IMPROVED SENSITIVITY IN SELECTIVE NMR CORRELATION SPECTROSCOPY AND APPLICATIONS TO THE DETERMINATION OF SCALAR COUPLINGS IN PEPTIDES AND PROTEINS

A new method, providing a significant improvement in sensitivity, is p resented for obtaining selective two-dimensional NMR correlation (COSY ) spectra. The method is demonstrated on a small molecule having nonli near optical properties and is further demonstrated by the precise det ermination of scalar coupling constants in the peptide gramicidin and in a 57 amino acid fragment of the nucleocapsid of the hepatitis C vir us, In these systems we observe sensitivity improvements of between a factor of 2 and 6. In the case of the protein it was not practical to record the selective COSY spectra using conventional techniques. The m ethod makes use of the recently introduced concept of ''linear'' selec tive pulses which provide a simple way of obtaining pure-phase band se lective excitation with an arbitrary shape for the response. In a sele ctive 2D COSY experiment the relative sensitivity improvement provided by these pulses is cubed with respect to a 1D experiment. The resulti ng pulse sequences are highly unusual in that they require a modulatio n of the pulse shape and pulse length as the indirect detection period is increased.