ROESY SPECTRA OF PARAMAGNETIC MODEL HEME COMPLEXES - UNAMBIGUOUS DIFFERENTIATION OF CROSS-PEAKS DUE TO CHEMICAL-EXCHANGE AND THE NUCLEAR OVERHAUSER EFFECT IN THE SAME MOLECULE

We have investigated the ROESY spectrum of [TMPFe(2-MeImH)(2)]+ClO4- o ver the temperature range from -55 to -75 degrees C using spin-lock fi elds of 4.1 and 10.3 kHz. The latter B-1 field is more typical of thos e used to observe TOCSY spectra, but is mon closely in line with the s pectral bandwidth required for this paramagnetic complex (12 kHz at 30 0 MHz). Chemical exchange cross-peaks were easily observed at both spi n-lock fields, but ROE cross-peaks were significantly more intense wit h the larger B-1 field. At -75 degrees C, all but one of the expected ROEs were observed; the absent ROE is that between the 2-CH3 of the ax ial ligands and either the ortho-CH3(2) or -(3) resonance. The absence of this ROE is probably due to the shalt T-1 of the ligand 2-CH3 prot ons (<10 ms) and the large difference in T-1 between it and the ortho- CH3 protons (25-35 ms at -75 degrees C). In spite of the absence of th is ROE, a nearly complete assignment of the ROESY spectrum in terms of the structure of the complex has been made. Minor interference from T OCSY effects, observed as a decrease in the expected intensity of the ROE cross-peaks between pyrrole-H resonances (2) and (4), was noted, b ut did not prevent observation of this ROE. For the diamagnetic Co(III ) analog, both this ROE (to ortho-CH3(3) in this case) and the ROE bet ween the ligand 4-H and ortho-CH3(4) resonances were observed, but the small separation between pyrrole-H signals precludes a complete assig nment of the ROESY spectrum of this complex. (C) 1998 Elsevier Science S.A. All rights reserved.