A 17-residue disulfide-bridged peptide (PAK 128-144) corresponding to the C-terminus of Pseudomonas aeruginosa pilin strain K has been studied by one- and two-dimensional nmr techniques. This synthetic immunogen has been found to exist as two distinct conformations in solution, which have been demonstrated to arise as a result of the isomerization of the I138-P139 amide bond. The two isomers occur in the ratio of 3:1 trans to cis at 5 degrees C. Sequential assignments for both forms have been accomplished through the use of nuclear Overhauser enhancement spectroscopy (NOESY) spectra and most side-chain resonances have been assigned using a combination of correlated spectroscopy, total correlated spectroscopy, and NOESY spectra. The presence of the cis isomer, which is considerably more predominant in the oxidized peptide, was confirmed by the observation of the characteristic NOEs between P139 and the preceding residue. Further corroboration was given by the disappearance of the cis resonances in the spectrum of the P139A analogue of PAK 128-144. From observation of the differences in the chemical shifts and amide proton temperature coefficients of the two isomers, it is apparent that the two forms differ markedly in their solution conformation. The biological implications of the isomerization are discussed.