Apparent Transverse Relaxation Rates in Systems with Scalar-Coupled Protons

The modulation of spin echoes by homonuclear scalar couplings render the determination of transverse relaxation rates of individual spins difficult, in particular for molecules that are isotopically enriched in 13C or 15N, and for all molecules with scalar-coupled protons. To avoid echo modulations, most studies using refocusing pulses have so far been restricted to isolated 1H, 13C, or 15N spins. We report measurements of apparent 1H transverse relaxation rates of backbone and side-chain protons in Cyclosporin A (CsA) determined by quenching the echo modulations that arise from homonuclear scalar couplings between protons.