Conformation of Antibiotic Protein, Neocarzinostatin, Studied by Plane Polarized Infrared Spectroscopy, Circular Dichroism and Optical Rotatory Dispersion

The peptide backbone and aromatic side chain conformations of antitumour antibiotic protein, neocarzinostatin and its derivative tetra‐S‐carboxymethyl neocarzinostatin, have been studied using the ultraviolet circular dichroism ( CD ), optical rotatory dispersion ( ORD ) and plane polarized infrared spectroscopy. The near ultraviolet CD and ORD indicated multiple contributions from side chain chromophores (π → π*) suggesting buried state ( resulting asymmetric property ) of phenylalanyl and tyrosyl and perhaps cystine residues in neocarzinostatin. These results are quite accordant with that of chemical inactivity of the tyrosyl and cystinyl residues in the antibiotic. No clear band was assigned for contribution of tryptophyl residue. The results of the infrared spectroscopy at amide I and II have been found more useful in the present study for the analysis of peptide backbone conformations, which indicated the presence of cross β‐conformation, while the far ultraviolet CD and ORD spectra were atypical and unable to be interpreted. The results of the examination of amino acid sequence and composition support the finding of infrared studies. The assumption that neocarzinostatin possesses a rigid overall conformation can be explained by the above results and by our separate findings of its peculiar resistances to disulfide bond reductions in aqueous system and hydrolyses by proteolytic enzymes.