Plastocyanin isolated from several species including spinach, poplar, and lettuce showed conformational changes both upon reduction and upon lowering the pH as determined by near-ultraviolet absorption and fluorescence measurements. The fluorescence excitation maximum was at 278 nm for all species of plastocyanin measured. In the case of spinach, the emission maximum was at 310-312 nm, similar to a tyrosine residue in solution. The fluorescence intensity increased 22% upon reduction of plastocyanin at pH 7.0. In poplar plastocyanin, the emission maximum was shifted to 335 nm and increased only 10% upon reduction. The 335 nm emission peak observed in poplar plastocyanin is attributed to Tyr 80 which is hydrogen bonded to a carbonyl group on the protein backbone. Tyr 83 was also shown to undergo fluorescence changes upon reduction since the redox state-dependent fluorescence changes decreased for a nitrotyrosine (nitrotyrosine-plastocyanin) derivative of this residue. These results show that the east face of the molecule, which contains both Tyr 80 and 83 as well as a possible binding site, undergoes conformational changes upon reduction. These conformational changes may be involved in promoting smooth electron transport between plastocyanin and its reaction partners. Both the absorption and fluorescence were found to be pH dependent. The quantum yield for fluorescence increased sharply below pH 6 for both oxidized and reduced spinach plastocyanin. This may be related to the appearance of a redox-inactive form of reduced plastocyanin. The conformational changes observed at low pH may provide a mechanism for control of electron transport by the proton gradient. Low concentrations of CaCl2 (10 mM) had no effect on plastocyanin fluorescence. However, addition of 2.7 M (NH4)2SO4 eliminated the redox-dependent fluorescence changes.