  The phase coherence relaxation times as long as $T_2\sim830-1030\pm20$ ns
were measured for the [2Fe-2S] cluster in the intrinsic protein environment.
This relaxation corresponds to a relatively long lasting coherence of the
low-spin $S=1/2$ state. For this biological cluster, the phase coherence
relaxation time was significantly affected by the nuclear hyperfine
interactions of $^{14}$N with $I=1$. After labeling the cluster environments
uniformly with the $^{15}$N isotope, $T_2$ exceeds $\sim1.1-1.4\pm0.1~\mu$s at
the canonical orientations. This is already an order of magnitude longer than
the duration of a single-spin qubit manipulation $\sim10-100$ ns. The transient
nutation experiment corresponds to the coherent manipulation of the electron
spin.
