Choosing a Spin-Lock Transmitter Position Which Minimizes HOHAHA Distortions of ROESY Spectra. Observation of a Correlation-Time Dependence of Frequency Offset

A technique for reliably selecting a transmitter position for rotating-frame NOE experiments which minimize scalar coupling artifacts is described. The technique makes use of the COSY spectrum of the molecule in question. The method is illustrated by application to thiostrepton, an antibiotic of molecular weight 1665. For an individual compound, the range of transmitter settings over which HOHAHA distortions are observed is found to depend on the molecular correlation time of the compound under study. The origin of this effect is discussed in terms of rotating-frame relaxation during the spin-lock period.