Resolution clusters

An unexpected result from X-ray structural analysis is that two [3Fe-4S] clusters and one isolated zinc center are observed in the three-dimensional structure determined at 2.0-A resolution cluster II was found as a cubane [3Fe-4S] cluster and was connected to the polypeptide chain by three cysteinyl residues, Cys-55, Cys-83, and Cys-89 (Fig. 2). The presence of two [3Fe-4S] clusters is very unusual in the bacterial-type dicluster ferredoxins. Biochemical and spectroscopic analyses of Sulfolobus sp. ferredoxin have shown that this 6Fe-containing species is an artifact of the crystallization procedure after protein isolation. Thus, the crystal structure provides the first structural evidence for a [4Fe-4S] [3Fe-4S] cluster conversion at the cluster n site in the dicluster-type ferredoxins. Interestingly, a potential ligand for a fourth site, Cys-86, is not bound in the crystal structure at 2.0-A resolution. In addition, the polypeptide conformation in vicinity of the cubane [3Fe-4S] cluster II and Cys-86 is reportedly an intermediate form between the [3Fe-4S] and [4Fe-4S] cluster conformations. ... 

These days, remarkably high-resolution spectra are obtained for positive and negative ions using coaxial-beam spectrometers and various microwave and IR absorption teclmiques as described earlier. Infonnation on molecular bond strengths, isomeric fonus and energetics may also be obtained from the teclmiques discussed earlier. The kinetics of cluster-ion fonuation, as studied in a selected-ion flow tube (SIFT) or by high-pressure... 

Nesbitt D J 1994 Fligh-resolution, direct infrared-laser absorption-spectroscopy in slit supersonic ]ets—intermolecular forces and unimolecular vibrational dynamics in clusters Ann. Rev. Phys. Chem. 45 367-99... 

Plenary 9. J W Nibler et al, e-mail address niblerj chem.orst.edu (CARS and SRS). High resolution studies of high lymg vibration-rotational transitions in molecules excited in electrical discharges and low density monomers and clusters in free jet expansions. Ionization detected (REMPI) SRS or IDSRS. Detect Raman... 

The reactivity of size-selected transition-metal cluster ions has been studied witli various types of mass spectrometric teclmiques [1 ]. Fourier-transfonn ion cyclotron resonance (FT-ICR) is a particularly powerful teclmique in which a cluster ion can be stored and cooled before experimentation. Thus, multiple reaction steps can be followed in FT-ICR, in addition to its high sensitivity and mass resolution. Many chemical reaction studies of transition-metal clusters witli simple reactants and hydrocarbons have been carried out using FT-ICR [49, 58]. 

It is also possible to measure microwave spectra of some more strongly bound Van der Waals complexes in a gas cell ratlier tlian a molecular beam. Indeed, tire first microwave studies on molecular clusters were of this type, on carboxylic acid dimers [jd]. The resolution tliat can be achieved is not as high as in a molecular beam, but bulk gas studies have tire advantage tliat vibrational satellites, due to pure rotational transitions in complexes witli intennolecular bending and stretching modes excited, can often be identified. The frequencies of tire vibrational satellites contain infonnation on how the vibrationally averaged stmcture changes in tire excited states, while their intensities allow tire vibrational frequencies to be estimated. 

Careful measurement of tlie kinetics of association of a molecule witli a surface can also yield stmctural infonnation at tliis level of resolution [27], and lateral clustering and crystallization can also be deduced. This is described in more detail in section C2.14.7.2. 

Future development of SAM-based analytical technology requires expansion of the size and shape selectivity of template stmctures, as well as introduction of advanced chemical and optical gating mechanisms. An important contribution of SAMs is in miniaturization of analytical instmmentation. This use may in turn have considerable importance in the biomedical analytical area, where miniature analytical probes will be introduced into the body and target-specific organs or even cell clusters. Advances in high resolution spatial patterning of SAMs open the way for such technologies (268,352). 

Fig. 2. Metrical data for [Fe3S4] clusters taken from the high-resolution crystsd structures of D. gigas Fdll (1.7 A resolution) (19, 61, 166), A. vinelandii Fdl (1.35 A resolution) (165), Emd the model complex (Et4N)3[Fe3S4(LSs)] (50).

By the application of modem spectroscopic techniques and through the recent availability of high-resolution structural information, several issues could be resolved, in particular the nature of the ligands of the Rieske cluster. Now that stmctures are available, the research will focus on new questions in order to provide a better understanding of the electronic properties of Rieske and Rieske-type clusters as well as of their function ... 

These were relatively low-resolution structures, and with refinement some errors in the initial structural assignments have been detected (4-7). Since the structures were first reported the subject has been extensively reviewed in this series (8) and elsewhere 9-15). This review will focus on the structure, biosynthesis, and function of the met-allosulfur clusters found in nitrogenases. This will require a broader overview of some functional aspects, particularly the involvement of MgATP in the enzymic reaction, and also some reference will be made to the extensive literature (9, 15) on biomimetic chemistry that has helped to illuminate possible modes of nitrogenase function, although a detailed review of this chemistry will not be attempted here. This review cannot be fully comprehensive in the space available, but concentrates on recent advances and attempts to describe the current level of our understanding. 

Figure 3 shows the three-dimensional structure of the MoFe protein from Klebsiella pneumoniae, Kpl, obtained at 1.65-A resolution (7). The overall structure of the polypeptides is frilly consistent with that reported earlier for Avl (3). The a and /8 subunits exhibit similar polypeptide folds with three domains of parallel /3 sheet/a helical type. At the interface between the three domains in the a subunit is a wide shallow cleft with the FeMoco at the bottom of the cleft about 10 A from the solvent. FeMoco is enclosed within the a subunit. The P cluster, however, is buried within the protein at the interface between the a and /8 subunits, being bound by cysteine residues from each subunit. A pseudo-twofold rotation axis passes between the two halves of the P cluster and relates the a and (3 subunits. Each af3 pair of subunits contains one FeMoco and one P cluster and thus appears... 

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