NMR studies of plastocyanin have centered on the ligands to the copper atom at the active site, particularly histidines-37 and -87. Heteronuclear ( 1 3 C, 1 H) J-connectivity spectroscopy has enabled cross assignment of 1 H and 1 3 C NMR resonances from the two copper-ligated histidines. In addition to providing assignments of the 1 3 C resonances, the two-dimensional Fourier transform NMR results require the reversal of the original 1 H NMR assignments to the ring protons of histidine-37. The line widths of the ring protons of histidine-87 are field-dependent leading to determination of the reduced lifetime of the proton on the N δ atom (about 400 μs).