Application of accordion excitation in H-1-N-15 long-range heteronuclear shift correlation experiments at natural abundance

Accordion excitation provides the means of sampling a broad range of potent ial long-range heteronuclear couplings in a single experiment. Two long-ran ge heteronuclear experiments, ACCORD-HMBC and IMPEACH-MBC, have been develo ped thus far that offer accordion excitation. The results obtained with the se experiments for H-1-N-15 heteronuclear shift correlation at natural abun dance are compared. Low digitization in the second (F-1) frequency domain o f H-1-N-15 ACCORD-HMBC potentially hampers this use of the experiment since it is prone to coupling modulation in the second frequency domain as a fun ction of the accordion excitation delay. The constant time variable delay u sed in lieu of a simple accordion delay in the IMPEACH-MBC experiment circu mvents homonuclear coupling modulation in F-1, thereby improving resolution relative to the ACCORD-HMBC experiment for the same number of increments o f the evolution period. Results obtained using accordion-optimized experime nts are contrasted with conventional gradient HMBC (GHMBC). Accordion-optim ized experiments offer a potential advantage for H-1-N-15 long-range hetero nuclear shift correlation experiments due to the wide variability of long-r ange H-1-N-15 couplings. Copyright (C) 2000 John Wiley & Sons, Ltd.