Applications of Resonance Enhanced Raman Spectroscopy: Electronic Structure Probe of Metal-Sulfur Interactions in Oxo-Molybdenum Ene-1,2-Dithiolate Systems Frank E. Inscore The University of Arizona C H3 (Tp*)MoO(bdt) O H3C N S N HB Mo NH3C N S C H3 C H 3 N N H3C Active Site of Sulfite Oxidase
The Raman Spectroscopic Technique General ConsiderationsResearch and Industrial Applications Structure/ Solid State/ Biological ChemistryUtility of Raman Spectroscopy Problematic: Inherent Weak Effect Problematic: Fluorescence Complications Problematic: Instrumental LimitationsDevelopment of New Techniques FT-RS/ SERS/ RERS
Resonance Raman Spectroscopy Characterizing Structure/ Monitoring Reactivity in Catalytic SystemsChemical and Petroleum/ Energy Production Industries• Catalyst Structure and Reactivity: Surface and In Situ StudiesHeterogeneous processes: Supported metal oxides (MoO/ WO) used as catalyst.Hydrodesulfurization catalyst: Removal of sulfur from petroleum feedstocks.Biological Systems• Structure/ Function In Situ StudiesProtonation in Biomolecules: S-H/ S-S conversion.Mechanistic insight into Carcinogenesis: Blue/green particle in tumors; Cu-S bonding.Structural Insight in Metalloproteins.
Overview of Presentation• Raman Applications • What we are doing?• Background• Electronic Structure • Why we are studying?• Raman Instrumentation• Resonance Raman Studies • How we will probe?• Implications for Mo Enzymes C H3 O H3C N S N HB Mo N H3C N S C H3 C H 3 N N H3C
The Importance of Metal 1,2-Dithiolene Complexes General ConsiderationsWhy the Interest in Transition Metal-Sulfur Complexes? Industrial Applications/ Commercial Uses: Vulcanization Accelerators for Rubber Wear Additive Inhibitors in Lubricants Catalytic Inhibitors /Oxidation Catalyst Mode-Locking Additives in Nd Lasers Potential Biological Activity: Correlations with Biological Systems containing Metal to Sulfur Bonds.What Is an Ene-1,2-Dithiolate Ligand ? Four Prototypical Ene-1,2-Dithiolate Systems: -S H -S -S N -S S -S M S = -S -S -S N -S S -S H SRelevance to structure, bonding and function of Metalloenzyme active site centers
Pyranopterin Molybdenum and Tungsten Enzymes Background and Significance3 Mo Families based on structure and reactivity2 W Families similar to DMSO reductase family O S O S er -O S O O Mo S S S Mo Mo S S - C ys S O H2 S S Sulfite Oxidase Xanthine Oxidase DMSO Reductase # X-ray crystallography reveals a common structural unit: Pyranopterin cofactor O S- H N S- HN H2N N N O OPO 2- H 3
The Resonance Raman Spectroscopic ProbeStructure/ Bonding in the Active Site of DMSO Reductase Single Metal Redox Center RO O (VI) • XAS [Mo(VI,V,IV)] S S M • MCD/ EPR [Mo(V)] S S (Mo-S) • Electronic Absorption (Mo=O) x • Resonance Raman Observe enhanced isotopic sensitive Mo=O and Mo-S vibrations. Parallel model studies on both relevant and simpler systems needed.
Outstanding Issues in Pyranopterin Mo Enzyme Catalysis Primary Issue What is Structural and Functional Role of the Pyranopterin Ene-1,2- Dithiolate Unit During Course of Catalysis? Research Goal Derive fundamental understanding at molecular level, into how the unique geometric and electronic structure of these enzyme active sites contribute to their reactivity.Research ObjectivesUtilize available physical characterization methods to determine thegeometric and electronic structure of small synthetic active site analogs.Derive key factors that define geometric/electronic structure relationshipsand correlate to the unique enzymatic spectroscopic features and theirelectronic contributions to structure-bonding/ function.
Chemical Evolution of Mo and W Dithiolene Systems The Reductionist Approach CH3 H3C O O -1, -2 O -1 N S RO HB N S S S S N N M M MH3C S S S S S CH3 -1 N CH3 OR N S S -1,0 M H3C S S S S CH3 M S S H3C S -2 N N Mo S OO HB S SH3C N N S M S S CH 0, +1 N 3CH3 N S M S -2 H3C OO CH3 S SR O M H3C 0, -1 S Y N N S HB N Mo OH3C N N S SS 0, -1, -2 -1 S SR S S M CH3 M S Y N CH3 S S N H3C
Minimal Structural Models/ Effective Spectroscopic Models Simple model; Mo coordinated by Ene-1,2-Dithiolate and terminal Oxo. Isolated Oxo-Mo-Dithiolate Center; Controlled six coordinate environment. Possess Mo(V) paramagnetic centers; Amenable to EPR/ MCD probes. C H3 E H3C N N S (S-S) HB M N S H3 C N Cl -S -S C H3 - S -S N CH C H 3 N 3 -S -S -S -S N N (Tp*)ME(S-S) Cl H3CProbe fundamental properties of Oxo-Mo mono-ene1,2-dithiolate complexes:Metal (M = Mo, W), axial (E = O, S, NO) and dithiolate (S-S) coordination effects.
Band Assignments from Combined Spectroscopic Approach y az2 Solution Electronic Absorption (DCE) y ax2-y2 4 6400y xz y ayz a 7 3 6 7 5600 4800 y axy 1 2 5 Epsilon (M cm ) -1 4000 6 y aop y aop -1 3200 2400 y aip 5 1600 y aip 800 1 2 (3, 4) axy + aip axy + aop ayz + aop O 0 M S 8000 12000 16000 20000 24000 28000 32000 = 90 0 Energy (wavenumbers) O M S > 90 0
Resonance Raman Scattering Enhancement of the Raman signalSensitive and selective probe of structure/ bonding Vibrational frequencies: sensitive to inner coordination environment. Intensitiy: selective enhancement associated with absorbing metal center. Resonance FC - A Term Normal Raman Rayleigh Raman O HT - B Term E’ E1 o o M S O SIR M S E0 SSelectivity based on resonant electronic transition and excited state distortion. Intensity depends on energy and intensity of electronic absorption band. Enhancement result of coupling with electronic excited state.
Raman Experimental Instrumentation and Techniques Design and MethodologyGoal: Obtain Low-frequency vibrational information regarding M-S bonding. Computer System Controller Interface Titanium Sapphire Laser Argon Ion Laser CCD Sample Krypton Ion Laser Illumination/ Collection Pre Monochromator Optics SPEX 1877 SPEX 1405 Triplemate
Collection Geometry Computer System Controller Interface Argon Ion Laser 90 degree geometry CCD Sample Illumination/ Collection Optics SPEX 1877135 degree back scattering geometry Triplemate
Vibrational Analysis (Tp*)MoO(bdt) in Benzene 2000 CH3 O H3C O z z S NN S 1900 Mo N N Mo S HB SRaman Intensity (cps) H3C 1800 O O CH3 CH N 3 1700 N M y (zy) M y B N N S S 1600 S x S 362 cm-1 H C 3 932 cm-1 x N -1 1500 ( A ) 393 cm ( A ) 6 3 ( A ) 1 1400 O O O 1300 300 400 500 600 700 800 900 1000 1100 M M M -1 S S S S S S Raman-shift (cm ) 1 ( A ) 2 ( A ) 3 ( A ) Key Points: 3 bands observed – polarized (A’ symmetry) O O O M M M Intensity enhancement patterns consistent S S S S S S With M-S/ M=O vibrational assignments 4 ( A ) 5 ( A ) ( A ) 6 Resonance Raman spectroscopy probes: Differences in bonding between ground and excited states via distortions along specific normal modes.
Solid-State Excitation ProfilesKey Points: Observe large differential enhancement of Mo=O Transitions probed are orthogonal (in-plane vs out-of-plane) O (Tp*)MoO(bdt): 8K PDMSO mull EA; 100K RR NaCl/ Na2SO4 M S S 3( A ) Sop Mo dxz,yz O Sip Mo dxy S M S 1( A ) O M S S 6( A )Conclusions:Sip Mo dxy CT probes covalent contributions to ground-state S Mo CT probes electronic contributions to redox potentials
Implications for Catalytic Reactivity in Enzymes Lowest energy (intense) CT must be Sip Mo dxy This CT transition probes covalency contributions to ET pathway. M=O aligns redox orbital for facile ET via unique 3-center 2-electron bond. O SO32- SO42- O O OH 2 S S yxya Mo (VI) Mo (IV) S S cys S S cys xya H+, e- H2O S-Moxy 3-center H+, e- pseudo- antibonding O OH S-Moxy 3-center Spseudo- bonding Mo (V) a ip S S cys Criteria for efficient ET yipa Reason Nature has chosen ene-1,2-dithiolate and M=O groups Good M-L overlap/ Minimize ROE
ConclusionsResonance Raman Important Probe of Ground and Excited State Structure State of the Art Equipment Necessary for probing M-S Bonding. Contributions of M-L Bonding to Electronic Structure Elucidated by RR Especially when Combined with other Spectroscopic Techniques. RR Spectroscopy Important Tool for Characterizing Enzyme Active Sites when Interpreted within Context of Well-Defined Small Molecular Models. Protocols Developed can be Applied to more Complicated Systems.
Acknowledgements and Funding * HeI Prof. John H. Enemark HOMO Pseudo anti-bonding Mo dxy h = 579 nm Enemark Research Group HOMO -1&-2 HeII Sip University of Arizona Pseudo bonding * HOMO-3 &-4 * * 10.5 10 9.5 9 8.5 8 7.5 7 6.5 HOMO-5 Ionization Energy (eV) Prof. Martin L. KirkNational Institutes of Health National Science Foundation Kirk Research Group University of New MexicoPetroleum Research Fund Sandia National Laboratories C16 O C14 C15 S2 C4 C2 C13 MO C17 C3 C24 C26 S1 C5 N21 C1 C6 B N31 C23 N32 C36 (Tp*)M oO(qdt) in Benze ne at 514.5nm C27 C33 C37 C34 4000 (Tp*)MoO(bdt) 3800 Raman Intensity (cps) 3600 3400 (Tp*)WO(bdt) 3200 3000 10 9.5 9 8.5 8 7.5 7 6.5 6 400 600 800 1000 1200 1400 Ionization Energy (eV) -1 Ram an S h ift (cm)