Before revision:  Motivated by a recent experiment reporting chiral transitions between right-handed B-DNA and left-handed Z-DNA  ,   we analyze the free energy for the particular case of linear DNA with an externally applied torque.
After revision:  Motivated by a recent experiment reporting chiral transitions between right-handed B-DNA and left-handed Z-DNA M. Lee, et. al., Proc. Natl. Acad. Sci. (USA) 107 , we analyze the free energy for the particular case of linear DNA with an externally applied torque.