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

Abstract

We have investigated the ROESY spectrum of [TMPFe(2-MelmH) 2] ClO 4 over the temperature range from −55 tc −75°C using spinlock fields of 4.1 and 10.3 kHz. The latter B 1 field is more typical of those used to observe TOCSY spectra, but is more closely in line with the spectral bandwidth required for this paramagnetic complex (12 kHz at 300 MHz). Chemical exchange cross-peaks were easily observed at both spin-lock fields, but ROE cross-peaks were significantly more intense with the larger B 1 field. At −75°C, all but one of the expected ROEs were observed; the absent ROE is that between the 2-CH 3 of the axial ligands and either the ortho-CH 3(2) or −(3) resonance. The absence of this ROE is probably due to the short T 1 of the ligand 2-CH 1 protons (<10 ms) and the large difference in T 1 between it and the ortho-CH 1 protons (25–35 ms at −75°C). In spite of the absence of this ROE a nearly complete assignment of the ROESY spectrum in terms of the structure of the complex has been made. Minor interference from TOCSY effects, observed as a decrease in the expected intensity of the ROE cross-peaks between pyrrole-H resonances (2) and (4), was noted, but did not prevent observation of this ROE. For the diamagnetic Co(III) analog, both this ROE (to ortho-CH 1(3) in this case) and the ROE between the ligand 4-H and ortho-CH 3(4) resonances were observed, but the small separation between pyrrole-H signals precludes a complete assignment of the ROESY spectrum of this complex.