1PIY:
reference: Variable coordination geometries at the diiron(II) active site of ribonucleotide reductase R2., Voegtli WC, Sommerhalter M, Saleh L, Baldwin J, Bollinger JM Jr, Rosenzweig AC, J Am Chem Soc 2003 Dec 24;125(51):15822-30. PMID: 14677973

This OCA Structure page
uses Jmol, developed by the Jmol Development Team (documentation).