Secondary structure of proteins associated in thin films

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Abstract
The solid state secondary structure of myoglobin, RNase A, concanavalin A (Con A), poly(L‐lysine), and two linear heterooligomeric peptides were examined by both far‐uv CD spectroscopy1 and by ir spectroscopy. The proteins associated from water solution on glass and mica surfaces into noncrystalline, amorphous films, as judged by transmission electron microscopy of carbon‐platinum replicas of surface and cross‐fractured layer. The association into the solid state induced insignificant changes in the amide CD spectra of all α‐helical myoglobin, decreased the molar ellipticity of the α/β RNase A, and increased the molar ellipticity of all‐β Con A with no change in the positions of the bands' maxima. High‐temperature exposure of the films induced permanent changes in the conformation of all proteins, resulting in less α‐helix and more β‐sheet structure. The results suggest that the protein α‐helices are less stable in films and that the secondary structure may rearrange into β‐sheets at high temperature. Two heterooligomeric peptides and poly (L‐lysine), all in solution at neutral pH with “random coil” conformation, formed films with variable degrees of their secondary structure in β‐sheets or β‐turns. The result corresponded to the protein‐derived Chou‐Fasman amino acid propensities, and depended on both temperature and solvent used. The ir and CD spectra correlations of the peptides in the solid state indicate that the CD spectrum of a “random” structure in films differs from random coil in solution. Formic acid treatment transformed the secondary structure of the protein and peptide films into a stable α‐helix or β‐sheet conformations. The results indicate that the proteins aggregate into a noncrystalline, glass‐like state with preserved secondary structure. The solid state secondary structure may undergo further irreversible transformations induced by heat or solvent. © 1993 John Wiley & Sons, Inc.