Conformational preference of dipeptide zwitterions in aqueous solvents.

Proper description of solvent effects is challenging for theoretical methods, particularly if the solute is a zwitterion. Here, a series of theoretical procedures are used to determine the preferred solvated conformations of twelve hydrophobic dipeptides (Leu-Leu, Leu-Phe, Phe-Leu, Ile-Leu, Phe-Phe, Ala-Val, Val-Ala, Ala-Ile, Ile-Ala, Ile-Val, Val-Ile and Val-Val) in the zwitterionic state. First, the accuracy of density functional theory (DFT), combined with different implicit solvent models, for describing zwitterions in aqueous solvent is assessed by comparing the predicted against the experimental glycine tautomerization energy, i.e., the energetic difference between canonical and zwitterionic glycine in aqueous solvents. It is found that among the tested solvation schemes, the charge-asymmetric nonlocally determined local-electric solvation model (CANDLE) predicts an energetic difference in excellent agreement with the experimental value. Next, DFT-CANDLE is used to determine the most favorable solvated conformation for each of the investigated dipeptide zwitterions. The CANDLE-solvated structures are obtained by exploring the conformational space of each dipeptide zwitterion concatenating DFT calculations, in vacuum, with classical molecular dynamics simulations, in explicit solvents, and DFT calculations including explicit water molecules. It is found that the energetically most favorable conformations are similar to those of the dipeptide zwitterions in their respective crystal structures. Such structural agreement is indicative of the DFT-CANDLE accomplishment of the description of solvated zwitterions, and suggests that these biomolecules self-assemble as quasi-rigid objects.

Establishing the Relationship between Quantum Capacitance and Softness of N-Doped Graphene/Electrolyte Interfaces within the Density Functional Theory Grand Canonical Kohn-Sham Formalism.

The joint density functional theory (JDFT) is applied in the context of the grand canonical Kohn-Sham theory to calculate the global and local softness of pristine and N-substituted graphene structures. A comparison is established between the different theoretical approaches to evaluate total capacitance, revealing that the JDFT approach presents the closest result of this property with experimental data. A model of series capacitors is used to determine the quantum and nonquantum contributions of total capacitance, which enables us to determine the limitations of the rigid band approximation for the studied systems. It is found that global chemical softness is proportional to the total capacitance measured in the experiments, when the geometry relaxation is neglected. In this context, it is possible to obtain quantum and total capacitance (and consequently softness) from an average number of electrons vs applied potential plots and the model of series capacitors. Likewise, the relation of capacitance and softness gives rise to a new definition of local capacitance within the JDFT formalism. The evaluation of global and local softness paves the way to analyze electrochemical surface reactivity as a function of applied potential for a solid-electrolyte interface.