Heavy atoms derivative preparation

Although the soaking method for heavy-atom derivative preparation is by far the simplest and most common, it is not the only method used. One can first derivatize the macromolecule, and then crystallize. This procedure is less frequently used because of drawbacks such as the inabihty to produce isomorphous crystals due to the disruption of intermolecular contacts by the heavy atoms. Other frequent problems are the introduction of additional heavy-atom sites (a potential complicating factor in phasing) by exposing sites hidden by crystal contacts, and changing the solubility of the derivatized macromolecule. 

A significant percentage of all the protein projects tackled will present the technical challenges of small crystal volume or problems of isomor-phous heavy atom derivative preparation for which the intensity and tunability of SR, respectively, will be harnessed. 

NCP crystals. There were two facets to this approach. First, it was necessary to reconstitute NCPs from a defined sequence DNA that phased precisely on the histone core to circumvent the random sequence disorder. It was obvious that the DNA was important for the quality of the diffraction from NCP crystals but the role of histone heterogeneity was not so clear. Heavy atom derivatives (i.e., electron rich elements bound in specific positions on the proteins) were not readily prepared by standard soaking experiments, due to a paucity of binding sites. Hence, it was necessary to selectively mutate amino acid residues in the histones to create binding sites for heavy atoms. 

Hatfull, G. R, Sanderson, M. R., Freemont, P. S., Raccuia, P. R., Grindley, N. D. F. and Steitz, T. A. (1989). Preparation of heavy-atom derivatives using site-directed mutagenesis introduction of cysteine residues into yS resolvase. /. Mol. Biol. 208,661-667. 

Petsko, G. A. (1985). Preparation of isomorphous heavy-atom derivatives. Method Enzymol. 114,147-156. 

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). 

X 77.8 X 51.4 A, a space group of P2i2i2j, and a Z value of 4 (molecules per unit cell) (46). Several heavy atom derivatives, whose crystalline structure are isomorphous with that of the native enzyme, have been prepared for the X-ray diffraction analysis of crystalline cytochrome c peroxidase. 

Prepare heavy-atom derivatives of the protein with heavy atoms in different positions in the unit celt. Measure an intensity data set for each heavy-atom derivative. 

A number of studies have been performed with methyl picolinimidate (Benisek and Richards 1968 Plapp et al. 1971) aimed at exploring the usefulness of the metal-chelating properties of such derivatives in the preparation of isomorphous heavy atom derivatives of proteins for X-ray diffraction studies. 

In our laboratory investigation a phase ambiguity arose when the experiment was performed only once. It was resolved simply by moving the oscillator to a new location and changing the phase of the reference wave. The analogous operation in X-ray crystallography is not quite so simple but still within reach. We must prepare a second (or even more, to be sure) heavy atom derivative bound at some other location in the unit cell. This second oscillator position generates then a second reference wave that yields, in the phase calculation, one phase solution in common with the first experiment. 

Heavy atom derivatives of a macromolecular crystal can be prepared (Green, Ingram and Perutz 1954) which for a minimum of two derivatives (and the native crystal) and in the absence of errors, leads to a unique determination of the phase ahkt in equation (2.7) (figure 2.13(a)). This requires the site and occupancy of the heavy atom to be known for the calculation of the vector FH (the heavy atom structure factor). In the absence of any starting phase information the heavy atom is located using an isomorphous difference Patterson synthesis P(u,v,w) where the isomorphous difference is given by... 

In particular the structure determination of cucumber basic protein has been made using MAD data recorded on this instrument around the CuK edge (Guss et al 1988, section 9.7.5). This protein crystal structure had previously defied solution for many years because of problems associated with preparing heavy atom derivatives. 

The electron density maps published so far have been based on the orthorhombic crystal form. The anions Pt(CN)4 - and Au (CNlz and the mercury complex ethyl mercurythiosalicylate were used to prepare heavy atom derivatives (106,107). The overall dimensions of the molecule... 

---