Sub-areas of research:

Solar Energy Materials
  • photoreactive thione compounds
  • voltaic devices

Photovoltaic materials enable light-to-electrical energy conversion by converting photon energy to ong-lived excited state species, thus trapping light energy in chemical form. Such species are typically characterized by the emission from the excited state, and this is used to define the stored chemical energy. Less studied are those in which the excited state is truly dark, i.e. non-emitting. We have found a family of heterocycles related to maltol which are “pre-aromatic”, in that oxidation generates pseudo-aromatic tautomers. These compounds demonstrate unusual aromatic-like photoreactivity in the presence of electron acceptors.

Related Pt(II)(bpy) complexes of the N, S, O-heterocycles are also strong photoreductants with > microsecond emission lifetimes. Homoleptic Zn(II), In(III), and Ga(III) complexes of these compounds are also powerful photoreductants but without related emissions, i.e. “dark” photoproducts; transient absorbance results suggest these compounds “photoeject an e- to attain the pseudoaromatic state. We hope to explore the application of these N, S, O-heterocyclic thiomaltols and their metal complexes towards applications in dye-sensitized solar cells (the so-called Gratzel cells).

Selected publications on thione chelators and inorganic photochemistry:

  • "Photo- and thermal-induced linkage isomerizations in a peroxy-dithiocarbamate-Ru complex" Ng, S.; Walker, M.B.; Farmer, P.J. Inorg Chim Acta 2009, 362, 4013-4016.
  • “Unexpected C-H activation of Ru(II)-dithiomaltol complexes upon oxidation" Backlund, M.; Ziller, J.; Farmer, P.J. Inorg. Chem. 2008, 47, 2864-2870.
  • “Melanosomal damage in normal human melanocytes induced by UVB and metal uptake, a basis for the pro-oxidant state of melanoma” Gidanian, S.; Mentelle, M.; Meyskens, F.L. Jr., Farmer, P.J. Photochem. Photobio. 2008, 84, 556-564.
  • “A novel heterocyclic atom exchange reaction with Lawesson’s reagent: one-pot synthesis of dithiomaltol” Brayton, D.; Faith E. Jacobsen, F.E.; Cohen, S.M.; Farmer, P.J. Chem. Comm., 2006, 206 – 208.
  • “The Effect of Stacking and Redox State on Optical Absorption Spectra of Melanins- a comparison of theoretical and experimental results.” Stark, K.B.; Gallas, J.M.; Zajac G.W.; Golab, J.T.; Gidanian, S.; McIntire, T.; Farmer, P.J. J. Phys. Chem. B. 2005 ,109, 1970-1977.

Photoactive heme proteins

Harry Gray pioneered the use of photo-active Ru(diimine) complexes to initiate and measure the rates of electron transfers in metalloproteins, and it has become an important tool in bioinorganic chemistry. Such use is based on the redox activity of a long-lived (ca. 700 ns) triplet excited state formed by photo-excitation into a metal-to-ligand charge transfer band. The triplet state can act as both a reductant (E = -0.77 V) or an oxidant (E = +.86 V); flash/quench techniques allow the more powerful redox species Ru3+ (E = +1.26 V) or Ru1+ (E = -1.35 V) to be generated.

We use these methods to initiate and follow redox-induced transformations important to heme-based catalysis. Our approach is to hardwire the Ru complex directly to the heme active site -- as in the heme protein myoglobin shown below.

First, we synthesized a pendant-arm heme cofactor that can be reconstituted with apomyoglobin, reforming the native myoglobin structure but with a photoactive Ru complex covalently linked --or hardwired-- to the heme active site.

Reaction scheme for making photoactive heme cofactors

When the photo-active heme is reconstituted into apomyoglobin, a hybrid RuC7Mb is obtained, with an absorbance spectra virtually identical to the sum of normal Mb and Ru(bpy)32+. With this hybrid, photo-initiation can generate FeIV=O formation and look at the ability of the ferryl and porphyrin radical cation to oxidize nearby protein residues.

Selected publications on heme protein photochemistry:

  • “Photolysis of the HNO Adduct of Myoglobin: Transient Generation of the Aminoxyl Radical” Pervitsky, D.; Immoos, C.; van der Veer, W.; Farmer, P.J. J. Am. Chem. Soc. 2007, 129, 9590-9591.
  • ”Electron Transfer Chemistry of Ru-linker-(heme)-modified Myoglobin: Rapid Intraprotein Reduction of a Photogenerated Porphyrin Cation Radical” Immoos, C.E.; Di Bilio, A.J.; Cohen, M.S.; Van der Veer, W.; Gray, H.B.; Farmer, P.J. Inorg Chem. 2004, 43, 3593-3596.
  • “Electron Transfer in the Ruthenated Heme Domain of Cytochrome P450BM-3” Sevrioukova, I.F.; Immoos, C.E.; Poulos, T.L.; Farmer, P.J. Isr. J. Chem. 2000, 40, 47-53.

Reconstituted oxochlorins

We use similar methods to make new protein catalysts (artificial enzymes) using heme-altered cofactors . Inspired by the oxygenated heme of the cd1 nitrite reductases (see NiR), we synthesized an oxochlorin-reconstituted cytochrome c peroxidase, MpCcP, which was the first structurally characterized oxochlorin in a protein (Immoos, JIB. 2002). Although a mixture of R- and S- isomers of the oxochlorin were used, only the S-isomer is found reconstituted in the crystallized protein. The hybrid MpCcP retained 99% wildtype peroxidase activity with cytochrome c.

We have synthesized several oxochlorin derivatives and reconstituted them into myoglobin and cytochrome c peroxidase. The reconstituted hybrids are green, and display altered ligand-binding and redox catalytitic properties. We have crystallographically characterized CcP hybrids using two different regioisomers of mesopone-- in both cases only one enantiomer of the oxochlorin is observed, i.e. the reconstitution itself is enantiomerically selective.

Another uncommon aspect of NiR enzymes is the presence of tyrosine residues within the active site. To model this, a distal pocket His52 Tyr mutant of CcP was generated (Bhaskar, JMB. 2003), whose structure revealed a novel covalent bond between the Tyr52 and the indole ring nitrogen of Trp51. The crosslink was shown to result from Fe activation of peroxide, and similar metal-dependent crosslinks have been found in cytochrome c oxidase and tyrosinase metalloproteins. In this case, the cross-link C-N bond itself is unusual, in that the Trp N is highly pyramidalized, with the NE1Trp-CE1Tyr bond bent ca. 600 from the aromatic plane. This, to our knowledge, is the largest deformation of an alkylated indole yet observed.

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Selected publications on heme protein engineering:

  • “High Temperature Electrocatalysis Using Thermophilic P450 CYP119: Dehalogenation of CCl4 to CH4” Blair, E.; Greaves, J.; Farmer, P.J. J. Am. Chem. Soc. 2004, 126, 8632-8633.
  • “Electrocatalytic reductions of nitrite, nitric oxide and nitrous oxide by Cytochrome P450 CYP 119” Immoos, C.E.; Chou, J.; Bayachou, M.; Blair, E.; Greaves, J.; Farmer, P.J. J. Am. Chem. Soc. 2004, 126, 4934-4942.
  • “A Novel Heme and Peroxide-Dependent Tryptophan-Tyrosine Cross-link in a Mutant of Cytochrome c Peroxidase” Bhaskar, B.; Immoos, C.E.; Shimizu, H.; Sulc, F.; Farmer, P.J.; Poulos, T.L. J. Mol. Biol. 2003, 328, 157-166.
  • "Mesopone Cytochrome C Peroxidase: Functional Model of Heme Oxygenated Oxidases" Immoos, C.E.; Bhaskar, B.; Cohen, M.S.; Barrows, T.P.; Farmer, P.J.; Poulos, T.L. J. Inorg. Biochem. 2002, 91, 635-643.