One of the main mechanisms of membrane protein folding is by spontaneous insertion into the lipid bilayer from the aqueous environment. The bacterial toxin, colicin la, is one such protein. To shed light on the conformational changes involved in this dramatic transfer from the polar to the hydrophobic milieu, we carried out 2D magic-angle spinning C-13 NMR experiments on the water-soluble and membrane-bound states of the channel-forming domain of colicin la. Proton-driven C-13 spin diffusion spectra of selectively C-13-labeled protein show unequivocal attenuation of cross-peaks after membrane binding. This attenuation can be assigned to distance increases but not reduction of the diffusion coefficient. Analysis of the statistics of the interhelical and intrahelical C-13-C-13 distances in the soluble protein structure indicates that the observed cross-peak reduction is well correlated with a high percentage of short interhelical contacts in the soluble protein. This suggests that colicin la channel domain becomes open and extended upon membrane binding, thus lengthening interhelical distances. In comparison, cross-peaks with similar intensities between the two states are dominated by intrahelical contacts in the soluble state. This suggests that the membrane-bound structure of colicin la channel domain may be described as a "molten globule", in which the helical secondary structure is retained while the tertiary structure is unfolded. This study demonstrates that C-13 spin diffusion NMR is a valuable tool for obtaining qualitative long-range distance constraints on membrane protein folding.