On the origin of pure optical rotation in twisted-cross metamaterials

We present an experimental and computational study of the response of twisted-cross metamaterials that provide near dispersionless optical rotation across a broad band of frequencies from 19 GHz to 37 GHz. We compare two distinct geometries: firstly, a bilayer structure comprised of arrays of metallic crosses where the crosses in the second layer are twisted about the layer normal; and secondly where the second layer is replaced by the complementary to the original, i.e. an array of cross-shaped holes. Through numerical modelling we determine the origin of rotatory effects in these two structures. In both, pure optical rotation occurs in a frequency band between two transmission minima, where alignment of electric and magnetic dipole moments occurs. In the cross/cross metamaterial, the transmission minima occur at the symmetric and antisymmetric resonances of the coupled crosses. By contrast, in the cross/complementary-cross structure the transmission minima are associated with the dipole and quadrupole modes of the cross, the frequencies of which appear intrinsic to the cross layer alone. Hence the bandwidth of optical rotation is found to be relatively independent of layer separation.The authors wish to thank Dr. Simon Horsley and Prof. Roy Sambles for their helpful discussions. A.D.-R., J.C. and J.S.-D. acknowledge the support by the Ministerio de Economica y Competitividad of the Spanish government, and the European Union FEDER through project TEC2014-53088-C3-1-R. L.E.B. and A.P.H. acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for Doctoral Training in Electromagnetic Metamaterials (Grant No. EP/L015331/1). E.H. wishes to acknowledge support from the EPSRC (Grant No. EP/K041215/1).Barr, LE.; Díaz Rubio, A.; Tremain, B.; Carbonell Olivares, J.; Sánchez-Dehesa Moreno-Cid, J.; Hendry, E.; Hibbins, AP. (2016). On the origin of pure optical rotation in twisted-cross metamaterials. Scientific Reports. 6:30307-30307. https://doi.org/10.1038/srep30307S30307303076Li, Z. et al. Coupling effect between two adjacent chiral structure layers. Opt. Exp. 18, 5375–5383 (2010).Rogacheva, A. V., Fedotov, V. A., Schwanecke, A. S. & Zheludev, N. I. Giant gyrotropy due to electromagnetic-field coupling in a bilayered chiral structure. Phys. Rev. Lett. 97, 177401 (2006).Barron, L. D. Molecular Light Scattering and Optical Activity 2nd Ed, Cambridge University Press (2004).Li, Z. et al. Chiral metamaterials with negative refractive index based on four “U” split ring resonators. App. Phys. Lett. 97, 081901 (2010).Monzon, C. & Forester, D. W. Negative refraction and focusing of circularly polarized waves in optically active media. Phys. Rev. Lett. 95, 123904 (2005).Pendry, J. B. A chiral route to negative refraction. Science 306, 1353–1355 (2004).Gao, W. & Tam, W. Y. Optical activities in complementary double layers of six-armed metallic gammadion structures. J. Opt. 13, 015101 (2011).Dong, J., Zhou, J., Koschny, T. & Soukoulis, C. Bi-layer cross chiral structure with strong optical activity and negative refractive index. Opt. Exp. 17, 14172–14179 (2009).Zhou, J. et al. Negative refractive index due to chirality. Phys. Rev. B. 79, 121104 (2009).Decker, M. et al. Strong optical activity from twisted-cross photonic metamaterials. Opt. Lett. 34, 2501–2503 (2009).Li, Z., Alici, K. B., Colak, E. & Ozbay, E. Complementary chiral metamaterials with giant optical activity and negative refractive index. App. Phys. Lett. 98, 161907 (2011).Hannam, K., Powell, D. A., Shadrivov, I. V. & Kivshar, Y. S. Dispersionless optical activity in metamaterials. App. Phys. Lett. 102, 201121 (2013).Hannam, K., Powell, D. A., Shadrivov, I. V. & Kivshar, Y. S. Broadband chiral metamaterials with large optical activity. Phys. Rev. B. 89, 125105 (2014).Li, Y. & Hung, Y. Dispersion-free broadband optical polarisation based on helix photonic metamaterials. Opt. Exp. 23, 16772 (2015).Zhu, W., Rukhlenko, I. D., Huang, Y., Wen, G. & Premaratne, M. Wideband giant optical activity and negligible circular dichroism of near-infrared chiral metamaterial based on a complementary twisted configuration. J. Opt. 15, 125101 (2013).ANSYS Electromagnetics Suite Release 15.0, ANSYS Inc., Pittsburgh, USA URL http://www.ansys.com .Luk’yanchuk, B. et al. The Fano Resonance in Plasmonic Nanostructures and Metamaterials. Nat. Mat. 9, 707–715 (2010).Kenanakis, G., Economou, E. N., Soukoulis, C. M. & Kafesaki, M. Controlling THz and Far-IR Waves with Chiral and Bianisotropic Metamaterials. EPJ Appl. Meta. 2, 1–12 (2015).Genet, C. & Ebbesen, T. W. Light in Tiny Holes. Nature 445, 39–46 (2007).Grigorenko, A. N., Nitkin, P. I. & Kabashin, A. V. Phase jumps and interferometric surface plasmon resonance imaging. App. Phys. Lett. 75, 3917–3919 (1999).Gorkunov, M. V. et al. Implications of the causality principle for ultra chiral metamaterials. Sci. Rep. 5, 1–5 (2015)

Mlh2 Is an Accessory Factor for DNA Mismatch Repair in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the essential mismatch repair (MMR) endonuclease Mlh1-Pms1 forms foci promoted by Msh2-Msh6 or Msh2-Msh3 in response to mispaired bases. Here we analyzed the Mlh1-Mlh2 complex, whose role in MMR has been unclear. Mlh1-Mlh2 formed foci that often colocalized with and had a longer lifetime than Mlh1-Pms1 foci. Mlh1-Mlh2 foci were similar to Mlh1-Pms1 foci: they required mispair recognition by Msh2-Msh6, increased in response to increased mispairs or downstream defects in MMR, and formed after induction of DNA damage by phleomycin but not double-stranded breaks by I-SceI. Mlh1-Mlh2 could be recruited to mispair-containing DNA in vitro by either Msh2-Msh6 or Msh2-Msh3. Deletion of MLH2 caused a synergistic increase in mutation rate in combination with deletion of MSH6 or reduced expression of Pms1. Phylogenetic analysis demonstrated that the S. cerevisiae Mlh2 protein and the mammalian PMS1 protein are homologs. These results support a hypothesis that Mlh1-Mlh2 is a non-essential accessory factor that acts to enhance the activity of Mlh1-Pms1

Distinct DNA-binding surfaces in the ATPase and linker domains of MutLγ determine its substrate specificities and exert separable functions in meiotic recombination and mismatch repair

<div><p>Mlh1-Mlh3 (MutLγ) is a mismatch repair factor with a central role in formation of meiotic crossovers, presumably through resolution of double Holliday junctions. MutLγ has DNA-binding, nuclease, and ATPase activities, but how these relate to one another and to <i>in vivo</i> functions are unclear. Here, we combine biochemical and genetic analyses to characterize <i>Saccharomyces cerevisiae</i> MutLγ. Limited proteolysis and atomic force microscopy showed that purified recombinant MutLγ undergoes ATP-driven conformational changes. <i>In vitro</i>, MutLγ displayed separable DNA-binding activities toward Holliday junctions (HJ) and, surprisingly, single-stranded DNA (ssDNA), which was not predicted from current models. MutLγ bound DNA cooperatively, could bind multiple substrates simultaneously, and formed higher-order complexes. FeBABE hydroxyl radical footprinting indicated that the DNA-binding interfaces of MutLγ for ssDNA and HJ substrates only partially overlap. Most contacts with HJ substrates were located in the linker regions of MutLγ, whereas ssDNA contacts mapped within linker regions as well as the N-terminal ATPase domains. Using yeast genetic assays for mismatch repair and meiotic recombination, we found that mutations within different DNA-binding surfaces exert separable effects <i>in vivo</i>. For example, mutations within the Mlh1 linker conferred little or no meiotic phenotype but led to mismatch repair deficiency. Interestingly, mutations in the N-terminal domain of Mlh1 caused a stronger meiotic defect than <i>mlh1Δ</i>, suggesting that the mutant proteins retain an activity that interferes with alternative recombination pathways. Furthermore, <i>mlh3Δ</i> caused more chromosome missegregation than <i>mlh1Δ</i>, whereas <i>mlh1Δ</i> but not <i>mlh3Δ</i> partially alleviated meiotic defects of <i>msh5Δ</i> mutants. These findings illustrate functional differences between Mlh1 and Mlh3 during meiosis and suggest that their absence impinges on chromosome segregation not only via reduced formation of crossovers. Taken together, our results offer insights into the structure-function relationships of the MutLγ complex and reveal unanticipated genetic relationships between components of the meiotic recombination machinery.</p></div