Alamethicin is an _-aminoisobutyric acid-containing antibiotic produced by the soil fungus 'Trichoderma viride'. It forms voltage-dependent ion channels in lipid with multiple conductance levels. Unlabeled and 15N labeled alamethicin dimers were synthesized in which two monomers are linked at their C-terminal ends by a flexible linker. Electrospray ionization mass spectrometry confirmed the identity of the dimers purified by high-performance liquid chromatography. The structure and dynamics of alamethicin dimers were studied by carrying out extensive nuclear magnetic resonance spectroscopy and circular dichroism experiments. The results suggest that the entire peptide exists in a helical conformation, the N-terminus (Aib1-Aib10) has a more stable helical structure than the C-terminus (Val15-Pho 20) and a flexible structure in the middle (Gly11-Pro 14) connects the helices. The results also suggest that the C-terminal helices associate but the N-termini do not and support a gating mechanism that undergoes a voltage-dependent conformational rearrangement of the peptide with respect to the bilayer. (Abstract shortened by UMI.)