The combined use of diamagnetic and paramagnetic chemical shift changes makes it possible to obtain detailed information about the structure of a transient complex of redox proteins. The structure suggests that the electrostatic interactions 'guide' the partners into a position that is optimal for electron transfer, and which may be stabilised by short-range interactions.

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The oxidized blue copper proteins azurin and stellacyanin have been investigated through 1H NMR at 800 MHz and the results compared with those for plastocyanin (Bertini, I.; Ciurli, S.; Dikiy, A.; Gasanov, R.; Luchinat, C.; Martini, G.; Safarov, N. J. Am. Chem. Soc. 1999, 121, 2037). By exploiting saturation transfer between the oxidized and the reduced forms, all the hyperfine shifted signals can be assigned, including the β-CH2 protons of the coordinated cysteines, which are so broad not to be detected under direct observation. Both hyperfine shifts and line widths of the latter signals differ dramatically from one protein to another:  average hyperfine shifts of about 850, 600, and 400 ppm and average line widths of 1.2, 0.45, and 0.25 MHz are observed for azurin, plastocyanin, and stellacyanin, in that order. The observation of a nuclear line width of 1.2 MHz is unprecedented in high-resolution NMR in solution. These data are interpreted as a measure of the out-of-plane displacement of the copper ion, which increases on passing from azurin to plastocyanin to stellacyanin. The present approach seems general for the investigation of blue copper proteins.