Tautomeric Effect of Histidine on the Monomeric Structure of Amyloid β‑Peptide(1–42)

Tautomeric state of histidine is one of the factors that influence the structural and aggregation properties of amyloid β (Aβ)-peptide in neutral state. It is worth it to uncover the monomeric properties of Aβ(1–42) peptide in comparison with Aβ(1–40) peptide. Our replica-exchange molecular dynamics simulations results show that the sheet content of each tautomeric isomer in Aβ(1–42) monomer is slightly higher than that in Aβ(1–40) monomer except His6­(δ)-His13­(δ)-His14­(δ) (δδδ) isomer, implying higher aggregation tendency in Aβ­(1–42), which is in agreement with previous experimental and theoretical studies. Further analysis indicates that (εεε), (εδε), (εδδ), and (δδε) isomers prefer sheet conformation although they are in nondominating states. Particularly, it is confirmed that antiparallel β-sheets of (εδδ) were formed at K16-E22 (22.0–43.9%), N27-A30 except G29 (21.9–40.2%), and M35-I41 except G37 (24.1–43.4%). Furthermore, (εδδ) may be the easiest one to overcome structural transformation due to nonobstructing interactions between K16 and/or L17 and histidine residues. The current study will help to understand the tautomeric effect of Aβ(1–42) peptide to overcome Alzheimer’s disease

Zinc-Porphyrin Based Dyes for Dye-Sensitized Solar Cells

We have designed seven efficient sensitizers based on the zinc-porphyrin structure for dye sensitized solar cells (DSSCs). The geometries, electronic properties, light harvesting efficiency (LHE), and electronic absorption spectra of these sensitizers are studied using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. We found that the designed sensitizers have smaller HOMO–LUMO energy with broadened and red-shifted absorption bands (300–1100 nm) having high molar extinction coefficient compared to the so far known best sensitizer (YD2-o-C8). The position of HOMO–LUMO energy level of these sensitizers ensures a positive effect on the process of electron injection and dye regeneration. Our theoretical calculations reveal that the new sensitizer can be used as a potential sensitizer for DSSCs compared to YD2-o-C8

Computational Study on Removal of Epoxide from Narrow Zigzag Graphene Nanoribbons

We performed computational studies on the removal of an epoxide group from oxidized zigzag graphene nanoribbons (ZGNRs). Using density functional theory (DFT) calculations, we investigated the reaction mechanisms for the two competing processes, migration and reduction of an epoxide group in the middle of a narrow ZGNR. We found that the relative magnitudes of the barriers for migration and reduction depend on the width of the ZGNR. The reaction barrier for the reduction of an epoxide by CO decreased as the width of the ZGNR increased, while the barrier for migration showed the opposite trend. Moreover, the transition state energies for migration and reduction decreased upon the applied electric field perpendicular to the surface of the ZGNR. Our results illustrate that the removal of an epoxide from a ZGNR by reduction with CO can be facilitated by the application of an external electric field. For narrow ZGNRs, epoxide migration to neighboring sites may compete with the reduction reaction

Doping Effect on Edge-Terminated Ferromagnetic Graphene Nanoribbons

The doping effect on intramolecular magnetic exchange coupling of an edge-terminated zigzag graphene nanoribbon (ZGNR) with organic radicals was studied with density functional theory calculation. We investigated magnetic behaviors of boron (B)- and nitrogen (N)-doped ZGNRs, terminated with trimethylenemethane (TMM) and 6-oxoverdazyl (OVER) radicals, that is, TMM-ZGNR-TMM, OVER-ZGNR-OVER, and TMM-ZGNR-OVER. A doping with B or N on the spin-coupling pathway of radical-ZGNR-radical changed the spin distribution pattern of each system and hence its magnetic ground configuration, magnetic coupling strength, and magnetic moment. The first doping switched the magnetic ground configuration of a system from antiferromagnetic (AFM) to ferromagnetic (FM) and vice versa. An additional doping switched it back to its original magnetic ground configuration. Moreover, N doping on a radical-terminated edge increased the magnetic coupling strength as compared with the undoped system, while B doping decreased it. Furthermore, B or N doping on a TMM-terminated edge increased the magnetic moment of the system, while the same doping on an OVER-terminated edge decreased it. Our results demonstrate a possibility of reversible spin control of organic magnetic materials from AFM to FM and vice versa by chemical doping and the enhancement of the magnetic coupling strength of edge-terminated ZGNRs

Tuning of the Band Structures of Zigzag Graphene Nanoribbons by an Electric Field and Adsorption of Pyridine and BF₃: A DFT Study

The influence of pyridine adsorption and the applied electric field on the band structure and metallicity of zigzag graphene nanoribbons (ZGNRs) was investigated by using density functional theory (DFT) calculations. The semiconducting ZGNRs became half-metallic or remained semiconducting depending on the configuration of N–C covalent bonds between pyridine and the outermost C atom of the ZGNRs. In addition, the band gap of the α- and β-spin states of the ZGNRs could be tuned by noncovalent bonds. This effect was enhanced when BF₃ was introduced simultaneously at the opposite edge. The applied external electric field also modulated the band structures of the ZGNRs, making them half-metallic or semiconducting to some extent. These features suggest that the well-arranged adsorption of pyridine and BF₃ could be used to tune the band structures of nanoscale electronic devices based on graphene

Scaling Approach for Intramolecular Magnetic Coupling Constants of Organic Diradicals

The intramolecular magnetic coupling constants (<i>J</i>) of 9 diradicals (<b>i</b>–<b>ix</b>) coupled with an aromatic ring were investigated by means of unrestricted density functional theory (DFT) calculations [UB3LYP/6-311++G­(d,p)]. For these diradicals, a remarkable linear relationship between the calculated and experimental <i>J</i> values was found. In this study, we suggest that the slope (0.380) of the linear relationship can be utilized as a scaling factor for estimating <i>J</i> values. By applying this scaling factor and calculating <i>J</i> values, we could predict the reliable <i>J</i> values of four dithiadiazolyl (<b>DTDA</b>) diradicals coupled with an aromatic ring. It was also found that this scaling scheme shows a dependence on the length of a coupler. Nevertheless, this scaling approach could be used to estimate <i>J</i> values for diverse diradical systems coupled with a particular coupler by DFT calculations

Tautomeric Effect of Histidine on the Monomeric Structure of Amyloid β‑Peptide(1–40)

Histidine state (deprotonated, neutral, and protonated) is considered an important factor influencing the structural properties and aggregation mechanisms in amyloid β-peptides (Aβ), which are associated with the pathogenesis of Alzheimer’s disease. Understanding the structural properties and aggregation mechanisms is a great challenge because two forms (the N^ε–H or N^δ–H tautomer) can exist in the free neutral state of histidine. Here, replica-exchange molecular dynamics simulation was performed to elucidate the changes in structure and the mechanism of aggregation influenced by tautomeric behaviors of histidine in Aβ(1–40). Our results show that sheet-dominating conformations can be found in the His6­(δ)–His13­(δ)–His14­(δ) (δδδ) isomer with significant antiparallel sheet structures between R5–D7 and L34–G38, as well as between L17–F20 and L34–G38, implying that a new aggregation mechanism may exist to promote the generation of oligomers and/or aggregates. This work is helpful in understanding the fundamental tautomeric behaviors of neutral histidine in the process of aggregation

Zn²⁺ Effect on Structure and Residual Hydrophobicity of Amyloid β‑Peptide Monomers

The aggregation of amyloid β-peptide (Aβ peptide) has been associated with the pathogenesis of Alzheimer’s disease (AD). In the present study, we aimed to disclose how Zn²⁺ affects the Aβ aggregation in detail. Thus, molecular dynamics simulation was implemented to elucidate the changes of structure and residual hydrophobicity upon Zn²⁺ coordination. Our results show that Zn²⁺ can strongly influence the structural properties of Aβ40 and Aβ42 by reducing helical formation and increasing turn formation to expose the hydrophobic regions. Furthermore, hydrophobicity of Zn²⁺-Aβ40 and Zn²⁺-Aβ42 was much higher than that of each monomer, since Zn²⁺ binding can significantly influence the hydrophilic domains of Aβ. The further analyses indicate that not only four residues (H6, E11, H13, and H14) but also R5, D7, K16, K28, and terminal residues influence hydrophobicity upon Zn²⁺ coordination. Importantly, R5, K16, and K28 play a crucial role to regulate solvation-free energies. This work is helpful to understand the fundamental role of Zn²⁺ in aggregation, which could be useful for further development of new drugs to inhibit Zn²⁺-Aβ aggregation

Factors Affecting Vote for the Conservative Presidential Candidate among Party Neutrals.

<p>Factors Affecting Vote for the Conservative Presidential Candidate among Party Neutrals.</p

Catalytic Mechanism for the Ruthenium-Complex-Catalyzed Synthesis of Amides from Alcohols and Amines: A DFT Study

Details of the reaction mechanism for the Ru–PNN pincer complex catalyzed amidation from an alcohol and an amine proposed by Milstein et al. was elucidated using M06 density functional theory calculations. In addition, the bifunctional double hydrogen transfer (BDHT) mechanism for the dehydrogenative oxidation step was investigated for comparison. Finally, the BDHT mechanism was found to be preferred over the β-H elimination pathway that was proposed by Milstein et al. On the basis of the analysis of NBO charges and orbital interactions of intermediates and transition states, we designed a new catalyst with the addition of an electron-donating substituent (−NEt₂), which provided much reduced energy barriers and a lower potential energy surface along both mechanisms