1H NMR studies of the phosphocarrier protein HPr and its three nitrotyrosyl derivatives revealed some structural features which may finally lead to an explanation of the mechanism of the phospho-transfer reaction. Titration studies on mononitrated, dinitrated, and trinitrated derivatives--i.e., derivatives with Tyr-56, Tyr-56 and Tyr-37, and Tyr-56, Tyr-37, and Tyr-6 modified--have been performed. The three tyrosyl residues seem to be in positions completely different from each other with respect to their solvent accessibility; Tyr-56 seems to be located near the surface of the protein, Tyr-6 seems to be completely buried, and Tyr-37 takes an intermediate position. Tyr-6 contributes to the core structure of the protein. A resonance at -0.18 ppm could be shown to correspond to a CH3 group of a valine. Nuclear Overhauser experiments revealed its being close to Tyr-6. One of the resonances tentatively assigned to methionine SCH3 groups titrates in the dinitrated derivative with the same pK as nitrotyrosyl residue 37. The titration behavior of the active-center histidyl residue suggests a hydrogen bond to the imidazole ring, possibly from Tyr-56 or Arg-17.