engleski

Electrical transport in dilute DNA 146bp solutions

We present initial investigations of electrical transport in pure water DNA solutions (no buffer or added salt), complemented by the results of the study of the DNA polyion diffusion constant D obtained by fluorescence correlation spectroscopy (FCS). The chosen range of DNA concentrations was 0.01-10 mg/ml, i.e. mostly in the dilute regime to simplify the interpretation of the conductivity (the crossover concentration for these 50 nm DNA fragments, N=146 bp, is ~2 mg/ml). Conductivity was extracted as the frequency-independent component from the dielectric spectra. We reveal that molar conductivity, \Lambda_p of DNA polyion attains 50% higher value below 0.05 mg/ml than above 0.5 mg/ml. The results for solutions of previously denatured DNA lacked this conductivity crossover. The observed conductivity crossover is interpreted basing on the assumption that only the free counterions and the polyions (whose charge is reduced for a factor f due to Manning condensation) contribute to the conductivity. At the lowest concentrations, DNA may denature and f may increase (theoretically, for ssDNA f=0.59 and for dsDNA f=0.24). Simply, the increased charge on DNA polyions will cause the increase in conductivity. Thus, we suggest a decrease in counterion binding as the plausible mechanism for the conductivity crossover. In this manner, we demonstrate the ability of DNA molecule to maintain the native state even without added salt or buffer, however only for concentrations above 0.5 mg/ml – i.e. the ability of DNA to act as its own salt. We note that conductometry in conjunction with FCS (\Lambda_p~f•D) may be used to quantitate Manning parameter \eta=1/f. Finally, checking the diffusion constant directly by FCS might reveal whether DNA actually separates in two strands, or it’s a dynamical process where the strands separate transiently.

polyelectrolytes; diffusion

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