ENHANCED SENSITIVITY OF RAPIDLY EXCHANGING AMIDE PROTONS BY IMPROVED PHASE CYCLING AND THE CONSTRUCTIVE USE OF RADIATION DAMPING

Two alternative, general methods are presented that lead to enhanced s ignal intensity of rapidly exchanging protons. Both methods work by av oiding saturation of the water resonance, and are convenient to implem ent since they do not use any selective pulses. One method carefully c hooses proton pulse phases and gradient strength and position in such a way that the water is realigned along the +z axis at the beginning o f the acquisition time. An alternative method is proposed for cases wh ere the pulse sequence does not allow such phase cycling. The latter u ses radiation damping to bring water back to the +z axis 20-30 ms afte r acquisition. The methods are applied to the triple-resonance experim ents HNCA, HNCO and HN(CO)CA. Both methods require pulsed B-0 field gr adients and can result in higher signal intensity by a factor of two o r more.