The lattice material surrounding brain coated vesicles is formed predominately from a M, 180000 polypeptide subunit structure termed clathrin. The vesicle coat is dissociated from the bilayer membrane by mild alkaline solutions (pH 8.5) to form clathrin trimers, or triskelions. In solution, at pH 6.5, the triskelions assemble into cagelike structures exhibiting lattices whose hexagonal and pentagonal polyhedral faces are similar to those associated with the coated vesicles. The secondary structures of these clathrin species in aqueous media were characterized through an examination of the infrared spectroscopic amide I and II vibrational frequency regions in an effort to monitor the structural changes that arise as the water-soluble triskelions assemble into cagelike networks. The reassembly process is accompanied by an increase in the peak height intensity ratio of the amide II to amide I region from 0.38 to 0.68 and a shift in the amide I frequency from 1648 to 1656 cm-1. These spectral changes indicate a decrease in the α-helical content of the triskelion polypeptide chains as they assemble to form cages. Other spectral features associated with the amide I and II bands indicate the presence of both β-sheet and β-turn protein conformations in the clathrin structures. The amide I and II frequency and intensity characteristics reflecting the clathrin coat of intact vesicles are nearly identical with analogous spectra of the clathrin cages reassembled from triskelions in the absence of membrane components. The spectral results suggest that for structural studies involving protein-protein and protein-lipid interactions, the reconstituted, water-soluble cages provide faithful model systems for the clathrin coat associated with intact membranes.