Shin’ichi Higai

Industry meets theory: computational R & D for innovative products

We introduce our activities for applications of first-principles theoretical calculations to various research and development processes for innovative electronic products. We present our recent selected results for four kinds of materials, which are the rare-earth element doped ceramic dielectric material BaTiO(3), BaTiO(3) ceramic nanoclusters and alkali-metal Li storage materials, i.e., graphite and sulfide Li(x)FeS(2).

Effect of D23N mutation on the dimer conformation of amyloid β-proteins: Ab initio molecular simulations in water

The molecular pathogenesis of Alzheimers disease (AD) is deeply involved in aggregations of amyloid β-proteins (Aβ) in a diseased brain. The recent experimental studies indicated that the mutation of Asp23 by Asn (D23N) within the coding sequence of Aβ increases the risk for the pathogeny of cerebral amyloid angiopathy and early-onset familial ADs. Fibrils of the D23N mutated Aβs can form both parallel and antiparallel structures, and the parallel one is considered to be associated with the pathogeny. However, the structure and the aggregation mechanism of the mutated Aβ fibrils are not elucidated at atomic and electronic levels. We here investigated solvated structures of the two types of Aβ dimers, each of which is composed of the wild-type or the D23N mutated Aβ, using classical molecular mechanics and ab initio fragment molecular orbital (FMO) methods, in order to reveal the effect of the D23N mutation on the structure of Aβ dimer as well as the specific interactions between the Aβ monomers. The results elucidate that the effect of the D23N mutation is significant for the parallel structure of Aβ dimer and that the solvating water molecules around the Aβ dimer have significant contribution to the stability of Aβ dimer.

Off-centering of rare-earth ion in (Ba,R)(Ti,Mg)O3 (R = Gd, Dy)

To investigate the off-centering behavior of rare-earth ions that were partially substituted for A site ions in ABO3-type crystals, accurate crystal structure analyses of (Ba,R)(Ti,Mg)O3 (R = Gd, Dy) were performed by high-energy synchrotron radiation X-ray powder diffraction analysis and the maximum entropy method (MEM)/Rietveld method. The R ions were observed at the six equivalent off-center sites in the 〈100〉 directions from the corner of a unit cell in the cubic phase. In the tetragonal phase, the R ions occupied one off-center site along the [001] direction. The R off-centering can be attributed to the covalent nature of the bond between the R and O ions in (Ba,R)(Ti,Mg)O3 crystals.

Proposal for an inhibitor of Alzheimer's disease blocking aggregation of amyloid-β peptides: ab initio molecular simulations

Aggregation of amyloid-β (Aβ) peptides is believed to play a key role in the mechanism of molecular pathogenesis of Alzheimers disease (AD). To inhibit the aggregation and prevent AD, numerous compounds have been synthesized. A previous experimental study elucidated that a triazine derivative AA3E2 has anti-amyloidogenic ability, while a triazine derivative AA3D2 having a different substituent has no inhibitory effect. However, the reason for this remarkable difference in the ability cannot be explained by the chemical structures of these derivatives. In the present study, we present stable structures of the solvated complexes with Aβ and AA3E2/AA3D2 obtained by classical molecular mechanics method. The specific interactions between Aβ and AA3E2/AA3D2 in the complexes are investigated by ab initio fragment molecular orbital calculations. Based on the results obtained, we attempt to propose new potent inhibitors for the Aβ aggregation.

Structure fluctuation in Gd- and Mg-substituted BaTiO3 with cubic structure

Gd- and Mg-substituted barium titanate crystals (Ba1− x Gd x )(Ti1− x /2Mg x /2)O3 with the Gd composition range of x > 0.1 show no ferroelectric phase transition. To discuss atomic thermal fluctuations in such unusual barium titanate crystals with cubic symmetry, accurate crystal structure analyses were performed by the maximum entropy method (MEM)/Rietveld method with the split-atom model and anharmonic temperature factor model using high-energy synchrotron radiation X-ray diffraction data. The off-centered positions of Gd ions at the Ba site in the probability density function were clearly observed at a low temperature but smeared in the high-temperature region by the over-barrier motion of Gd ions.

Theoretical study on low dielectric loss perovskites Ba(Zn1/3Ta2/3)O3 by first-principles calculations

We performed first-principles theoretical calculations on microwave dielectric compounds Ba(Zn1/3Ta2/3)O3, to clarify the origin of high Q characteristics in the Zn/Ta ordered structure. It was found that Zn atoms suppress the intrinsic ferroelectric nature of Slater mode vibration in TaO6 octahedra. This results in the improvement of harmonicity in lattice vibration. There are two important mechanisms. One is the compression of TaO6 octahedra by adjacent ZnO6, and then the double-well potential that TaO6 originally has is narrowed to be single-well with higher harmonicity. The other is the weakening of the covalent character of Ta–O bonds by adjacent Zn atoms, which is the origin of the double-well potential. In the fully ordered Ba(Zn1/3Ta2/3)O3, all Ta atoms are adjacent to Zn atoms, where Zn atoms have the maximum effect on improving harmonicity. Thus, highly ordered Ba(Zn1/3Ta2/3)O3 with prolonged heat treatment exhibits a high Q value.