Abstract:The central domain of cytoplasmic intermediate filament (IF) proteins from vertebrates contains some 310 residues and forms a double stranded coiled coil (rod) with a length of about 46 nm. The flanking terminal domains show a high cell type specific variability both in sequence and in length. Using Fourier Transform Infra Red (FTIR) spectroscopy we measured secondary structures of isolated domains of type III and IV IF proteins and of the soluble tetramers and the filaments formed by type III IF-proteins. The amide I spectrum of the desmin rod is virtually identical to the spectra of other coiled coil proteins such as tropomyosin and the myosin rod. All these double stranded coiled coils reveal spectra distinctly different from classical a-helical spectra. The spectrum of coiled coils is a triplet of approximately equally strong bands. One band occurs at normal a helix position, while the other two are found at lower wave numbers. Theoretical aspects of these findings are discussed in the accompanying paper by W.C. Reisdorf and S.Krimm. The aminoterminal head domain of desmin has a multicomponent spectrum with major fractions of b-sheet, b-turns and random structure. The carboxyterminal tail domains of desmin and the neurofilament proteins L and H, the latter in the phosphorylated and in the dephosphorylated form, have very similar FTIR spectra, indicating mostly random structure. The spectrum of desmin type III protofilaments is very similar to the sum of the spectra of the three isolated domains. Polymerization into filaments seems to induce a small change in secondary structure.