C-13 natural abundance (SE)-E-3 and (SCT)-C-3 experiments for measurement of J coupling constants between C-13(alpha) or H-1(alpha) and other protonsin a protein

It is demonstrated that the spin-state-selective pulse sequence elements, ( SE)-E-3 and (SCT)-C-3, previously introduced for measurement of J coupling constants in N-15-labeled proteins can be applied for work with peptides an d proteins with C-13 at the natural abundance level, In addition, a method is described for suppression of crosstalk caused by passive spin flips and pulse imperfections, which otherwise results in systematically underestimat ed J coupling constants and thereby inaccurate structural constraints, This method is also applicable for crosstalk suppression in applications of S3E and (SCT)-C-3 to C-13- or N-15-labeled samples. Experimental confirmation is obtained using a 10 mM BPTI sample focusing on C-13 in the a position. T he measured J coupling constants include (3)J(H-N-H-alpha) and (3)J(H-alpha -H-beta) related to the phi and chi(1) angles, respectively, (c) 1999 Acade mic Press.