Infrared spectra at room temperature under relative humidity (RH) of 0% and 90% have been measured on dry salmon-sperm DNA incorporating various metallic ions (M-DNA, M=Li, Na, Mg, Ca, Mn, Fe, and Zn); the effects of which are examined from the molecular-vibrational points of view. Among many absorption bands, the antisymmetric and symmetric stretching modes of the phosphate group (PO 2-) are found to show distinct spectral change depending on M. At RH=90%, the band shifts appear for both modes, reflecting that the magnitude of the electric dipole of PO 2- is affected by M. At RH=0%, a band splitting induced by the spatial anisotropy around PO 2- is detected in the antisymmetric stretching mode for Mg-DNA, Ca-DNA, Mn-DNA, Zn-DNA with divalent counterions, and Fe-DNA with trivalent counterions. The configurational change in M leads to these shifts and splittings; the divalent or trivalent ions do not connect to PO 2- but are incorporated into the base-pair sequence as already pointed out by the previous measurements of NMR, ESR, and fluorescence. In M-DNA with M=Mg, Ca, Mn, Fe, and Zn, a kink appears around 3100 cm-1 corresponding to the NH stretching, and additionally a slight minimum disappears around 310 cm-1. These phenomena are caused by a generation of N-M stretching vibrations due to the replacement of proton in the imino group to M.
|Physical Review B - Condensed Matter and Materials Physics
|Published - 2009 Jun 3