Sharon J. Nieter Burgmayer

DNA binding by Ru(II)–bis(bipyridine)–pteridinyl complexes

The interactions of five bis(bipyridyl) Ru(II) complexes of pteridinyl-phenanthroline ligands with calf thymus DNA have been studied. The pteridinyl extensions were selected to provide hydrogen-bonding patterns complementary to the purine and pyrimidine bases of DNA and RNA. The study includes three new complexes [Ru(bpy)2(L-pterin)]2+, [Ru(bpy)2(L-amino)]2+, and [Ru(bpy)2(L-diamino)]2+ (bpy is 2,2′-bipyridine and L-pterin, L-amino, and L-diamino are phenanthroline fused to pterin, 4-aminopteridine, and 2,4-diaminopteridine), two previously reported complexes [Ru(bpy)2(L-allox)]2+ and [Ru(bpy)2(L-Me2allox)]2+ (L-allox and L-Me2allox are phenanthroline fused to alloxazine and 1,3-dimethyalloxazine), the well-known DNA intercalator [Ru(bpy)2(dppz)]2+ (dppz is dipyridophenazine), and the negative control [Ru(bpy)3]2+. Reported are the syntheses of the three new Ru–pteridinyl complexes and the results of calf thymus DNA binding experiments as probed by absorption and fluorescence spectroscopy, viscometry, and thermal denaturation titrations. All Ru–pteridine complexes bind to DNA via an intercalative mode of comparable strength. Two of these four complexes—[Ru(bpy)2(L-pterin)]2+ and [Ru(bpy)2(L-allox)]2+—exhibit biphasic DNA melting curves interpreted as reflecting exceptionally stable surface binding. Three new complexes—[Ru(bpy)2(L-diamino)]2+, [Ru(bpy)2(L-amino)]2 and [Ru(bpy)2(L-pterin)]2+—behave as DNA molecular “light switches.”

Redox reactions of the pyranopterin system of the molybdenum cofactor

This work provides the first extensive study of the redox reactivity of the pyranopterin system that is a component of the catalytic site of all molybdenum and tungsten enzymes possessing molybdopterin. The pyranopterin system possesses certain characteristics typical of tetrahydropterins, such as a reduced pyrazine ring; however, it behaves as a dihydropterin in redox reactions with oxidants. Titrations using ferricyanide and dichloroindophenol (DCIP) prove a 2e−/2H+ stoichiometry for pyranopterin oxidations. Oxidations of pyranopterin by Fe(CN)63− or DCIP are slower than tetrahydropterin oxidation under a variety of conditions, but are considerably faster than observed for oxidations of dihydropterin. The rate of pyranopterin oxidation by DCIP was studied in a variety of media. In aqueous buffered solution the pyranopterin oxidation rate has minimal pH dependence, whereas the rate of tetrahydropterin oxidation decreases 100-fold over the pH range 7.4–8.5. Although pyranopterin reacts as a dihydropterin with oxidants, it resists further reduction to a tetrahydropterin. No reduction was achieved by catalytic hydrogenation, even after several days. The reducing ability of the commonly used biological reductants dithionite and methyl viologen radical cation was investigated, but experiments showed no evidence of pyranopterin reduction by any of these reducing agents. This study illustrates the dual personalities of pyranopterin and underscores the unique place that the pyranopterin system holds in the spectrum of pterin redox reactions. The work presented here has important implications for understanding the biosynthesis and reaction chemistry of the pyranopterin cofactor in molybdenum and tungsten enzymes.

Noninnocent Dithiolene Ligands: A New Oxomolybdenum Complex Possessing a Donor−Acceptor Dithiolene Ligand

A new monoanionic dithiolene ligand is found in Tp*MoO(S(2)BMOQO). A combination of X-ray crystallography, electronic absorption spectroscopy, resonance Raman spectroscopy, and bonding calculations reveal that the monoanionic dithiolene ligand possesses considerable thiolate-thione character resulting from an admixture of an intraligand charge transfer excited state into the ground state wave function. The unusual dithiolene exhibits a highly versatile donor-acceptor character that dramatically affects the Mo(IV/V) redox couple and points to a potentially noninnocent role of the pterin fragment in pyranopterin Mo enzymes.

Study of molybdenum(4+) quinoxalyldithiolenes as models for the noninnocent pyranopterin in the molybdenum cofactor.

A model system for the molybdenum cofactor has been developed that illustrates the noninnocent behavior of an N-heterocycle appended to a dithiolene chelate on molybdenum. The pyranopterin of the molybdenum cofactor is modeled by a quinoxalyldithiolene ligand (S(2)BMOQO) formed from the reaction of molybdenum tetrasulfide and quinoxalylalkyne. The resulting complexes TEA[Tp*MoX(S(2)BMOQO)] [1, X = S; 3, X = O; TEA = tetraethylammonium; Tp* = hydrotris(3,5-dimethylpyrazolyl)borate] undergo a dehydration-driven intramolecular cyclization within quinoxalyldithiolene, forming Tp*MoX(pyrrolo-S(2)BMOQO) (2, X = S; 4, X = O). 4 can be oxidized by one electron to produce the molybdenum(5+) complex 5. In a preliminary report of this work, evidence from X-ray crystallography, electronic absorption and resonance Raman spectroscopies, and density functional theory (DFT) bonding calculations revealed that 4 possesses an unusual asymmetric dithiolene chelate with significant thione-thiolate character. The results described here provide a detailed description of the reaction conditions that lead to the formation of 4. Data from cyclic voltammetry, additional DFT calculations, and several spectroscopic methods (IR, electronic absorption, resonance Raman, and electron paramagnetic resonance) have been used to characterize the properties of members in this suite of five Mo(S(2)BMOQO) complexes and further substantiate the highly electron-withdrawing character of the pyrrolo-S(2)BMOQO ligand in 2, 4, and 5. This study of the unique noninnocent ligand S(2)BMOQO provides examples of the roles that the N-heterocycle pterin can play as an essential part of the molybdenum cofactor. The versatile nature of a dithiolene appended by heterocycles may aid in modulating the redox processes of the molybdenum center during the course of enzyme catalysis.

Structure and reversible pyran formation in molybdenum pyranopterin dithiolene models of the molybdenum cofactor.

The syntheses and X-ray structures of two molybdenum pyranopterin dithiolene complexes in biologically relevant Mo(4+) and Mo(5+) states are reported. Crystallography reveals that these complexes possess a pyran ring formed through a spontaneous cyclization reaction of a dithiolene side-chain hydroxyl group at a C═N bond of the pterin. NMR data on the Mo(4+) complex suggest that a reversible pyran ring cyclization occurs in solution. These results provide experimental evidence that the pyranopterin dithiolene ligand in molybdenum and tungsten enzymes could participate in catalysis through dynamic processes modulated by the protein.

Pteridine cleavage facilitates DNA photocleavage by Ru(II) polypyridyl compounds.

The synthesis, characterization, binding to calf thymus DNA, and plasmid DNA photocleavage studies of two ruthenium(II) pteridinylphenanthroline complexes are reported where the new pteridinylphenantholine ligands in these complexes are additions to a larger family designed to resemble DNA bases. [Ru(bpy)(2)(L-keto)](PF(6))(2)1 is synthesized from ligand substitution of Ru(bpy)(2)Cl(2) by 4-keto-pteridino[6,7-f]phenanthroline (L-keto). Increasing the reaction temperature during synthesis of 1 causes a ring scission of the L-keto ligand within the pyrimidine ring yielding a second Ru complex, [Ru(bpy)(2)(L-aap)](PF(6))(2)2 where L-aap is 2-amino-3-amidopyrazino[5,6-f]phenanthroline. The ring cleavage reaction is accompanied by the loss of one carbon in the pyrimidine ring. Complexes 1 and 2 are characterized by (1)H NMR, UV/visible absorption and FT-IR spectroscopies and by cyclic voltammetry, and these results are presented in comparison to the previously reported related complexes [Ru(bpy)(2)(L-allox)](PF(6))(2), [Ru(bpy)(2)(L-amino)](PF(6))(2), and [Ru(bpy)(2)(dppz)](PF(6))(2). In addition, 2 has been structurally characterized by X-ray diffraction. Both 1 and 2 are good intercalators of calf thymus DNA as determined by viscometry and binding constants obtained from absorption titrations. Only the ring-cleaved complex 2 exhibits a high degree of pBR322 plasmid photocleavage in contrast to the other pteridinyl-phenanthroline complexes, which exhibit no plasmid DNA photocleavage. Complex 1, however, decomposes in buffer forming the photocleaver 2, demonstrating that sample age and reactivity can affect observed photocleavage. Complex 2 appears to photocleave DNA through a singlet oxygen mechanism.

Recent developments in the study of molybdoenzyme models

Over the past two decades, a plethora of crystal structures of molybdenum enzymes has appeared in the literature providing a clearer picture of the enzymatic active sites and increasing the challenge to chemists to develop accurate models for those sites. In this minireview we discuss the most recent model studies aimed to reproduce detailed features of the pterin–dithiolene ligand, both as the uncoordinated form and as a chelate coordinated to molybdenum.

Solvent-Dependent Pyranopterin Cyclization in Molybdenum Cofactor Model Complexes

The conserved pterin dithiolene ligand that coordinates molybdenum (Mo) in the cofactor (Moco) of mononuclear Mo enzymes can exist in both a tricyclic pyranopterin dithiolene form and as a bicyclic pterin-dithiolene form as observed in protein crystal structures of several bacterial molybdoenzymes. Interconversion between the tricyclic and bicyclic forms via pyran scission and cyclization has been hypothesized to play a role in the catalytic mechanism of Moco. Therefore, understanding the interconversion between the tricyclic and bicyclic forms, a type of ring-chain tautomerism, is an important aspect of study to understand its role in catalysis. In this study, equilibrium constants (K(eq)) as well as enthalpy, entropy, and free energy values are obtained for pyran ring tautomerism exhibited by two Moco model complexes, namely, (Et4N)[Tp*Mo(O)(S2BMOPP)] (1) and (Et4N)[Tp*Mo(O)(S2PEOPP)] (2), as a solvent-dependent equilibrium process. Keq values obtained from (1)H NMR data in seven deuterated solvents show a correlation between solvent polarity and tautomer form, where solvents with higher polarity parameters favor the pyran form.

Implications of Pyran Cyclization and Pterin Conformation on Oxidized Forms of the Molybdenum Cofactor

The large family of mononuclear molybdenum and tungsten enzymes all possess the special ligand molybdopterin (MPT), which consists of a metal-binding dithiolene chelate covalently bound to a pyranopterin group. MPT pyran cyclization/scission processes have been proposed to modulate the reactivity of the metal center during catalysis. We have designed several small-molecule models for the Mo-MPT cofactor that allow detailed investigation into how pyran cyclization modulates electronic communication between the dithiolene and pterin moieties and how this cyclization alters the electronic environment of the molybdenum catalytic site. Using a combination of cyclic voltammetry, vibrational spectroscopy (FT-IR and rR), electronic absorption spectroscopy, and X-ray absorption spectroscopy, distinct changes in the Mo≡O stretching frequency, Mo(V/IV) reduction potential, and electronic structure across the pterin-dithiolene ligand are observed as a function of pyran ring closure. The results are significant, for they reveal that a dihydropyranopterin is electronically coupled into the Mo-dithiolene group due to a coplanar conformation of the pterin and dithiolene units, providing a mechanism for the electron-deficient pterin to modulate the Mo environment. A spectroscopic signature identified for the dihydropyranopterin-dithiolene ligand on Mo is a strong dithiolene → pterin charge transfer transition. In the absence of a pyran group bridge between pterin and dithiolene, the pterin rotates out of plane, largely decoupling the system. The results support a hypothesis that pyran cyclization/scission processes in MPT may function as a molecular switch to electronically couple and decouple the pterin and dithiolene to adjust the redox properties in certain pyranopterin molybdenum enzymes.

CHAPTER 2:Pterin-Inspired Model Compounds of Molybdenum Enzymes

The molybdenum cofactor at the catalytic heart of mononuclear molybdoenzymes comprises a molybdenum ion coordinated by one or two dithiolene ligands containing an N-heterocyclic structure known as pterin. Understanding the details of the unusual combination of molybdenum with pterin and dithiolene is the impetus behind employing model chemistry to investigate the cofactor’s broad redox capabilities. This chapter highlights the major efforts to synthesize pterin-containing models and study their chemical properties. The history of identification of the cofactor’s pyranopterin dithiolene ligand and the details of pterin redox chemistry are reviewed, followed by an account of the synthesis and analysis of pterin-inspired chemical models. The implications of these models’ chemical reactivity and redox features that provide a fundamental basis for understanding the molybdenum cofactor are included. In addition, we highlight the potential directions of the field.