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M-DNA: synthesis, chemical structure and physical properties Aleš Omerzu1, Bernarda Anželak2, Iztok Turel2, Janez Štrancar1 and Denis Arčon1 1Jozef 2Faculty Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia of Chemistry and Chemical Technology, Aškerčeva 5 , 1000 Ljubljana, Slovenia In 1993 Lee and co-workers (J. S. Lee et al., Biochem. Cell. Biol. 71, (1993) 162) have reported a discovery of novel DNA complexes which DNA forms with divalent transition metal cations Zn2+, Co2+ and Ni2+ at pH > 8. The new complex forms when an imino proton from A-T or G-C hydrogen bonds is replaced with the metal cation as confirmed by NMR measurements and pH titration. The presence of metal ions in DNA structure would reduce the total charge of the helix and consequently compact the helix. In other aspects, the structure of so called M-DNA doesn’t deviate much form that of B-DNA and can be reversibly converted back to B-DNA by adding a chelate ligand ethylenediaminetetraacetic acid (EDTA) which strongly binds M2+. The electron transfer and electric conductivity properties of M-DNA have been studied trough fluorescence quenching experiments and by electrochemical methods in solution. They all show a large increase in conductivity compared to native DNA. The only experiment on conductivity in dry M-DNA so far was an I-V characteristic measurement on dry M-DNA molecules suspended between electrical contacts. It has also suggested a metallic-like conduction in M-DNA but its mechanism has remained unknown. Our aim was to synthesise bulk quantities of dry M-DNA for spectroscopic measurements which would elucidate a mechanism of electronic conductivity in the complex. For that purpose we have employed a lyophilization method (freeze-drying) because no other method of extraction of a solid material from a solution can assure that zinc ions would remain in the interior of the double helix in a dry form. We have observed, by measuring a strong ESR signal, that zinc divalent cations Zn2+ (which are diamagnetic) when trapped in a dry DNA structure reduce to the monovalent state Zn+. The ESR linewidth (~ 1000 G), it’s asymmetric (dysonian) shape and the temperature dependence of the ESR `signal suggest that the zinc monocations, Zn + embedded in the DNA structure form an 1D metallic chain. We suggest that the metallic conductivity is realized through mobile unpaired 4s1 electrons from each Zn+ cation in the chain.