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E A Stern

Publications and source records attributed to E A Stern.

29 records · Page 2Linked to original sources
PubMed1993

Microsecond-resolved XAFS of the triplet excited state of Pt2(P2O5H2)4(4-)

Little is known about the excited-state structures of most inorganic compounds. Time-resolved resonance Raman and time-resolved infrared spectroscopies can provide only indirect structural information for short-lived excited species in solution at room temperature. Time-resolved X-ray diffraction has the potential to give more direct information, but no excited-state structures have yet been reported; picosecond gas-phase electron diffraction has been proposed recently, but not yet demonstrated. Here we report a technique that combines the X-ray absorption fine-structure (XAFS) method with rapid-flow laser spectroscopy to measure structural changes in a solution-phase excited-state transition-metal complex with microsecond resolution. We find that the triplet excited state of Pt2(P2O5H2)4(4-), with a lifetime of about 4 microseconds, undergoes a contraction in the Pt-Pt distance of 0.52 +/- 0.13 A relative to the ground state. We anticipate that time-resolved XAFS will have broad applications in chemistry and biology.

Absorptiometry, Photon↗
PubMed1991

Comparison of the effects of saccharide binding and crystallization on the zinc transition-metal site of concanavalin A.

The Zn site in concanavalin A solution was studied by X-ray absorption fine structure spectroscopy (XAFS) with and without the saccharide methyl alpha-D-glucoside (aMG) bound to the protein. No structural change occurs in the metal-binding site when the saccharide is bound to the protein. There is, however, evidence for structural change remote from the metal site. This is in contrast to the significant changes that we have previously found to occur in the near neighborhood of the Zn atom when an aqueous solution of Zn concanavalin A crystallizes. We propose a structural explanation of these facts based on the known crystal structure of concanavalin A.

Concanavalin A↗
PubMed1991

Structural differences in solution and crystalline forms of met-myoglobin.

For several decades X-ray diffraction studies have been the paragon of biological structure studies at atomic resolution. Diffraction provides three-dimensional structure information, which is essential to our fundamental understanding of protein function. However, since X-ray diffraction cannot be done to atomic resolution on proteins in their native solution or membrane-bound state, the possibility exists that the conformations of the protein in crystals are slightly different from the conformations in solution, and attempts to interpret details of the structure may be misleading and without physiological relevance. In this paper, we show that this concern is justified for a familiar protein, myoglobin. Performing X-ray absorption fine structure experiments on both solution and crystalline met-myoglobin (met-Mb), we find significant differences in the local environment of the iron between the two states. Specifically, the average iron-nearest neighbor atom distance in the crystalline form is 0.05 A shorter than that in the solution form, and the iron-nearest neighbor bond is more rigid in the crystalline met-Mb. Possible artifactual explanations for the differences have been ruled out.

Animals↗
PubMed1991

X-ray absorption fine structure investigation of the zinc transition metal binding site of Zn concanavalin A in solution and in the crystal.

We report details on measurements by the X-ray absorption fine structure (XAFS) technique of the conformational changes around the transition metal binding site (S1) of the protein concanavalin A induced by crystallization when that site is occupied by Zn. A change from hexa- to tetracoordination occurs at the S1 site on crystallization when the calcium-binding site (S2) is occupied by a calcium atom. When the S2 site is unoccupied, the Zn is pentacoordinated both in solution and in the crystal. The average distance to the coordination shell increases with coordination number as expected. Conformational changes are detected up to 4.5 A from the Zn, the limit of sensitivity of the XAFS technique. When the Zn is hexacoordinated, the ligands around the Zn, as determined by XAFS, are consistent with the crystal structure determination results of five oxygens and one nitrogen. The atom that is released in the tetracoordinated Zn. decreases to five is an oxygen atom, and, in addition, the nitrogen is released in the tetracoordinated Zn. Thus, when S2 is emptied, the protein gains a ligand about the Zn site in the crystal and loses one in solution. These results provide direct evidence that the protein conformation can be altered by the intermolecular forces of crystallization.

Binding Sites↗
PubMed1990

Evidence from X-ray absorption fine structure spectroscopy for significant differences in the structure of concanavalin A in solution and in the crystal.

We have used X-ray absorption fine structure spectroscopy (XAFS) to study and compare the structure of concanavalin A in crystals and in aqueous solution. Significant differences were found between crystal and solution in the configuration of the transition-metal site of the protein. The metal has six ligands in solution but only five in the crystal. The ligand bond lengths are shorter in the crystal than in solution. The vibrational disorder in the crystal and possibly the corresponding bond length show a negative temperature dependence whereas in solution they vary normally with temperature. The anomalous temperature dependence in the crystal suggests that as the temperature decreases the protein molecules are subject to additional stresses, which are transmitted as a tensile stress at the metal site leading to distorted geometry and lengthening and weakening of metal-ligand bonds.

Calcium↗
PubMed1988

The active site of hemerythrin as determined by X-ray absorption fine structure.

Extensive X-ray absorption fine structure measurements and analysis have been made on azidomet- and methemerythrin and on the native forms of oxy- and deoxyhemerythrin. Due to the availability of models that have been synthesized to mimic the active site of hemerythrin, it was possible to make a thorough assessment of the various errors in the structural parameters determined by the analysis. It is found that the largest source of error is the lack of complete transferability of amplitude and phase between the standards and hemerythrin. This is of particular importance in distinguishing the contributions of the second-shell low-Z atoms and, thus, has a substantial influence on the determination of the iron-iron distance. The internal consistencies of the various checks and a new formulation of error analysis for the structural parameters give us confidence in the structure determined for the active site. The main result is that as O2 is released from oxyhemerythrin, the mu-oxo bridge between the two iron atoms in the active site with an Fe-O distance of 1.8 A converts to a mu-hydroxo bridge in deoxyhemerythrin, expanding the Fe-O distance to 2.0 A. The Fe-Fe distance expands proportionally from 3.24 A in oxyhemerythrin to 3.57 A in deoxyhemerythrin so as to keep the Fe-O-Fe bridging angle approximately constant. These conclusions provide experimental support for the structures of oxy- and deoxyhemerythrin proposed previously on the basis of spectroscopic and preliminary X-ray crystallographic data.

Animals↗
PubMed1982

An x-ray absorption study of the iron site in bacterial photosynthetic reaction centers.

Measurements were made of the extended x-ray absorption fine structure (EXAFS) of the iron site in photosynthetic reaction centers from the bacterium Rhodopseudomonas sphaeroides. Forms with two quinones, two quinones with added o-phenanthroline, and one quinone were studied. Only the two forms containing two quinones maintained their integrity and were analyzed. The spectra show directly that the added o-phenanthroline does not chelate the iron atom. Further analysis indicates that the iron is octahedrally coordinated by nitrogen and/or oxygen atoms located at various distances, with the average value of about 2.14 A. The analysis suggests that most of the ligands are nitrogens and that three of the nitrogen ligands belong to histidine rings. This interpretation accounts for several unusual features of the EXAFS spectrum. We speculate that the quinones are bound to the histidine rings in some manner. Qualitative features of the absorption edge spectra also are discussed and are related to the Fe-ligand distance.

Iron↗
PubMed1977

The iron-sulfur environment in rubredoxin.

The atomic environment around the iron site in the nonheme iron sulfur protein rubredoxin was studied by the extended X-ray absorption fine structure (EXAFS) technique. Within experimental error, the Fe-S bonds in oxidized Clostridium pasteurianum rubredoxin are the same as in the analogue anion [Fe(S2-o-xyl)2]-synthesized by Holm. The average Fe-S bond length is 2.267 +/- 0.003A and the root mean square deviation about this average due to structural disorder is 0.032 + 0.013 - 0.032.

Absorption↗