Chemische und physikalische Untersuchungen an Keratinfasern

Study by PMR spectroscopy and gel chromatography of the unfolding of substituted kerateines in 8 M urea

Various S-substituted derivatives of the reduced low sulphur and high proteins from wool have been prepared in which the substituted group is hydrogen, carboxymethyl, carboxethyl, methyl, carbamidomethyl, cyanoethyl and aminoethyl. The proton magnetic resonance (PMR) spectra and gel filtration chromatography of these proteins have been examined in 8 M urea solution as a function of pH in order to determine conditions under which the proteins occur as random coils in solution with no evidence for the occurrence of non-covalent interactions. The PMR method described in an earlier paper (1) provides an easier and much more sensitive method for the observation of non-covalent interactions in random coil proteins than does the measurement of elution volumes in gel chromatography. The results obtained by both methods are consistent and show that the widest range of pH for which unfolding occurs in 8 M urea is obtained with the S-carboxymethyl, S-carboxyethyl, S-methyl and S-carbamidomethyl derivatives.

Conformations of hydrophobic polyelectrolytes

Recent studies on the (hypercoiled) compact conformation and the pH-induced conformational transition to the extended coil of the hydrophobic poly(acids or bases), which have been well used in industrial and medical circles, are summarized, especially on the basis of our studies on poly(alpha-alkyl acrylic acids) and maleic acid copolymers with hydrophobic aromatic monomers. The stability of the compact conformation and values of various thermodynamic parameters of the transition of the hydrophobic polyelectrolytes suggest the importance of hydrophobic interaction among the side chains to form the compact conformation. Characterization of various physicochemical methods, including 1H-NMR measurements, gives a model of the compact conformation with a more expanded structure than in globular proteins, a free-draining property of the solvent, the conformation fluctuation, etc. Also, the apparent two-state character of each segment in the transition region, a life-time in each state and the cooperative parameter for the compact form formation are discussed. A review of our conformational studies on the hydrophobic-hydrophilic polypeptides is also given. For the anion-induced conformation transition in basic homopoly(alpha-amino acids), hydrophobic characters of the anion and the side chain in the polypeptide are shown to be important, especially in terms of solvation. The difference in the induced conformation by the anion between random and alternating basic copolypeptides is explained by introducing a model which shows the importance of the sequence of hydrophobic and hydrophilic residues in the polypeptide to induce the ordered conformation of the anions. Also, we attempt to explain the difference between the induction of ordered conformation in the basic homo- or copolypeptide in reversed micelles with a large sulfonate (AOT) vs. that in aqueous AOT in terms of the hydrophobic character of the side chains in the polypeptides, AOT and the medium.

The formation and cleavage of lysinoalanine crosslinks

Treatment of lysinoalanine (LAL) with methyl iodide at pH-values above 8 leads to cleavage to dehydroalanine (DHA) and Nepsilon-di- or/and trimethyllysine, as was shown by ultraviolet spectra and amino acid analysis. This reaction is due to methylation of the crosslinking NH group of LAL to the quaternary ammonium compound; nucleophilic beta-elimination follows. By this reaction, the epsilon-amino groups of polylysine and its copolymers can be methylated to form di- and/or trimetryl-lysine residues. Chemically modified polypeptides containing Nepsilon-dimethyllysine can be made in this way.

On the specific cleavage of cysteine containing peptides and proteins

Addition of cysteine to the double bonds of polydehydroalanine and copolymers of dehydroalanine (DHA) and methylcysteine, alanine, leucine or gamma- methyl-L-glutamate is accompanied by increased solubility and decreased molecular weight. This result is due to a peptide bond cleavage caused by formation of a thiazolidine as a consequence of nucleophilic attack by the sulfur atom on the preceding C=O group and subsequent splitting of the peptide bond by nucleophilic attack of an OH ion. This mechanism is predominant in alkaline media; in acid another mechanism is favoured; carbonyl-oxygen attacks the carbonyl-C-atom of the cysteine residue, forming a second ring system. Addition of one water molecule then yields two peptide fragments, one of them a terminal cysteine residue. Both mechanisms could be confirmed in the case of gamma-L-glutamyl-dehydroalanyl-glycine by adding cysteine. Furthermore, it could be shown that SH-glutathione is decomposed at alevated temperatures according to the two mechanisms mentioned. This SH-induced peptide bond cleavage can be used for selective peptide chain splitting of cysteine-containing polypeptides and proteins under relatively mild conditions.