X-ray Spectroscopy A Critical Look at the Past Accomplishments and Future Prospects James Penner-Hahn jeph@umich.edu Room 243 Monday Wednesday 3-5 PM and by appointment http://www.chem.usyd.edu.au/~penner_j/index.htm Lecture plan. Basic Physical Principles 2. Practical aspects of x-ray absorption 3. Data analysis 4. Near edge structure 5. Spatially and temporally resolved methods 6. Exotic x-ray spectroscopies Grading Participation 2% Draft research proposal 3% Final research proposal 5% Original Feasible Well presented Proposal forms and guidelines at http://www-ssrl.slac.stanford.edu/users/user_admin/ xray_vuv_proposal_guide.html Proposal body not more than 5 pages A. DESCRIPTION OF EXPERIMENT I. Background II. Previous results III. Proposed Experiments IV. Literature cited B. PREVIOUS EXPERIENCE WITH THE TECHNIQUES AND FACILITY C. DETAILED SAFETY CONCERNS D. EQUIPMENT DEVELOPMENT SCHEDULE E. RESOURCES FOR PROGRAM PROJECT Techniques for studying metal sites in proteins UV-visible spectroscopy EPR spectroscopy Magnetic susceptibility MCD NMR spectroscopy X-ray crystallography X-ray spectroscopy Require open d shell Requires I=/2 nucleus Requires crystals
http://www.coe.berkeley.edu/ast/sxreuv Absorption of x-rays by Pb Electron binding energies of the elements Absorbance (cm 2 /g) L 3 L L 2 M L 2 K. Energy (kev) Center for Synchrotron Radiation Research and Instrumentation http://www.csrri.iit.edu/ XAS XAFS NEXAFS X-ray absorption spectroscopy XANES EXAFS A S Absorption E E2 A S 95 96 97 98 99 Energy (ev) 2
EXAFS k 3 X-ray absorption spectroscopy 5-5 - Phase scatterer Frequency bondlength Shape scatterer 2 4 6 8 2 k (Å -) Amplitude coordination number Information Content of EXAFS Bond length ±.2 Å (accuracy) Bond length ±.5 Å (precision) Coordination number (lower limit) ± Ligation type (Z) ± R max < ~ 4 Å Scott, R. A. "Measurement of Metal-Ligand Distances by EXAFS" Methods Enzymol. 985, 7, 44-459. Teo, B. K. EXAFS: Basic Principles and Data Analysis; Springer-Verlag: New York, 986. Scott, R.A., X-Ray Absorption Spectroscopy in Physical Methods in Bioinorganic Chemistry, Que, L. (Ed)., 2, University Science Books. Penner-Hahn, J.E., X-Ray Absorption Spectroscopy, in Comp. Coord. Chem. II, Vol. 2, 24. Levina A, Armstrong R.S., Lay P.A., Three-dimensional structure determination using multiple-scattering analysis of XAFS: applications to metalloproteins and coordination chemistry Coord. Chem. Rev. 25, 249, 4-6. Dependence of XANES on Oxidation State Normalized Absorption 8 4 II/II II/III III/III III/IV 6535 6555 6575 6595 Energy (ev) Mn(V)=O has intense pre-edge transition, not seen in Mn(III) analog NiN 4 vs NiS 4 NiN 6 vs NiS 6 NiN 4 ; Td vs D 4h 3
X-ray Fluorescence X-ray fluorescence lines N 7 N M 5 M L K K β K α Kα α 2 3 2 β β β M α β L α 2 β l 2,5 L 3 L 2 L 2p 3 2p 2s 2 2 K Counts/sec X-ray fluorescence spectra give element sensitivity 2 5 5 Ar Elastic scatter Zn Co Fe Cu Advantages of XAFS Direct structural determination for: Any form of matter Any isotope Any spin state Direct determination of oxidation state 2 4 6 8 Energy (ev) Disadvantages of XAFS Bulk spectroscopy (average structure) Little angular information Gives only local structural information Limited sensitivity Requires synchrotron x-ray source http://learntech.uwe.ac.uk/radscience/x-ray_tube 4
Synchrotrons produce intense, tunable x-ray beams Bremsstrahlung radiation 5
22 July 25 http://www.synchrotron.vic.gov.au MetE (cobalamin independent MetSyn) contains Zn Zn is tightly bound Zn is required for activity Is Zn involved in reaction, or does it play a structural role? The Zn site in MetE has ZnS 2 (O/N) 2 ligation. Addition of homocysteine changes ligation to ZnS 3 (O/N). Fourier Transform Magnitude EXAFS k 3 2 5 5 2 3 4 5 6 7 R + α (Å) 8 4-4 Native +Hcy 2 4 6 8 2 k (Å - ) Fourier Transform Magnitude Changes in ligation are due to homocysteine binding to Zn 6 MetH (ZnS O) 3 5 4 3 2 MetE (ZnS NO) 2 Native +Hcy +SeHcy 2 3 4 5 6 7 R + α (Å) Combination of Zn + Se EXAFS consistent with only a small distortion from tetrahedral geometry in substrate-bound enzyme 6
J. Am. Chem. Soc., 2 () 99 p. 43-432 p. 432-434 EXAFS shows that CN does not remain bound Fourier Transform Magnitude CuCN 2Li + 2 MeLi CuCN 2Li + MeLi CuCN 2Li Cu-C?N Cu-C?N-Cu Cu-Nearest Neighbor.5 3 4.5 6 7.5 R+ (Å) Structures of cyanocuprates in THF N Cu C N Cu Cu C MeLi Me Cu C N MeLi Li C N Li Me Cu Me Solution speciation of CuI+PhLi PhLi + CuI phenylcopper 2PhLi+ CuI diphenylcuprate Crystalline phenyl:copper species : Cu 4 Ph 4 (Me 2 S) 2 Cu 5 Mes 5.2: [Cu 5 Ph 6 ].5: [Cu 4 LiPh 6 ] [Cu 4 MgPh 6 ] 2: [CuPh 2 ] [CuPh 2 Li] 2 [Cu 3 Li 2 Ph 6 ] Titration of CuI+ n PhLi shows isosbestic behavior up to.2 equivalents 3 Titration of CuI+ n PhLi shows isosbestic behavior from.2-2. equivalents 3 Normalized Absorbance 25 2 5 5 n=. n=.2 n=.4 n=.6 n=.8 n=. n=. n=.2 897 898 899 9 9 92 Energy ( ev ) Normalized Absorbance 25 2 5 5 n=2. n=.8 n=.7 n=.5 n=.4 n=.3 n=.2 897 898 899 9 9 92 Energy ( ev ) 7
EXAFS data support XANES speciation Cu Composition CuI - CuPh - 2 2.8 Cu 5 Ph - 6.6.4.2.5.2.5 2 PhLi / CuI Ratio [CuI 2 ] [Cu 5 Ph 6 ] [CuPh 2 ] Cu Cu + Cu 8