The 'Blue' Copper Centres

Such centres owe their name to the intense blue colouration of the corresponding Cu(II) proteins. The colour is particularly distinctive since the metal centres are so optically diluted in these metalloenzymes that only intense absorption in the visible region, resulting from symmetry allowed electronic transitions, can give rise to conspicuous colours. In contrast, the comparatively pale blue colour of normal Cu(II)) is the result of forbidden electronic transitions between d-orbitals of different symmetry; in Cu²⁺_(aq) this gives a molar extinction coefficient of 10M^-1cm^-1 from a broad absorption between 10,000cm^-1 and 15,000cm^-1 compared to about 3000 M^-1cm^-1 observed for blue Cu(II) centres.  For the T1 centres the intense absorption is attributed to a ligand-to-metal charge transfer between the Cu²⁺ and a bonded cysteinate ligand.  Typically, as in azurin or plastocyanin this occurs around 16,000cm^-1. Caeruloplasmin has three T1 centres, and the blue absorption is at 16,400cm^-1 (610nm).

Crystal structures show a very irregular 'tetrahedral' coordination with two sulphurs from methionine and cysteinate, and two histidine nitrogens. However a comparison of azurin with plastocyanin shows that the geometry is in some ways closer to a trigonal bipyramid, with and without one extra apicial ligand, so that azurin has a weakly bound glutamine oxygen, and plastocyanine does not. The T1 coppers in caeruloplasmin are in plastocyanine-type domains. Each of these are coordinated to two histadines and a cysteine, in two of the T1 domains there is also a methionine residue, the third T1 domain has a leucine residue which may only have a van der Waals type contact with the copper.

T1 copper centres are functional in the reversible electron transfer:

Cu²⁺ + e^-   =   Cu⁺

The strongly distorted geometry represents a compromise (entactic-state situation) between d¹⁰ Cu(I), with its preferred tetrahedral or trigonal coordination through soft sulfur ligands, and d⁹ Cu(II) with preferential square planar or square pyramidal geometry and nitrogen ligand coordination.   This irregular, high energy arrangement at the metal centre resembles the transition-state geometry between the tetrahedral and square planar equilibrium configurations of the two oxidation states involved and permits enhanced rates of electron transfer. The potential range for proteins with T1 copper centres runs from 180 mV in stellacyanin to 680 mV in rusticyanin.