(Phys.org)—Macrocyclic and chelating molecules are important in physiological processes and pharmaceuticals. Hemoglobin, for example, is a chelating molecule in which iron is coordinated to donor atoms in a porphyrin ring. Often the donor atoms are some combination of nitrogen, oxygen, phosphorous, and sulfur. For example, nitrogen serves as the donor atoms in hemoglobin. Sometimes carbon acts as a donor atom, but it is usually as a heterocycle with one of these other atoms.