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ALSNews           Vol. 77           May 14, 1997

Table of Contents


    1. OPERATIONS UPDATE
    2. BIOMOLECULAR STUDIES WITH CIRCULARLY POLARIZED LIGHT
    3. JULY 15 IS NEXT DEADLINE FOR INDEPENDENT INVESTIGATOR PROPOSALS

1. OPERATIONS UPDATE
(contact: rmmiller@lbl.gov)

The ALS is in a planned shutdown for equipment installation and maintenance. (For details, see ALSNews Vol. 74, April 2, 1997.) User beamtime is scheduled to resume at 00:00 on June 10.

Shutdown maintenance and installation activities are progressing well, with all major tasks either on schedule or running ahead of it. Notable accomplishments include the temporary removal of the U5 undulator in sector 8 and the successful replacement of its vacuum chamber with the new narrow-gap chamber, as well as installation and alignment of the pinger in sector 2.

Weekly operations scheduling meetings will resume on Friday, June 6 at 3:30 p.m. in the Building 6 conference room.

2. BIOMOLECULAR STUDIES WITH CIRCULARLY POLARIZED LIGHT
(Contact: Stephen Cramer, cramer@lbl.gov)

What do bacteria and hydrogen fuel cells have in common? They both get energy from reactions that can be catalyzed by proteins containing metal clusters (metalloproteins). In order to understand how the catalysis works, a group of innovative researchers is using circularly polarized light to study these bio-inorganic molecules. In experiments at the ALS and the Stanford Synchrotron Radiation Laboratory, they are using x-ray magnetic circular dichroism (XMCD) to determine spin states in the proteins' metal atoms. Their efforts have already led to results that challenge conventional ideas about how some metalloproteins catalyze reactions. The group plans to tackle more difficult samples with light from the ALS elliptical polarization undulator when it begins operating in 1998.

Circular dichroism is an increasingly useful phenomenon for studying atoms with a magnetic moment (spin). These atoms yield different spectra from light with left and right circular polarization. The greater the magnetic moment on a given atom, the stronger this difference (dichroism) will be. Investigators can therefore derive the spin state of an atom in a sample by measuring the difference between spectra taken with left and right circularly polarized light and comparing it to dichroisms in well characterized model compounds.

X-ray magnetic circular dichroism has been used mainly to study magnetic materials, such as those of interest to the computer industry, but it is now being applied to the seemingly unlikely study of biological molecules. The key, of course, is that these biomolecules contain clusters of metal ions. The group has focused its recent work on metalloproteins that contain metal centers with nickel and iron atoms. Such molecules have traditionally been studied by EXAFS (extended x-ray absorption fine structure), but this technique has not allowed researchers to resolve the different oxidation states of the nickel ions. Soft x-ray XMCD at the nickel L edge with fluorescence detection allows resolution of oxidation state as well as spin state.

One metalloprotein successfully studied with XMCD is hydrogenase, an enzyme found in bacteria and algae that catalyzes the oxidation of molecular hydrogen and the reduction of hydrogen ions back to hydrogen molecules. This chemical reaction is important in anaerobic metabolism and happens to be a key reaction in hydrogen fuel cells as well. In studies on hydrogenase enzyme from Desulfovibrio baculatus bacteria, the group was able to associate specific spin and oxidation states of the nickel atoms with the various stages of activation of the enzyme. What the group found stands on its head the conventional picture of how the enzyme works. As the enzyme is activated with hydrogen, the nickel in it cycles through various oxidation and spin states. The conventional wisdom is that a paramagnetic species (spin=1/2) representing either Ni(I) or Ni(III) is dominant in the fully activated enzyme. However, XMCD studies show that the dominant species is high-spin (spin=1) Ni(II) and that the active site (the Ni and associated ligands) as a whole has a lower spin (spin=1/2). The Ni(II) must therefore be spin coupled with another atom (such as a ligand) that brings the total spin down to 1/2. Thus, it is probable that ligand atoms such as sulfur or selenium participate in the activation.

For studies of this kind, the group currently obtains circularly polarized light from a bend magnet source by selecting light above or below the plane of the synchrotron ring, where the light has natural left or right circular polarization. While having access to circularly polarized light has opened up a world of possible studies, resolving samples with a smaller proportion of magnetic atoms calls for a higher flux of circularly polarized light than is currently available in only part of a bend magnet beam. The users will get the flux they need with the ALS's elliptical polarization undulator (EPU), due to begin providing light to Beamline 4.0 in 1998. The new insertion device will give the researchers more complete polarization than a bend magnet (with percentages of right or left circular polarization approaching 100%), ten times the usable flux, and rapid switching between left and right circular polarizations. To keep pace with these improvements, the group will update its endstation with faster electronics (to handle higher count rates) and a higher-resolution detector when it moves to Beamline 4.0.

Experiment performed by Stephen Cramer (principal investigator, University of California Davis and Berkeley Lab); Corie Ralston, Craig Bryant, and Hong-Xin Wang (University of California Davis); and Daulat Patil (Berkeley Lab) using the L-Edge/XMCD Workstation at Stanford Synchrotron Radiation Laboratory.
Funding: Office of Basic Energy Sciences of the U.S. Department of Energy, National Institutes of Health, National Science Foundation, and U.S. Department of Agriculture.

3. JULY 15 IS NEXT DEADLINE FOR INDEPENDENT INVESTIGATOR PROPOSALS

There has been a change in this year's schedule for submitting Independent Investigator proposals. The deadline for proposals for December 1997 - March 1998 beamtime will be July 15, 1997, not June 1 as in our usual cycle. Because the December-March period is short and includes a scheduled shutdown in January, all proposals will automatically roll over for consideration in the following semester (April-September 1998), along with proposals submitted by that semester's usual December 1 deadline.

The proposal form for independent investigators is now available in Portable Document Format (PDF) on the World Wide Web; point your browser to http://www-als.lbl.gov/als/quickguide/independinvest.html. Information on the proposal process is available at the same location.

To request a proposal form by mail, contact:
Elizabeth Saucier, ALS Administrator
Tel: (510) 486-6166
Fax: (510) 486-4960
Email: alsuser@lbl.gov

For information on beamlines available to independent investigators, contact:
Fred Schlachter, ALS Program Support
Tel: (510) 486-4892
Fax: (510) 486-6499
Email: fred_schlachter@lbl.gov