SynopsisPreviously used procedures for processing the amino acids from 6N hydrochloric acid hydrolysis of poly[N5-(4-hydroxybutyl)-L-glutamine], poly[N5-(3-hydr~xypr~py1)-~-glutamine], and several random copolymers derived from these, led to the formation of spurious products. These have been isolated and characterized as the y-ester of glutamic acid and the hydroxyalkyl amine, and chloro-alkyl amine hydrochloride. The former reduces the observed values for glutamic acid, but the latter has no effect on them. A method is used to avoid formation of these artifacts in the amino-acid analysis. Of all the copolymers studied previously in this series, the compositions of only those containing L-serine are in error as a result of the formation of the y-ester. A redetermination of the amino-acid compositions of the copolymer fractions studied earlier leads to slightly revised values for the Zimm-Bragg parameters u and s of serine.
Water-soluble copolymers containing L-arginine and N5-(4-hydroxybutyl)-L-glutamine were prepared by copolymerization of the N-carboxy-alpha-amino acid anhydrides of Ndelta-tert-butyloxycarbonyl-L-ornithine and gamma-benzyl L-glutamate, followed by aminolysis with 4-amino-1-butanol, by removal of the tert-butyloxycarbonyl protecting group, and by treatment with O-methylisourea. The copolymers were fractionated and characterized, and the thermally induced helix-coil transitions of these copolymers were studied in water at neutral pH in the presence and in the absence of KCl. The Zimm-Bragg parameters sigma and s for the helix-coil transition in poly(L-arginine) in aqueous solution were deduced from an analysis of the melting curves of the copolymers in the manner described in earlier papers. The computed values of s indicate that L-arginine is a weak helix-making residue at low temperature and a weak helix-breaking residue at high temperature in aqueous solution. The results were found to be in good agreement with those obtained earlier in conformational analyses of arginyl residues in proteins.
Stepwise solid phase peptide synthesis, Fmoc‐approach, of 88 peptides varying in length from 4 to 24 amino acid residues was performed using a uniform procedure for coupling, monitoring and deprotection steps. The data of 696 couplings have been incorporated into a computer programme in order to study whether the degree of coupling can be predicted. Parameters studied are the nature of the amino acid to be coupled, the amino acid to be acylated and the length of the growing peptide chain. Using this information, prediction of “good” or “difficult” sequences, that is, peptides that can be synthesized without appreciable repeated couplings or the opposite, seems possible.
Two side‐reactions occur during the DCC‐mediated attachment of Fmoc‐amino acids to p‐alkoxybenzyl alcohol resins viz. dipeptide formation and racemization. The addition of 1‐hydroxybenzotriazole in combination with a low reaction temperature is shown to decrease both side‐reactions to an acceptable level without lowering the degree of incorporation.
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