Unveiling the three-dimensional structure of the green pigment of nitrite-cured meat.

The 1.70 Å-resolution X-ray crystal structure of the green pigment occasionally present in nitrite-cured meat reveals a regiospecific nitration of the heme at the 2-vinyl position; the regiospecificity is favored on steric grounds, and results in movement of the 2-nitrovinyl moiety into the plane of the 24-atom heme macrocycle.