Magnesium: density electronic structure

Interpretation of the electron density maps showed that the large subunit could not be modelled beyond His536 (Fig. 6.10), that is fifteen amino acids short of the 551 residues predicted by the nucleotide sequence (Table 6.2). At about the same time, the cleavage of this fifteen-residue stretch, which is performed by a specific protease, was reported to be an obligatory step for the maturation of the enzyme (Menon et al. 1993). It is also of interest to note that in all [NiFe] hydrogenase crystal structures this buried C-terminal histidine is ligated to a metal atom which is either a magnesium or an iron (see above). 

A portion of the structure model and the 2F0-FC electron density map contoured at 1.0 standard deviation. The region containing the inhibitor, magnesium, parts of AdoMet, and the residue Glul99 is shown. The sphere marked with W represents a water molecule. 

Analyses of electron density distributions have enabled the positions of major elements of high atomic weights such as iron and other transition elements to be located relative to lighter elements such as magnesium and aluminium in mineral crystal structures. The widespread availability of automated X-ray dif-fractometry and least squares refinement programs have increased the availability of site occupancy data for transition metal ions in most contemporary crystal structure refinements. 

The terminal carbonyl groups may also play the role of a base if there is considerable concentration of electron density on them. Examples are [Co(CO)4]", [Fe(CO)4]" , [Mn(CO)5]-, [Mocp(CO)3]-, and [Fecp(CO)2]- which, with Mg", and Mn -", form the M-CO-M bonds. In [ Mocp(CO)3 2 Mg(py)4] the magnesium atom is six-coordinate it is bonded to four pyridine molecules as well as to two oxygen atoms of carbonyl groups (in trans orientation). A similar structure is adopted by [V(THF)4][V(C0)6]2- Four molecules of tetrahydrofuran are coordinated to... 

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