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UV resonance Raman bands of Cu-bound and protonated histidine residues have been detected in 2H2O solutions of poplar plastocyanin. For the Cu(II) protein, slow NH–2H exchange of the His37 ligand was monitored via the growth of bands at 1389 and 1344 cm−1 when Pcy was exchanged into 2H2O, or via their diminution when the protein was exchanged back into H2O; the rate constant is 7×10−4/s at pH (p2H) 7.4 at room temperature. The slow exchange is attributed to imidazole H-bonding to a backbone carbonyl. Nearby bands at 1397 and 1354 cm−1, appear and disappear within the mixing time, and are assigned to the solvent-exposed His87 ligand. The ∼10 cm−1 differences between His37 and His87 are attributed to the effect of H-bonding on the imidazole ring modes. The UVRR spectra of the Cu(I) protein in 2H2O reveal a 1408 cm−1 band, characteristic of NH–2H-exchanged histidinium, which grows in as the p2H is lowered. Its intensity follows a titration curve with pKa=4.6. This protonation is assigned to the His87 residue, whose bond to the Cu(I) is known from crystallography to be broken at low pH. As the 1408 cm−1 band grows, a band at 1345 cm−1 diminishes, while another, at 1337 cm−1 stays constant. These are assigned to modes of bound His87 and His37, respectively, shifted down 7–9 cm−1 from their Cu(II) positions. [ABSTRACT FROM AUTHOR]