic hydrocarbons and fullerenes.

In this work we quantify the local aromaticity of six-membered rings in a series of planar and bowl-shaped polycyclic aromatic hydrocarbons (PAHs) and fullerenes. The evaluation of local aromaticity has been carried out through the use of structurally (HOMA) and magnetically (NICS) based measures, as well as by the use of a new electronically based indicator of aromaticity, the para delocalization index (PDI), which is defined as the average of all the Bader delocalization indices between para-related carbon atoms in six-membered rings. The series of PAHs selected includes C(10)H(8), C(12)H(8), C(14)H(8), C(20)H(10), C(26)H(12), and C(30)H(12), with benzene and C(60) taken as references. The change in the local aromaticity of the six-membered rings on going from benzene to C(60) is analyzed. Finally, we also compare the aromaticity of C(60) with that of C(70), open [5,6]- and closed [6,6]-C(60)NH systems, and C(60)F(18).

Electrons↗

The delocalization index as an electronic aromaticity criterion: application to a series of planar polycyclic aromatic hydrocarbons.

This work introduces a new local aromaticity measure, defined as the mean of Bader's electron delocalization index (DI) of para-related carbon atoms in six-membered rings. This new electronic criterion of aromaticity is based on the fact that aromaticity is related to the cyclic delocalized distribution of pi-electrons. We have found that this DI and the harmonic oscillator model of aromaticity (HOMA) index are strongly correlated for a series of six-membered rings in eleven planar polycyclic aromatic hydrocarbons. The correlation between the DI and the nucleus-independent chemical shift (NICS) values is less remarkable, although in general six-membered rings with larger DI values also have more negative NICS indices. We have shown that this index can also be applied, with some modifications, to study of the aromaticity in five-membered rings.

Journal Article↗

Electron localization and delocalization in open-shell molecules.

Localization and delocalization indices derived in the framework of the quantum Atoms in Molecules theory have recently been used to analyze the electron-pair structure of closed-shell molecules. Here we report calculations of localization and delocalization indices for open-shell molecules at the Hartree-Fock (HF) level. Several simple doublet and triplet radical molecules are studied. In general, interatomic delocalization between bonded atoms is heavily dependent on the order and polarity of the bond. Unpaired electrons also have a significant effect on the interatomic delocalization indices. Indeed, for many radicals, the analysis of the spin components reveals that the interatomic delocalization is very different for alpha and beta spin electrons in many cases. In general, at the HF level, the results can be rationalized in terms of orbital contributions. However, the definition of localization and delocalization indices is completely general, and they could be calculated at any level of theory, provided that the one- and two-electron densities are available.

Journal Article↗

Ligand-induced changes in the binding sites of proteins.

Classical molecular interaction potentials, in conjunction with other theoretical techniques, are used to analyze the dependence of the binding sites of representative proteins on the bound ligand. It is found that the ligand bound introduces in general small structural perturbations at the binding site of the protein. However, such small structural changes can lead to important alterations in the recognition pattern of the protein. The impact of these findings in docking procedures is discussed.

Amino Acid Sequence↗