Protein, copper test structure

The metalloproteins that have attracted most attention are those whose properties are most obviously different from those observed in the normal classical aqueous coordination chemistry of the metal ions. The challenge is to account for (initially) unique spectral or chemical properties in terms of the coordination chemistry of the metalloprotein active site, as moderated by the protein environment. With increasing frequency, as in the case of type 1 copper (Section IIIB), crystallography reveals the active site structure with sufficient clarity to provide strong clues as to the origin of the unusual spectroscopy. However, an important test of the structural and spectroscopic analyses is to reproduce the same effects in a model complex. On other occasions, as with the [4Fe-4S] proteins (Section VC), many questions remained even after the structures were known. In spite of the very impressive achievements of protein crystallography, there remain many metalloproteins for which structural data are either not available or inconclusive. 

According to the induced-rack and the entatic state hypotheses [10,12], the Cu(II) coordination sphere in the blue copper proteins is strained into a Cu(I)-like structure. Such hypotheses are hard to test experimentally, but with theoretical methods it is quite straightforward. The actual coordination preferences of the copper ion can be determined by optimising the geometry of the ion and its ligands in vacuum if the optimised structure is almost the same as in the proteins, strain is probably of minor importance for the geometry. 

This formula is eoneeptually simple, and its application can be facilitated, in the ease of Type 1 copper proteins, by referral to x-ray crystallographic structures. One therefore might test this and similar models by comparing the experimentally measured NQI parameters with estimates derived from the known x-ray structures and routine computational chemistry procedures. 

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