Sensitivity improvement and new acquisition scheme of heteronuclear active-coupling-pattern-tilting spectroscopy

A simplified phase-cycling scheme for heteronuclear active-coupling-pattern tilting (ACT) spectroscopy is presented. It is demonstrated that the theor etically expected twofold sensitivity gain over earlier implementations can be experimentally realized. A further intensity increase by a factor of 2 is obtained with standard sensitivity-enhancement pulse-sequence elements. The HSQC-HECADE sequence presented is designed for an accurate determinatio n of heteronuclear one-bond and, with subsequent I-spin isotropic mixing, l ong-range coupling constants. As an exemplary application, the determinatio n of the (3)J(N,H beta) coupling constants in a peptide at natural isotope abundance is demonstrated, Additionally, a new polarization-transfer step f or the long-range HSQC-HECADE experiment is proposed which avoids a fixed d elay tuned to a specific coupling-constant value. Thus, the long-range corr elation experiment does not require prior knowledge of the coupling constan ts to be measured and yields more uniform cross-peak intensity for a broad range of active coupling constants. (C) 2000 Academic Press.