NaDNA–bipyridyl-(ethylenediamine)platinum (II) complex: Structure in oriented wet-spun films and fibers

Complexes of NaDNA with bipyridyl-(ethylenediamine)platinum(II) (abbreviated $\left[\right(\mathrm{bipy}\left)\mathrm{Pt}\right(\mathrm{en}\left){]}^{2+}\right)$ in solid, oriented films, prepared with a wet-spinning method, have been studied using x-ray diffraction, elastic neutron scattering, two-dimensional magic-angle-spinning nuclear magnetic resonance (NMR), infrared (IR) linear dichroism, and IR absorption. All of these experiments indicate that the DNA in this complex is in the B conformation. The neutron diffraction experiments reveal that the rise per residue is $3.31\AA ,$ indicating that the $\left[\right(\mathrm{bipy}\left)\mathrm{Pt}\right(\mathrm{en}){]}^{2+}$ molecular ion causes a small distortion of the B conformation. The neutron data in the direction perpendicular to the helical axis are consistent with a centered orthorhombic unit cell with $a=22.65\mathrm{}\AA$ and $b=32.2\mathrm{}\AA .$ The NMR and IR experiments show that the orientation of phosphate groups in the $\mathrm{DNA}\cdot \left[\right(\mathrm{bipy}\left)\mathrm{Pt}\right(\mathrm{en}){]}^{2+}$ complex is the same as that observed for pure DNA in the B conformation. The IR experiments also show that the $\left[\right(\mathrm{bipy}\left)\mathrm{Pt}\right(\mathrm{en}){]}^{2+}$ molecular ion stabilizes the B conformation of DNA down to $59\%$ relative humidity, a low water activity. Mechanochemical experiments on wet-spun NaDNA fibers in $68\%$ ethanol with and without $\left[\right(\mathrm{bipy}\left)\mathrm{Pt}\right(\mathrm{en}){]}^{2+}$ reveal a $9\%$ elongation of the DNA fibers as the complex is formed.