Download M-DNA: synthesis, chemical structure and physical properties

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.