Molecular Design of Interfaces of Model Food Nanoemulsions: A Combined Experimental and Theoretical Approach.

The composition and structure of the interfacial region of emulsions frequently determine its functionality and practical applications. In this work, we have integrated theory and experiments to enable a detailed description of the location and orientation of antioxidants in the interfacial region of olive-oil-in-water nanoemulsions (O/W) loaded with the model gallic acid (GA) antioxidant. For the purpose, we determined the distribution of GA in the intact emulsions by employing the well-developed pseudophase kinetic model, as well as their oxidative stability. We also determined, by employing an in silico design, the radial distribution functions of GA to gain insights on its insertion depth and on its orientation in the interfacial region. Both theoretical and experimental methods provide comparable and complementary results, indicating that most GA is located in the interfacial region (~81.2%) with a small fraction in the aqueous (~18.82%). Thus, GA is an effective antioxidant to inhibit lipid oxidation in emulsions not only because of the energy required for its reaction with peroxyl radical is much lower than that between the peroxyl radical and the unsaturated lipid but also because its effective concentration in the interfacial region is much higher than the stoichiometric concentration. The results demonstrate that the hybrid approach of experiments and simulations constitutes a complementary and useful pathway to design new, tailored, functionalized emulsions to minimize lipid oxidation.

Synthesis of Novel Dinuclear N-Substituted 4-(Dimethylamino)benzaldehyde Thiosemicarbazonates of Rhenium(I): Formation of Four- and/or Five-Membered Chelate Rings, Conformational Analysis, and Reactivity.

The reaction of fac-[ReX(CH3CN)2(CO)3] (X = Cl, Br) with N-phenyl-[4-(dimethylamino)benzaldehyde] thiosemicarbazone (HLA) or N-4-methoxybenzyl-[4-(dimethylamino)benzaldehyde] thiosemicarbazone (HLB) under controlled synthetic conditions gave 4 mononuclear [ReX(HL)(CO)3] (X = Cl, Br) and 16 dinuclear [Re2L2(CO)6] compounds. These complexes were obtained as single crystals, and their structures were established by X-ray diffraction. The structural study of these dimers showed the formation of several solvates, the presence of linkage isomerism, and the stabilization of four- and/or five-membered chelate rings. The different ligand coordination modes (a new µ-κ2-S,N2:κ-N3 coordination mode for a thiosemicarbazone ligand was observed), the conformation of the thiosemicarbazone chain in each case, the formal symmetry of the dimers, and the role of the synthetic procedure in the stability of the different chelate rings were analyzed and are discussed. Theoretical calculations in the gas phase were performed for the dimers with the HLA ligand in order to identify the thermodynamically most stable species. The behavior and structural stability of dimers in dimethyl sulfoxide and acetone solutions was investigated by 1H NMR spectroscopy. The strength of the ReI-L bond in solution was evidenced by the formation of [Re2(LNO2)2(CO)6] and [Re(LA)(py)(CO)3] upon reaction of the corresponding dimer with concentrated nitric acid and pyridine, respectively.

Stacking and hydrogen bond interactions between adenine and gallic acid.

We have performed DFT and DFT-SAPT calculations on dimers of gallic acid, the model system for plant polyphenols, and the DNA base adenine. These dimers were selected for this study as they exhibit simultaneously hydrogen bonds and stacking interactions and it allows to quantify the relative values of these interactions. We calculate the relationships between the stability of the complexes, the charge transfer between monomers and the properties of the intermolecular bonds including hydrogen bonds and other bonds that do not involve hydrogen atoms. DFT-SAPT calculations were also performed to obtain the participation of the different types of energy and so the resulting physical effects. The results show that the presence of hydrogen bonds is the main stabilizing factor for dimers: the higher number and strength, the lower the dimer energy. The contribution of stacking to the stabilization is related to the strength and number of bonds between non-hydrogen atoms and quantified by the contribution of the dispersion terms to the interaction energy. Dimers I and II are mainly stabilized due to hydrogen bonds whereas dimer III is mainly stabilized by stacking interactions.

A QTAIM-based energy partitioning for understanding the physical origin of conformational preferences: application to the Z effect in O=C-X-R and related units.

A quantum theory of atoms in molecules-based energy partitioning was carried out for Z and E conformers of a series of O=C-X-R containing compounds. The results obtained for the simplest compound (formic acid) indicate that the attraction of the electron density within carbonyl oxygen by the nucleus of the acid hydrogen is the most important energy term for Z preference. This conclusion can be extended (mutatis mutandis) to larger carboxylic acids, esters, sulfur derivatives, secondary amides, and carbonyl isocyanates, and even explains the sequence of relative conformational energies in the HCXOH series (X = O, S, Se). In contrast, although the hyperconjugative model has been traditionally employed to explain this preference, we observe it is incompatible with: (i) relative values of diverse QTAIM atomic populations for the Z/E conformational equilibrium; (ii) conformational energies in the HCXOH series.

How does aromaticity rule the thermodynamic stability of hydroporphyrins?

Several measures of aromaticity including energetic, magnetic, and electron density criteria are employed to show how aromatic stabilization can explain the stability sequence of hydroporphyrins, ranging from porphin to octahydroporphin, and their preferred hydrogenation paths. The methods employed involve topological resonance energies and their circuit energy effects, bond resonance energies, multicenter delocalization indices, ring current maps, magnetic susceptibilities, and nuclear-independent chemical shifts. To compare the information obtained by the different methods, the results have been put in the same scale by using recently proposed approaches. It is found that all of them provide essentially the same information and lead to similar conclusions. Also, hydrogenation energies along different hydrogenation paths connecting porphin with octahydroporphin have been calculated with density functional theory. It is shown by using the methods mentioned above that the relative stability of different hydroporphyrin isomers and the observed inaccessibility of octahydroporphin both synthetically and in nature can be perfectly rationalized in terms of aromaticity.

Electron density analysis on the protonation of nitriles.

The applicability of the resonance model to explain the evolution of electron density was tested for a set of 15 nitriles whose protonation processes were studied by means of the quantum theory of atoms in molecules (QTAIM). The electron densities were obtained at the B3LYP/6-31++G**//B3LYP/6-31++G** and HF/6-31++G**//HF/6-31++G** levels. QTAIM atomic and bond properties do not follow the trends that should be expected according to the resonance model and our results are more in line with a H(+)-N[triple bond]C-R Lewis structure than with the H-N(+)[triple bond]C-R and H-N[triple bond]C(+)-R ones. Also, reasonable agreement between experimental and calculated PA values as well as good correlations between variations in atomic energies and populations as a result of protonation were found.

Aromaticity in spin-polarized systems: can rings be simultaneously alpha aromatic and beta antiaromatic?

The partition of the multicenter electron delocalization indices and the nucleus independent chemical shift indices into alpha and beta contributions in open-shell systems has been performed. In general it is shown that a full understanding of aromaticity in these systems cannot be achieved by restricting the calculations to the global properties but by dissecting these properties into alpha and beta terms. The 4n+2- and 4n-aromaticity rules for singlet and triplet annulenes, respectively, reduce to a general aromaticity rule when the alpha and beta terms are studied separately. This new rule allows us to extend the concept of conflicting aromaticities to radical systems that are simultaneously alpha-aromatic and beta-antiaromatic or vice versa. The existence of such systems is demonstrated here by means of multicenter electron delocalization indices and nucleus independent chemical shifts. Finally, the global aromatic/antiaromatic character of these radical systems is estimated by means of aromatic stabilization energy, which is shown to be either slightly positive or slightly negative, thus reflecting the small aromatic/antiaromatic character of these radicals and reinforcing the conclusions obtained with aromaticity indices.

Blue-shifting hydrogen bond in the benzene-benzene and benzene-naphthalene complexes.

Ab initio complete optimizations at MP2/6-31++G** level have been performed in the T-shaped geometry of the benzene-benzene and benzene-naphthalene complexes. To check the effect of the basis set superposition error (BSSE), optimizations have been done in the BSSE corrected and BSSE uncorrected potential energy surfaces. The BSSE effect in the calculation of the Hessian has also been evaluated to check its influence in the frequency values. Quantum theory atoms in molecules (QTAIM) calculations have also been performed on both dimers. Intermolecular energies differ around a 25% when the optimization is performed with or without counterpoise corrected gradients. The influence of BSSE is also noticeable in the distances. Frequency shifts show big changes because of the BSSE. Thus, uncorrected values are up 350% larger than corrected ones. The hypotheses given in the literature to explain the origin of the blue-shifting hydrogen bond do not seem to give a suitable explanation for all characteristics of the behavior found in the studied systems.

Theoretical study of the electronic structure of CnS (n=1-6) thiocumulenes.

Linear sulfur-carbon chains C(n)S (n=1-6) of astronomical interest were examined by means of several theoretical methods. The three smallest compounds of the series were chosen to evaluate the performance of several computational models, including Hartree-Fock theory, density functional theory with the Becke's three parameter exchange functional and the correlation functional of Lee, Yang, and Parr (B3LYP), and electron-correlated methods (second-order Moller-Plesset perturbation method (MP2), configuration interaction method including single and double excitations (CISD), and quadratic configuration interaction method including single and double excitations (QCISD) in combination with a large variety of basis sets. The systematic comparison between the experiment and theory indicates that the B3LYP/6-311G** method can be considered suitable for the study of the electronic structures of the C(n)S compounds. The electronic ground states of the C(n)S molecules alternate between 1Sigma and 3Sigma for odd and even values of n, respectively. The B3LYP/6-311G** wave functions for these electronic ground states were analyzed by means of the atoms in molecules (AIM) and natural bond orbital (NBO) methods. Both approaches suggest that the electronic structures for the singlet and triplet compounds must be considered separately. According to the NBO method, singlet compounds can be properly represented by acetylenic structures with alternating single and triple bonds (S[triple bond]C-C[triple bond]C...). However, triplet compounds are better described by means of double bond-double bond cumulenic structures (S=C=C=C=C...) as a consequence of the average between different alpha and beta electronic densities. AIM delocalization indexes and NBO interactions between localized orbitals also indicate that these structures are strongly pi delocalized. Finally, the different singlet and triplet structures proposed provide a consistent explanation for the geometries, dipole moments, and spin-density values of the C(n)S compounds studied.