Postdoc Ph D Research Fei 2002
Transcript of Postdoc Ph D Research Fei 2002
S
S
O
M oN
N
NN
N
N
HB
H3C
H3C
H3C
C H3
C H3C H3
(Tp*)MoO(bdt)
Active Site of Sulfite Oxidase
Applications of Resonance Enhanced Raman Spectroscopy: Electronic Structure Probe of Metal-Sulfur Interactions in Oxo-Molybdenum
Ene-1,2-Dithiolate Systems
Frank E. InscoreThe University of Arizona
The Raman Spectroscopic TechniqueGeneral Considerations
Problematic: Inherent Weak Effect
Problematic: Fluorescence Complications
Problematic: Instrumental Limitations
Utility of Raman Spectroscopy
Research and Industrial Applications
Structure/ Solid State/ Biological Chemistry
Development of New Techniques
FT-RS/ SERS/ RERS
Resonance Raman Spectroscopy
• Catalyst Structure and Reactivity: Surface and In Situ Studies Heterogeneous processes: Supported metal oxides (MoO/ WO) used as catalyst. Hydrodesulfurization catalyst: Removal of sulfur from petroleum feedstocks.
Characterizing Structure/ Monitoring Reactivity in Catalytic Systems
• Structure/ Function In Situ Studies Protonation in Biomolecules: S-H/ S-S conversion. Mechanistic insight into Carcinogenesis: Blue/green particle in tumors; Cu-S bonding.
Structural Insight in Metalloproteins.
Biological Systems
Chemical and Petroleum/ Energy Production Industries
Overview of Presentation
• What we are doing?
• Why we are studying?
• How we will probe?
• Raman Applications• Background• Electronic Structure• Raman Instrumentation• Resonance Raman Studies• Implications for Mo Enzymes
S
S
O
M oN
N
NN
N
N
HB
H3C
H3C
H3C
C H3
C H3C H3
The Importance of Metal 1,2-Dithiolene ComplexesGeneral Considerations
Industrial Applications/ Commercial Uses:
Potential Biological Activity:
What Is an Ene-1,2-Dithiolate Ligand ?
Vulcanization Accelerators for Rubber Wear Additive Inhibitors in LubricantsCatalytic Inhibitors /Oxidation Catalyst Mode-Locking Additives in Nd Lasers
Correlations with Biological Systems containing Metal to Sulfur Bonds.
Relevance to structure, bonding and function of Metalloenzyme active site centers
Four Prototypical Ene-1,2-Dithiolate Systems:
S
S-S
-S
-S
-S
H
H N
N
-S
-S-S
-S- S
- SM
S
S =
Why the Interest in Transition Metal-Sulfur Complexes?
# X-ray crystallography reveals a common structural unit: Pyranopterin cofactor
HN
N N
N
OH2N OPO32-
S-S-
O H
H
Pyranopterin Molybdenum and Tungsten EnzymesBackground and Significance
2 W Families similar to DMSO reductase family
MoS
S
O
MoS
S
S
MoS
S
O
S
SO
S - C ys
OS er-O
O H2
Sulfite Oxidase Xanthine Oxidase DMSO Reductase
3 Mo Families based on structure and reactivity
The Resonance Raman Spectroscopic Probe Structure/ Bonding in the Active Site of DMSO Reductase
• XAS [Mo(VI,V,IV)]• MCD/ EPR [Mo(V)]• Electronic Absorption• Resonance Raman
Single Metal Redox Center
MSS
O
SS
RO(VI)
(Mo=O)
(Mo-S)
x
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
Research ObjectivesUtilize available physical characterization methods to determine the geometric and electronic structure of small synthetic active site analogs.
Derive key factors that define geometric/electronic structure relationships and correlate to the unique enzymatic spectroscopic features and their electronic contributions to structure-bonding/ function.
Primary IssueWhat is Structural and Functional Role of the Pyranopterin Ene-1,2-
Dithiolate Unit During Course of Catalysis?
Research GoalDerive fundamental understanding at molecular level, into how the unique geometric and electronic structure of these enzyme active sites contribute to their reactivity.
Chemical Evolution of Mo and W Dithiolene Systems
The Reductionist Approach
S
SO
MN NN N
NN
S
SMoN N
N N
NN
S
S
SMoN N
N N
NN
NO
HB
H3C
H3C
H3C
CH3
CH3CH3
HB
H3C
H3C
H3C
CH3
CH3CH3
HB
H3C
H3C
H3C
CH3
CH3CH3
0, -1
MS
S
MS
SS
S
MS
SO
S
S
MS
SS
S
S
S
0, +1
-1, -2
-1,0
0, -1, -2
MS
SO
S
S
MS
SOR
S
S
MS
SO
S
S
MY
SRO
S
S
MY
SRO
S
S
RO
O
O
-1
-1
-2
-2
-1
Possess Mo(V) paramagnetic centers; Amenable to EPR/ MCD probes.
Minimal Structural Models/ Effective Spectroscopic Models
- S
- S N
N
- S
- S- S
- S
- S
- S
C H3
C l
C l
S
S
E
MN N
NN
N
N
HB
H3C
H 3 C
C H 3
C H 3CH 3
H3C
Probe 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.
(Tp*)ME(S-S)
(S-S)
Isolated Oxo-Mo-Dithiolate Center; Controlled six coordinate environment.
Simple model; Mo coordinated by Ene-1,2-Dithiolate and terminal Oxo.
-4 10-6
-3 10-6
-2 10 -6
-1 10-6
0
1 10-6
2 10 -6
3 10-6
4 10-6
-1500.0-1000.0-500.000.0000500.001000.01500.0
(Tp*)MoO(bdt) (1)I
II
1 5.7 14.7 13.7 12.7 11.7 10.7 9.7 8.7 7.7 6.7 5.7
Ionization Energy (eV)
C27
C23
C24
C17C13
C14
C16
O
MOC15
C26N21B
N32
C37C33
C34
C36
N31
S1 C1 C6C5
C4
C3
C2S2
Resonance Raman Spectroscopy
Electrochemistry Magnetic Circular Dichroism
Electronic AbsorptionPhotoelectron Spectroscopy
Electron Paramagnetic Resonance
X-ray Crystallography
Density Functional Theory
Synthesis/ Purification
Characterization:NMR, IR, HR-MS, XAS
Minimal Structural/ Effective
Spectroscopic Active SiteModels
-1 105
-5 104
0
5 104
1 105
1.5 105
3100 3200 3300 3400 3500 3600 3700Field (Gauss)
n= 9.4510 GHz
(3)
Physical Characterization: The (Tp*)MoO(bdt) Benchmark
MCD and Electronic Absorption Spectroscopy
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
-1000
-500
0
500
1000
8000 12000 16000 20000 24000 28000 32000
Ab
sorb
ance
MC
D In
tensity (m
deg)
Energy (wavenumbers)
1 2 (3, 4) 5 6 7
Complimentary selection rules: resolve electronic transitions in spectra
MCD (5K /7T)
Low Temperature Solid-State (PDMSO Mull) Studies
Absorption (5K)
Band Assignments from Combined Spectroscopic Approach
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800
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4800
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6400
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Ep
silo
n (
M-1
cm
-1)
Energy (wavenumbers)
1 2 (3, 4)
5
6
7y
1 2
3
5
6 7
4
ya''op
ya''ip
ya'op
ya'ip
ya'z2
ya''x2-y2
a''x'z ya'
y'z
y a'xy
O
O
M= 90 0
M
a'xy + a'
ip
S
S
> 90 0
a'xy + a'
op a'yz + a'
op
Solution Electronic Absorption (DCE)
Resonance Raman Scattering
Enhancement of the Raman signal
RayleighNormal RamanResonance Raman
nn'
nono
IR
E0
E1
FC - A TermHT - B Term
E’
Intensitiy: selective enhancement associated with absorbing metal center.
Selectivity based on resonant electronic transition and excited state distortion.
Vibrational frequencies: sensitive to inner coordination environment.
Sensitive and selective probe of structure/ bonding
Intensity depends on energy and intensity of electronic absorption band.
Enhancement result of coupling with electronic excited state.
M
O
S
S
M
S
O
S
Raman Experimental Instrumentation and Techniques Design and Methodology
CCD
Computer Controller
System Interface
Argon Ion LaserTitanium Sapphire Laser
Krypton Ion Laser
Pre Monochromator SPEX 1405
Sample
Illumination/ Collection Optics
Goal: Obtain Low-frequency vibrational information regarding M-S bonding.
SPEX 1877Triplemate
CCD
Computer Controller
System Interface
Argon Ion Laser
Sample
Illumination/ Collection Optics SPEX 1877
Triplemate135 degree back scattering geometry
90 degree geometry
Collection Geometry
The Resonance Raman Experiment
Laser Enhanced Raman Spectroscopy
Sample Illumination and Collection Optics
Sample Handling, Detection and Dispersal System
Samples Problematic: Photo Decomposition/ Thermal Degradation?
Vibrational Raman Spectroscopy
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7000
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8500
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Ra
ma
n I
nte
ns
ity
(c
ps
)
Raman-shift (wavenumbers)
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1400
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2000
300 400 500 600 700 800 900 1000 1100
n6
n1 n 3
n 6
n1
n1 n6
n3
n3
140K 528.7 nm ~40 mW
(Tp*)MoO(bdt) in NaCl/ Na2SO4
(Tp*)MoO(bdt) in Benzene
293K 514.5 nm ~75 mW
3 vibrational bands observed
= 932 cm-1= 393 cm-1= 362 cm-1
Identify normal modes coupled to electronic transitions
S
S
O
MoN
N
NN
N
N
HB
H3C
H3C
H3C
CH3
CH3CH3
Solution Raman Depolarization Studies
Ram
an I
nte
nsi
ty (
cps)
= 932 cm-1
= 393 cm-1
= 362 cm-1
1300
1400
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1600
1700
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1900
2000
300 400 500 600 700 800 900 1000 1100
n3 n1 n6
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1400
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1600
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1900
2000
300 400 500 600 700 800 900 1000 1100
Raman-shift (wavenumbers)
n1 n6 n3
(Tp*)MoO(bdt) in Benzene293K 496.5 nm ~75 mW
n6 ( A' )
n3 ( A' )
n1 ( A' )
I
I
Parallel polarization
Perpendicular polarization
Depolarization Ratio
= I/ I 0 3/4
Totally symmetric (polarized)
¾
Non-totally symmetric (depolarized)
Ratio indicates 3 modes are totally symmetric
Vibrational Analysis
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2000
300 400 500 600 700 800 900 1000 1100
Ram
an I
nte
nsi
ty (
cps)
Raman-shift (cm-1
)
Key Points:3 bands observed – polarized (A’ symmetry)
Resonance Raman spectroscopy probes: Differences in bonding between ground and excited states via distortions along specific normal modes.
Intensity enhancement patterns consistentWith M-S/ M=O vibrational assignments
M
O
S SM
O
S S
M
O
S S
M
O
S SM
O
S SM
O
S S
SS
M
O
1 ( A' ) 2 ( A'' )
4 ( A' )
3 ( A' )
5 ( A'' )
z
y
x
(zy)SS
M
O
z
y
x N
NB N
n6 ( A' )
932 cm-1
393 cm-1
362 cm-1
n6 ( A' ) n3 ( A' )n1 ( A' )
S
SO
MoS
SO
MoN NN N
NN
HB
H3C
H3C
H3C
CH3
CH3CH3
(Tp*)MoO(bdt) in Benzene
Solid-State Excitation Profiles
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0
0.5
1
1.5
2
16000 18000 20000 22000 24000
Energy (wavenumbers)
S
S
M
O
n3 ( A' )
S
S
M
O
S
S
M
O
n1 ( A' )
n6 ( A' )
(Tp*)MoO(bdt): 8K PDMSO mull EA; 100K RR NaCl/ Na2SO4
Observe large differential enhancement of Mo=O
Key Points:
Transitions probed are orthogonal (in-plane vs out-of-plane)
Conclusions:
Sip Mo dxy CT probes covalent contributions to ground-state
S Mo CT probes electronic contributions to redox potentials
Sip Mo dxy
Sop Mo dxz,yz
Implications for Catalytic Reactivity in Enzymes
xya'
ipa'
xya'
ipa'
S-Moxy3-center
pseudo- antibonding
S-Moxy3-center
pseudo- bonding
S
S
S
O
c ys
Mo
OH2
(IV)
S
S
S
O
c ys
Mo
OH
(V)
S
S
S
O
c ys
Mo (VI)
OSO3
2- SO42-
H2O
H+, e-H+, e-
Lowest energy (intense) CT must be Sip Mo dxy
This CT transition probes covalency contributions to ET pathway.
Criteria for efficient ET
Good M-L overlap/ Minimize ROE
Reason Nature has chosen ene-1,2-dithiolate and M=O groups
M=O aligns redox orbital for facile ET via unique 3-center 2-electron bond.
Conclusions
Resonance 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 RREspecially when Combined with other Spectroscopic Techniques.
RR Spectroscopy Important Tool for Characterizing Enzyme Active Siteswhen Interpreted within Context of Well-Defined Small Molecular Models.
Protocols Developed can be Applied to more Complicated Systems.
3000
3200
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3600
3800
4000
400 600 800 1000 1200 1400
Ram
an I
nte
nsi
ty (
cps)
Raman Shift (cm -1)
(Tp*)MoO(qdt) in Benzene at 514.5nm
Acknowledgements and Funding
Mo
E
S
S C
C
z
y
x
E = O, NO
= 42.1
(Tp*)MoO(bdt) (Tp*)MoO(bdtCl2)
(Tp*)Mo(NO)(bdt) (Tp*)Mo(NO)(bdtCl2)
= 6.9 = 21.3
= 44.4
14
2
21 37
C27
C23
C24
C17C13
C14
C16
O
MOC15
C26N21B
N32
C37C33
C34
C36
N31
S1 C1 C6C5
C4
C3
C2S2
Small change with changeof dithiolate ligand
Significantchangewithchange ofaxial ligand
10.5 10 9.5 9 8.5 8 7.5 7 6.5 Ionization Energy (eV)
HeI
HeII
HOMO
HOMO -1&-2
HOMO-3 &-4
HOMO-5
10 9.5 9 8.5 8 7.5 7 6.5 6
Ionization Energy (eV)
(Tp*)MoO(bdt)
(Tp*)WO(bdt)
C27
C23
C24
C17C13
C14
C16
O
MOC15
C26N21B
N32
C37C33
C34
C36
N31
S1 C1 C6C5
C4
C3
C2S2
Mo dxy
Pseudo anti-bonding
Pseudo bonding
Sip
**
*
*
h = 579 nmEnemark Research Group
University of Arizona
Kirk Research GroupUniversity of New Mexico
National Institutes of Health National Science Foundation
Petroleum Research Fund Sandia National Laboratories
Prof. John H. Enemark
Prof. Martin L. Kirk