Heavy atom clusters

It is clear that since supported heterogeneous catalysts frequently comprise heavy atom clusters, such as Pt and Pd, distributed over light supports such as charcoal, silica and alumina, Z contrast imaging could be very useful in catalyst studies, particularly for detecting the smaller clusters (< 10A) which are frequently missed by the conventional imaging methods. 

Because surfaces of ribosomal particles have a variety of potential binding sites for such clusters, attempts are in progress to bind heavy-atoms covalently to a few specific sites on the ribosomal particles prior to crystallization. This may be achieved either by direct interaction of a heavy-atom cluster with chemically active groups such as -SH or the ends of rRNA on the intact particles or by covalent attachment of a cluster to natural or tailor-made carriers that bind specifically to ribosomes. 

Recently von Boehlen et al. (1991) have obtained an improved crystal form of the large (50S) ribosomal subunit of H. marismortui, which diffracts to 3 A resolution, by the addition of 1 mAf Cd2+ to a crystallization medium that contained over 1.9 M of other salts. These improved crystals are isomorphous with the previously reported ones, and, as was the case for the previous crystals, they show no measurable decay after a few days of synchrotron irradiation at cryotemperatures. The new crystals are of adequate mechanical strength. Initial phasing studies by specific and quantitative deriva-tization with super-dense heavy-atom clusters and by real- and reciprocal-space rotation searches are in progress. 

This work also highlights the central problem of theoretical treatments of heavy-atom clusters (which is somewhat less pressing for lighter atoms) An ap-... 

Figure 3.43 Comparison ofthe penetrative polyhedra in 14 (a) and 15 (b) that form the heavy atom cluster cores. Cu gray spheres S black spheres P white spheres. The drawn lines only represent polyhedral edges and do not specify binding interactions.

The main difficulty in the theoretical study of clusters of heavy atoms is that the number of electrons is large and grows rapidly with cluster size. Consequently, ab initio "brute force" calculations soon meet insuperable computational problems. To simplify the approach, conserving atomic concept as far as possible, it is useful to exploit the classical separation of the electrons into "core" and "valence" electrons and to treat explicitly only the wavefunction of the latter. A convenient way of doing so, without introducing empirical parameters, is provided by the use of generalyzed product function, in which the total electronic wave function is built up as antisymmetrized product of many group functions [2-6]. 

In the elucidation of the X-ray structure of hCP by the method of isomorphous replacement, gold and mercury heavy atom derivatives were utilized. In the case of the mercury derivative, p-chloromercury-benzoate, the heavy atom bound to the free sulphydryl residue, C221, but for the gold cyanide derivative the gold atom was found to bind in the vicinity of the trinuclear copper cluster. A realistic explanation of this... 

The allyl-resonance stabilized E- and Z-pent-l,3-dienyl-2-cations (22 and 23) are the smallest member of vinyl cations observed as persistent species in superacid solution 49 These are difficult to generate experimentally50 but structures with only five heavy atoms are suitable candidates for coupled cluster model calculations. A challenging task of quantum chemistry was to assign the 13C NMR spectrum of the mixture of isomers (Fig. 3), which exhibits pairs of signals of 22 and 23 which differ only by a few ppm, to the chemical shifts for the specific carbon atoms of the E- and Z-isomers, respectively. 

Fig. 9. Approximate Bragg resolution for the first exposure of each of about 200 crystals from H. marismortui SOS subunits that were investigated at XI1, EMBL/DESY, Hamburg (ERG), in August 1986 at —4° to 19 °C. Shading indicates heavy-atom derivative test crystals (undecagold-cluster and tetrakis(acetoxymercuri)methane (TAMM) see paragraph 4 Phase Determination)...

Heavy-atom derivation of an object as large as a ribosomal particle requires the use of extremely dense and ultraheavy compounds. Examples of such compounds are a) tetrakis(acetoxy-mercuri)methane (TAMM) which was the key heavy atom derivative in the structure determination of nucleosomes and the membrane reaction center and b) an undecagold cluster in which the gold core has a diameter of 8.2 A (Fig. 14 and in and ). Several variations of this cluster, modified with different ligands, have been prepared The cluster compounds, in which all the moieties R (Fig. 14) are amine or alcohol, are soluble in the crystallization solution of SOS subunits from H. marismortui. Thus, they could be used for soaking. Crystallographic data (to 18 A resolution) show isomorphous unit cell constants with observable differences in the intensity (Fig. 15). 

In this paper, the main features of the two-step method are presented and PNC calculations are discussed, both those without accounting for correlation effects (PbF and HgF) and those in which electron correlations are taken into account by a combined method of the second-order perturbation theory (PT2) and configuration interaction (Cl), or PT2/CI [100] (for BaF and YbF), by the relativistic coupled cluster (RCC) method [101, 102] (for TIF, PbO, and HI+), and by the spin-orbit direct-CI method [103, 104, 105] (for PbO). In the ab initio calculations discussed here, the best accuracy of any current method has been attained for the hyperfine constants and P,T-odd parameters regarding the molecules containing heavy atoms. 

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