  A key limitation of electron paramagnetic resonance (EPR), an established and
powerful tool for studying atomic-scale biomolecular structure and dynamics is
its poor sensitivity, samples containing in excess of 10^12 labeled
biomolecules are required in typical experiments. In contrast, single molecule
measurements provide improved insights into heterogeneous behaviors that can be
masked by ensemble measurements and are often essential for illuminating the
molecular mechanisms behind the function of a biomolecule. We report EPR
measurements of a single labeled biomolecule that merge these two powerful
techniques. We selectively label an individual double-stranded DNA molecule
with a single nanodiamond containing nitrogen-vacancy (NV) centers, and
optically detect the paramagnetic resonance of NV spins in the nanodiamond
probe. Analysis of the spectrum reveals that the nanodiamond probe has complete
rotational freedom and that the characteristic time scale for reorientation of
the nanodiamond probe is slow compared to the transverse spin relaxation time.
This demonstration of EPR spectroscopy of a single nanodiamond labeled DNA
provides the foundation for the development of single molecule magnetic
resonance studies of complex biomolecular systems.
