'Chirality switching' is one of the most important chemical processes controlling many biological systems. DNAs and proteins often work as time-programmed functional helices, in which specific external stimuli alter the helical direction and tune the time scale of subsequent events. Although a variety of organic foldamers and their hybrids with natural helices have been developed, we highlight coordination chemistry strategies for development of structurally and functionally defined metal helicates. These metal helicates have characteristic coordination geometries, redox reactivities and spectroscopic/magnetic properties as well as complex chiralities. Several kinds of inert metal helicates maintain rigid helical structures and their stereoisomers are separable by optical resolution techniques, while labile metal helicates offer dynamic inversion of their helical structures via non-covalent interactions with external chemical signals. The latter particularly have dynamically ordered helical structures, which are controlled by the combinations of metal centres and chiral ligands. They further function as time-programmable switches of chirality-derived dynamic rotations, translations, stretching and shape flipping, which are useful applications in nanoscience and related technology.

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Three zinc(II) complexes of picolinic acid and its derivatives with a Zn(N2O2) coordination mode were prepared and evaluated for their insulinomimetic activities by in vitro and in vivo studies. By introducing an electron-donating methyl group into the picolinate ligand (pic), bis(6- or 3-methylpicolinato)zinc(II) complexes [Zn(6-mpa)2 or Zn(3-mpa)2, respectively] were prepared. The Zn(6-mpa)(2) complex was crystallized as a water adduct [Zn(6-mpa)2(H2O)].H2O, in which two carboxylate oxygens and two pyridine nitrogens of 6-mpa and a water oxygen coordinate to a zinc(II) with a trigonal bipyramidal geometry. By in vitro evaluation of the inhibition of free fatty acid (FFA) release from isolated rat adipocytes in the presence of epinephrine, the insulinomimetic activities of Zn(pic)2, Zn(6-mpa)2, and Zn(3-mpa)2 (IC50=0.64 +/- 0.13, 0.31 +/- 0.05, and 0.40 +/- 0.07 mM, respectively) were found to be higher than those of VOSO(4) (IC50=1.00 mM) and ZnSO(4) (IC50=1.58 +/- 0.05 mM) in terms of IC50 value, the 50% inhibition concentrations for the FFA release from the adipocytes. Then, Zn(6-mpa)2, which exhibited the highest in vitro insulinomimetic activity among three complexes examined, was given at a dose of 3.0 mg (45.9 micromol) Zn/kg body weight to KK-A(y) mice with type 2 diabetes mellitus by daily intraperitoneal injections for 14 days and it was found that the hereditary high blood glucose levels were lowered during the administration of the complex. The improvement of diabetes mellitus was confirmed with the oral glucose tolerance test.

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The cis-dioxo-molybdenum(VI) complexes, [MoO2(L(H))2]2- (1b), [MoO2(L(S))(2)]2- (2b), and [MoO2(L(O))2]2- (3b) (L(H) = cyclohexene-1,2-dithiolate, L(S) = 2,3-dihydro-2H-thiopyran-4,5-dithiolate, and L(O) = 2,3-dihydro-2H-pyran-4,5-dithiolate), with new aliphatic dithiolene ligands were prepared and investigated by infrared (IR) and UV-vis spectroscopic and electrochemical methods. The mono-oxo-molybdenum(IV) complexes, [MoO(L(H))2]2- (1a), [MoO(L(S))2]2- (2a), and [MoO(L(O))2]2- (3a), were further characterized by X-ray crystal structural determinations. The IR and resonance Raman spectroscopic studies suggested that these cis-dioxo molybdenum(VI) complexes (1b-3b) had weaker Mo=O bonds than the common Mo(VI)O2 complexes. Complexes 1b-3b also exhibited strong absorption bands in the visible regions assigned as charge-transfer bands from the dithiolene ligands to the cis-MoO2 cores. Because the oxygen atoms of the cis-Mo(VI)O2 cores are relatively nucleophilic, these complexes were unstable in protic solvents and protonation might occur to produce Mo(VI)O(OH), as observed with the oxidized state of arsenite oxidase.

Dynamic and efficient inversion of peptide helices by an achiral NO3- anion was programmed in terms of time scale from milliseconds to hours.

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The helical chirality of a Co(II) complex with a chiral tetradentate ligand is completely inverted from Lambda to Delta by the addition of achiral NO3- anion as an external stimulus.