Flow dichroism evidence for tilting of the bases when DNA is in solution

Flow linear dichroism studies of DNAs from Clostridium perfringens, Escherichia coli, and Micrococcus luteus are extended into the vacuum UV region to 175 nm. The short-wavelength pi pi region shows a less negative reduced linear dichroism than the well-studied long-wavelength pi pi region. These results are independent of the type of DNA and the concentration of salt over the range 0.001 to 1.0 M. This proves that the bases are not perpendicular to the flow orientation. Assuming that the DNA tends to align along its helix axis, the largest angle that the bases can make with the helix axis is 73 +/- 3 degrees.