Twist in Chiral Interaction between Biological Helices

Using an exact solution for the pair interaction potential, we show that long, rigid, chiral molecules with helical surface charge patterns have a preferential interaxial angle $\sim \sqrt{\mathrm{RH}}/L$, where $L$ is the length of the molecules, $R$ is the closest distance between their axes, and $H$ is the helical pitch. Estimates based on this formula suggest a solution for the puzzle of small interaxial angles in $\alpha$-helix bundles and in cholesteric phases of DNA.