In a preliminary report, we have previously shown that N-[(2,2,5,5-tetramethyl-1-oxypyrrolidin-3-yl)carbonyl]-L-phenyl ala nine tert-butyl ester (SL-Phe) exhibits specific binding to hemoglobin and an antiaggregation activity more than 2 orders of magnitude greater than that of phenylalanine [Lu, H.-Z., Currie, B. L., & Johnson, M. E. (1984) FEBS Lett. 173, 259-263]. Transverse 1H NMR relaxation measurements have been used to investigate the interaction of SL-Phe with hemoglobin molecules by use of the resonances assigned to the C2 protons of the beta 2 His, the beta 143 His, and the beta 146 or beta 97 His residues as intrinsic probes. Distance calculations using the paramagnetically induced relaxation data suggest that the SL-Phe binding site is approximately 12-16 A away from the C2 protons of the beta 2 His and the beta 146 or beta 97 His residues in the (carbonmonoxy)hemoglobin tetramer; for deoxyhemoglobin, the distances are approximately 14-17 A between the SL-Phe binding site and the C2 protons of the beta 2 His, the beta 143 His, and the beta 146 His residues. Calculations using the (carbonmonoxy)hemoglobin crystal atomic coordinates only restrict the probable SL-Phe binding region to the full F and H helices of the beta-chain and a small section of the alpha-chain. For deoxyhemoglobin, the distance calculations provide greater restrictions on the probable binding region, limiting it to small sections of the beta-chain F, G, and H helices near the EF bend and to a few residues on the alpha-chain.(ABSTRACT TRUNCATED AT 250 WORDS)