Automation of Hubble Space Telescope Mission Operations

On June 13, 2011, after more than 21 years, 115 thousand orbits, and nearly 1 million exposures taken, the operation of the Hubble Space Telescope successfully transitioned from 24x7x365 staffing to 815 staffing. This required the automation of routine mission operations including telemetry and forward link acquisition, data dumping and solid-state recorder management, stored command loading, and health and safety monitoring of both the observatory and the HST Ground System. These changes were driven by budget reductions, and required ground system and onboard spacecraft enhancements across the entire operations spectrum, from planning and scheduling systems to payload flight software. Changes in personnel and staffing were required in order to adapt to the new roles and responsibilities required in the new automated operations era. This paper will provide a high level overview of the obstacles to automating nominal HST mission operations, both technical and cultural, and how those obstacles were overcome

Multifrequency EPR investigations into the origin of the S2-state signal at g = 4 of the O2-evolving complex

The low-temperature S2-state EPR signal at g = 4 from the oxygen-evolving complex (OEC) of spinach Photosystem-II-enriched membranes is examined at three frequencies, 4 GHz (S-band), 9 GHz (X-band) and 16 GHz (P-band). While no hyperfine structure is observed at 4 GHz, the signal shows little narrowing and may mask underlying hyperline structure. At 16 GHz, the signal shows g-anisotropy and a shift in g-components. The middle Kramers doublet of a near rhombic S = 1/2 system is found to be the only possible origin consistent with the frequency dependence of the signal. Computer simulations incorporating underlying hyperfine structure from an Mn monomer or dimer are employed to characterize the system. The low zero field splitting (ZFS) of D = 0.43 cm and near rhombocity of E/D = 0.25 lead to the observed X-band g value of 4.1. Treatment with F or NH3, which compete with Cl for a binding site, increases the ZFS and rhombicity of the signal. These results indicate that the origin of the OEC signal at g = 4 is either an Mn(II) monomer or a coupled Mn multimer. The likelihood of a multimer is favored over that of a monomer.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30241/1/0000635.pd

On the structure of the iron-sulfur complex in the two-iron ferredoxins

Recent spectroscopic and magnetic susceptibility studies of the iron center in the two-iron ferredoxins provide criteria which any model for the iron-sulfur complex in these proteins must satisfy. These criteria are most stringent for parsley and spinach ferredoxin: the reduced proteins contain a high-spin ferric atom antiferromagnetically exchange-coupled (presumably via sulfide bridging ligands) to a high-spin ferrous atom. In the oxidized proteins the iron atoms are antiferromagnetically spin-coupled, high-spin ferric atoms. Arguments are given to substantiate the claim that the ferrous atom in the reduced protein is ligated by four sulfur atoms in a distorted tetrahedral configuration: two are the bridging sulfides, two are cysteinyl sulfurs. A treatment of proton contact shifts based upon the above model is pertinent to proton magnetic resonance data already available and provides a means to identify directly the ligands at both iron atoms via further PMR experiments.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33507/1/0000004.pd

Multiple frequency EPR studies on three forms of oxidized cytochrome c oxidase

Bovine heart mitochondrial cytochrome c oxidase (cytochrome aa3) (EC 1.9.3.1) has been demonstrated to occur in several forms when the redox centers in the protein are thought to be fully oxidized. We report here the results of extensive EPR studies at 3, 8.9, 9.2, 9.4, 15 and 34 GHz on the resting state, the alternative resting state (with g = 12 at 9 GHz) and pulsed state (with g = 5 signal at 9 GHz). Theoretical consideration is given to all binary spin-coupling possibilities under the constraint that the iron atoms are either ferric or ferrous and the copper atoms are either cupric or cuprous. We conclude that the g = 12 signal can arise from any spin system with S > 1 and |D| = 0.15 cm-1. The g = 5 signals originate from an excited, integer-spin system with |D| = 0.035 cm-1, which is approximately 7 cm-1 above the ground state (not observed in EPR). It is pointed out that in interpretations of data and elaboration of suitable models in this field, the implications of spin-coupling should be considered in a comprehensive and not in a selective way. At 3 GHz, EPR spectra of CuA in the resting, pulsed and anaerobically oxidized states show that this center is identical in its EPR for all three states.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25085/1/0000516.pd

A statistical theory for powder EPR in distributed systems

A statistical interpretation is presented for "g strain," the dominant broadening in the EPR spectra of metallo-proteins. The direct cause of g strain is described by a three-dimensional tensor p, whose principal elements are random variables. The p and g tensors are not necessarily colinear. The observed EPR linewidth results from a distribution in the effective g value as a function of (a) the joint distribution function of the elements of the p tensor and (b) the spatial relationship between the two principal axis systems involved. The theory is reformulated in terms of matrices that facilitate a direct comparison with earlier work. Two previous theories of g strain represent different subsets of the general theory, namely, the case of zero rotation between axis systems and the case with nonzero rotation and full correlation between elements of the p tensor.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25870/1/0000433.pd

Studies on the spin-spin interaction between flavin and iron-sulfur cluster in an iron-sulfur flavoprotein

When the di- or trimethylamine dehydrogenases (trimethylamine:(acceptor) oxidoreductase (demethylating), EC 1.5.99.7) of certain methylotrophic bacteria are reduced by two electrons with substrate unusual EPR signals arise at g = 2 and g = 4 (Steenkamp, D.J. and Beinert, H. (1982) Biochem, J. 207, 233-239; 241-252) indicative of spin-spin interaction between the EMN and iron-sulfur compounds of these enzymes. An attempt is made to understand, describe and simulate these spectra in terms of a triplet state with possible contributions from both dipolar and anisotropic exchange (J) interactions. No direct measurement of J is available, but various approaches to setting limits to J are outlined. According to these, J [approximate] 0.4 to 3 cm-1 or 15 to 50 cm-1. The spectra show, in the g = 2 region, a pair of rather sharp inner and a pair of broad outer lines; the latter broaden as well as move out from the center with increasing time (after substrate addition) and substrate concentration, while there is little change of g = 4. The best fits to such a spectra were obtained by assuming distribution of D and E values, depending on substrate effects and arriving presumably from `g-strain'. The fact that both shapes and intensities at g = 2 and g = 4 could be reproduced simultaneously at two frequencies indicates that the assumptions underlying our approaches and interpretations are permissible and reasonable, although we cannot claim their uniqueness. The distance between the centers of the spin densities of the flavin radical and the Fe-S cluster is thought to lie between the limits 3 to 5 A if the asymmetries in the spin-spin interaction are magnetic dipole-dipole in origin. Because there is an indication that the interaction is anisotropic exchange, the upper limit is less stringent.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26294/1/0000379.pd

On the nature of the iron sulfur cluster in a deuterated algal ferredoxin

A protonated and a completely deuterated two-iron algal ferredoxin from Synechococcus lividus have been studied by optical, electron paramagnetic resonance, electron-nuclear double resonance, proton magnetic resonance and Mossbauer spectroscopies; temperature dependent magnetic susceptibility measurements are reported as well. These studies have confirmed the electron localized model of the active center in the two-iron ferredoxins, as previously deduced from studies of spinach ferredoxin, have yielded much more precise spectroscopic parameters for this center, and have thus greatly increased the confidence in this model.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/21928/1/0000335.pd

Quarter field resonance and integer-spin/half-spin interaction in the EPR of Thermus thermophilus ferrodoxin. Possible new fingerprints for three iron clusters

We describe two new characteristics of the EPR of the seven-iron containing ferrodoxin from Thermus thermophilus. First, the reduced state of the 3Fe center, which has traditionally been considered to be EPR-silent, has been found to exhibit [Delta]m = 4 transition, which is unique for Fe-S centers. This signal is similar to that of high-spin Fe2+-EDTA and supports the suggestion that the ground electronic state of the 3Fe cluster is S = 2. Second, we have recorded the EPR spectrum of the fully reduced protein at 9 and 15 GHz and found that changes occur in the signal which are consistent with a weak electronic spin-spin interaction between the [4Fe-4S]+ (S = 1/2) and the reduced 3Fe center. A theoretical explanation is given for the observation of interaction signals with constant effective g values.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25700/1/0000254.pd

Quantitative numerical analysis of g strain in the EPR of distributed systems and its importance for multicenter metalloproteins

A method for simulation of inhomogeneously broadened EPR of metallo-proteins based on recent theoretical advances is surveyed critically in terms of efficiency and accuracy. From the quality of the experimental spectrum, minimal boundary conditions are established for the spatial integration over the g-strained polycrystal. Computational efficiency is achieved by generating the spectrum as an absorption in g space, reducing the number of molecular orientations computed by filtering mosaic artifacts from the Fourier-transformed spectrum, and generating the lineshape due to g strain from a tabulated distribution function. These techniques provide a reduction in computation time by some two orders of magnitude and make the data analysis of EPR of metalloproteins by minimization practical. The resulting simulation program is superior to current approaches in that it does not introduce artifactual multiplicities, and it is expected to require a smaller number of fitting parameters for the quantitative analysis of most cases. To illustrate its potential, the method is applied to EPR data from the iron-sulfur centers in NADH:Q oxidoreductase and in QH2:ferricytochrome c oxidoreductase, clarifying existing controversies on the stoichiometries of these centers.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25873/1/0000436.pd

An investigation of Chromatium vinosum high-potential irondashsulfur protein by EPR and Mossbauer spectroscopy; evidence for a freezing-induced dimerization in NaCl solutions

The high-potential irondashsulfur protein (HiPIP) from Chromatium vinosum contains a cubane prosthetic group that shuttles between the [4Fe-4S]3+,2+ states. We find that the EPR spectra from this protein can be explained as a sum of two components, a major one with g=2.02; 2.04; 2.12, and a minor one with g=2.04; 2.07; ~2.13. In the presence of 0.1-2.0 M NaCl, freezing induces polymerization of the protein (presumably dimers), which is detected as intercluster spindashspin interaction in the EPR. The observed spindashspin interactions are interpreted as being due to two very similar dimeric structures in an approx. 1:2 ratio. Computer simulation of the X- and Q-band EPR spectra shows that the z-components of the g-tensors in each dimer pair must be co-linear, with center-to-center distances between the clusters of ~ 13 A and ~ 16 A. Inspection of possible dimeric structures of C. vinosum HiPIP by standard molecular graphics procedures revealed that the Fe/S cluster is exposed toward a flattened surface and is accessible to solvent. Moreover, the Fe/S clusters in two HiPIP molecules can easily achieve a center-to-center distance of ~ 14 A when approaching along a common 3-fold axis that extends through the S4 sulfur atom of the cubane; the z-component of the EPR g-tensor is co-linear with this symmetry axis.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29129/1/0000168.pd