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 , 4985 (2010) we analyze the free energy for the particular case of linear DNA with an externally applied torque.