13C Direct detected experiments: Optimization for paramagnetic signals

Abstract

To optimize 13C direct detected experiments for the observation of signals close to a paramagnetic center, we have assessed the sensitivity of different sequences based on C O–C ali coherence transfer. Features of CACO experiments were tested for Calbindin D 9k, in which one of the two native Ca 2+ ions is replaced by the paramagnetic Ce 3+ ion. We have studied the comparison of single vs multiple quantum coherence transfer evolution as well as the influence of in-phase vs anti-phase detection of 13C O signals and finally the comparison of a coherence transfer step based on a C y O in plane with respect to a C y ali in plane. The acquisition of the anti-phase component of the signal, accomplished by the removal of the last refocusing steps, allowed the identification of some signals unobserved with other pathways. The structural dependency of paramagnetism-induced nuclear relaxation is such that the identification of the most suitable coherence transfer pathway is not known “a priori” but it is driven by the relative proximity of C ali and C O to the paramagnetic center.