Fig. 7.

Effect of PROXYL spin labeling at Cys223 on backbone amide signals in HNCO spectrum for the C-terminal 71 residues of subunit a. The ratio of peak volumes (V_ox/V_red) before and after reduction of the spin label with phenylhydrazine is plotted versus residue number. The peak volumes of residues marked X could not be calculated confidently because of signal overlap, peak splitting, or significant chemical shift changes caused by the addition of phenylhydrazine. The ratio of peak volumes for residue 264 was 1.98 as indicated by the *. The brackets indicate the TMHs predicted in Fig. 1.