13C Direct detected experiments:: Optimization for paramagnetic signals

To optimize C-13 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-U-ali coherence transfer. Features of CACO experiments were tested for Calbindin D-9k, in which one of the two native Ca2+ ions is replaced by the paramagnetic Ce3+ 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 C-13(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. (c) 2005 Elsevier Inc. All rights reserved.