RESUMO
Nematic liquid crystals (LCs) are known to transform into chiral nematic LCs with helical structures upon doping with enantiomeric compounds (called chiral dopants or guests). Here, we investigated the mechanism of host LC- and guest-dependent helix inversion by using octahedral metal complexes as guests. The helical twisting powers (HTPs/µm-1) of eight metal complexes with Δ, Λ chirality were examined in five nematic LC hosts. For example, Δ-[Ru(acac)2(trop)] (Ru-trop, Hacac = 2,4-pentanedione, Htrop = tropolone) induces a left-handed ( M) helix upon doping with N-(4-methoxybenzylidene)-4-butylaniline (MBBA), whereas it induces an opposite right-handed ( P) helix in 4-cyano-4'-pentylbiphenyl (5CB). The monobrominated complex of Ru-trop, Δ-[Ru(acac)2(Brtrop)] (Ru-Br1trop, HBr1trop = 5-bromotropolone), induces P helices in both MBBA and 5CB, whereas the dibrominated complex, Δ-[Ru(acac)2(Br2trop)] (Ru-Br2trop, HBr2trop = 3,7-dibromotroplone) induces M helices in both the media. The molecular dynamics simulation performed in parallel confirmed the drastic effect of the bromo groups on the microscopic ordering direction of guests in nematics. Further, HTPs of all Δ isomers of metal complexes investigated were found to shift in the positive direction as the dielectric constant anisotropy (Δε) of the host LCs increases (and vice versa for Λ isomers). These experimental and calculated results highlight the interplay of steric (excluded volume effect) and electrostatic (dipole-dielectric body) interactions between the host LCs and guest metal complexes.
RESUMO
Oxidation of low-density lipoprotein (LDL) by reactive oxygen species (ROS) and reactive nitrogen species (RNS) has been suggested to be involved in the onset of atherosclerosis. Oolong tea contains unique polyphenols including oolonghomobisflavan A (OFA). In this study, the effects of OFA on LDL oxidation by ROS and RNS were investigated in vitro. OFA suppressed formation of cholesterol ester hydroperoxides in LDL oxidized by peroxyl radical and peroxynitrite, and formation of thiobarbituric acid reactive substances in LDL oxidized by Cu2+. In addition, OFA inhibited fragmentation, carbonylation, and nitration of apolipoprotein B-100 (apo B-100) in the oxidized LDL, in which heparin-binding activity of apo B-100 was protected by OFA. Our results suggest that OFA exhibits antioxidant activity against both lipid peroxidation and oxidative modification of apo B-100 in LDL oxidized by ROS and RNS. Polyphenols in oolong tea may prevent atherosclerosis by reducing oxidative stress.