  DNA flexibility is a key determinant of biological function, from nucleosome
positioning to transcriptional regulation, motivating a direct measurement of
the bend-torque response of individual DNA molecules. In this work, DNA bending
is detected using a nano-mechanical torque balance formed by tethering a
ferromagnetic nanoparticle probe by an individual DNA molecule to a diamond
magnetic field imager. The torque exerted by the DNA in response to bending
caused by an applied magnetic torque is measured using wide-field imaging of
quantum defects near the surface of the diamond. Qualitative measurements of
differences in DNA bio-mechanical binding configuration are demonstrated, and
as a proof-of-principle, a quantitative measurement of the bend response is
made for individual DNA molecules. This quantum-enabled measurement approach
could be applied to characterize the bend response of biophysically relevant
short DNA molecules as well as the sequence dependence of DNA bending energy.
