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Biophysical Journal 86:3822-3827 (2004)
© 2004 The Biophysical Society

Hydration of ds-DNA and ss-DNA by Neutron Quasielastic Scattering

M. Bastos *, V. Castro *, G. Mrevlishvili * and J. Teixeira {dagger}

* Centro de Investigação em Química (Universidade do Porto), Department of Chemistry, Faculty of Sciences, University of Porto, P-4169-007 Porto, Portugal; and {dagger} Laboratoire Léon Brillouin (Commissariat à I'Énergie Atomique-Centre National de la Recherche Scientific), CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France

Correspondence: Address reprint requests to Margarida Bastos, Dept. of Chemistry, R. Campo Alegre, 687 P-4169-007 Porto, Portugal. Tel.: 351-22-6082811; E-mail: mbastos{at}fc.up.pt.

Quasielastic neutron scattering measurements were performed in hydrated samples of ds-DNA and ss-DNA. The samples were hydrated in a high relative humidity atmosphere, and their final water content was 0.559 g H2O/g ds-DNA and 0.434 g H2O/g ss-DNA. The measurements were performed at 8 and 5.2 Å for the ds-DNA sample, and at 5.2 Å for the ss-DNA sample. The temperature was in both cases 298 K. Analysis of the obtained data indicates that in the ds-DNA sample we can distinguish two types of protons—those belonging to water molecules strongly attached to the ds-DNA surface and another fraction belonging to water that diffuses isotropically in a sphere of radius 2.8 Å, with a local diffusion coefficient of 2.2 x 10–5 cm2 s–1. For ss-DNA, on the other hand, no indication was found of motionally restricted or confined water. Further, the fraction of protons strongly attached to the ds-DNA surface corresponds to 0.16 g H2O/g ds-DNA, which equals the amount of water that is released by ds-DNA upon thermal denaturation, as studied by one of us (G.M.) by differential scanning calorimetry. This value also equals the difference between the critical hydration values of ds-DNA and ss-DNA, also determined by DSC. These results represent, thus, a completely independent measurement of water characteristics and behavior in ds- and ss-DNA at critical hydration values, and therefore substantiate the previous suggestions/conclusions of the results obtained by calorimetry.




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