To answer this question, we mutated the proximal serine to cysteine in the engineered P411 and performed 200 ns of MD simulation. Interestingly, now, the tosyl azide ligand never approaches the heme-porphyrin during the entire 200 ns of simulation of cysteine ligated P411 complex. As can be seen in Figure , the average distance between Fe and N1 is ~7 Å and the lowest possible distance is 4.7 Å. In fact, the QM/MM optimization (see Figure ) also reveals that the ligand moves away from its original position by a large distance much the same as the MD results. To pinpoint the trigger of this change in the distance of Fe II ---TAZ when serine is replaced by cysteine, we plotted in Figure the molecular orbitals which are responsible for the Fe II -N1  bonds between the ferrous ion and TAZ. Thus, the serine-ligated complex exhibits a bond-making orbital which is well-located on the Fe II ion (see Figure ; the weight contribution of Fe to the dz 2 MO is 0.63). In contrast, the cysteine-ligated ferrous complex has a quintet ground spin state (cf.