ELECTRON TRANSFER IN
TRANSITION METAL-PTERIDINE SYSTEMS
Sharon J. Nieter Burgmayer
Department of Chemistry, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010,
e-mail: sburgmay@brynmawr.edu
The combination of a pterin and a transition metal in many enzymes is the motivation for exploring the chemistry of pteridine complexes in detail. Unlike other biological ligands for essential transition metals, pterin is unique in displaying multi-electron redox reactivity, an ability that resembles the redox capabilities of transition metals. It is perhaps because these two partners, metal and pterin, have this chemical similarity that their compounds defy traditional categorization by formal oxidation number. The result challenges the chemist to formulate fresh interpretations of these deceptively ordinary complexes.
This review concerns reports of metal-pterin complexes that appeared from early 1980's through 1996. In a few cases older literature is briefly mentioned to build a context for the newer work. The review comprises four sections. Section 1 introduces the pteridine family and its important contributions to biochemistry. Section 2 is devoted to studies of molybdenum(6+) complexes reacted with reduced pterins. Section 3 describes redox interactions between reduced pterins and the first row metals copper and iron. Finally Section 4 turns to a discussion of the electronic interactions in flavin complexes of various metals. An Epilogue closes the review.keywords: metalloenzyme, molybdenum, pteridine, tetrahydropterin
1 Introduction
1.1 Pterins and Pteridines
1.1.1 Structure
1.1.2 Pteridine Redox Relatives
1.2 Use of Pteridine Redox in Biological Systems
1.2.1 Aromatic Amino Acid Hydroxylases
1.2.2 Oxo-Transferases
1.2.3 Flavins
1.2.4 Folates
2 Synthetic Molybdenum-Pterin Systems
2.1 Dioxo-Molybdenum(6+) Reactions with Tetrahydropterins
2.1.1 Hypotheses
2.1.2 Preparation of Molybdenum Complexes of Reduced Pterins
2.1.3 Reactivity of Molybdenum Complexes of Reduced Pterins
2.1.4 Theoretical Approaches to Determining Formal Oxidation States
2.1.5 Requirements for the Synthesis of Molybdenum Complexes of Reduced Pterins
2.2 Complexes of Molybdenum Coordinated by Pterin-Substituted Dithiolene Ligands
2.3 Pterin Function in Molybdenum and Tungsten Enzymes
3 Model Systems for the Metal Site in Phenylalanine Hydroxylase
3.1 Copper Reactions with Tetrahydropterins
3.1.1 A Functional Model for the Copper Site in ChromatiumViolaceum.
3.1.2 Formation of a Cupric Complex of Tetrahydropterin
3.2 Iron Reactions with Tetrahydropterins
4 Transition Metal Complexes of Flavins
4.1 Early Flavin Studies
4.2 Ruthenium Flavin Chemistry
4.3 Molybdenum Complexes of Flavin and Pterin
5 Epilogue
6 References