SynopsisThe nature of peptide bonds in poly(aspartic acid) prepared by debenzylation of poly(0-benzyl-L-aspartate) under various conditions has been studied by means of nmr spectroscopy. It was established that the majority of the polymers prepared, as well as the commercially obtained polymer, contained aspartic acid linked in both a-and 0-peptide bonds. The purest polymer, having practically undetectable amounts of 0-bond, was prepared by debenzylation by HBr in trifluoroacetic acid. It was established that the 0-bonds are formed via succinimides.
INTRODUCTIONAspartic acid, as a trifunctional monomer, may be linked in the main chain of poly(aspartic acid) [poly(Asp)] by a-or 0-peptide bonds (see compound I in Fig. 1). The common synthetic route for preparing poly(Asp) is polycondensation of monomers protected at one carboxyl and activated for the polymerization at the other (e.g., as N-carboxy anhydride or activated ester), followed by deblocking of the protected carboxyl on the resulting po1ymer.l It is generally accepted that this procedure gives rise to a polymer with one type of peptide bond, but unequivocal evidence has never been presented. On the contrary, chemical degradation method^^,^ and potentiometric titration3y4 provide some indirect indication that this is not always the case. However, substantial objections may be raised against both these r n e t h~d s .~~~ Synthetic poly(Asp) is used in various physicochemical model studies of the secondary structure of proteins. As even minor irregularities7 may affect the conformation of a polypeptide, the nature of the peptide bonds in poly(Asp) should be clearly characterized.It has been shown in the preceding paper on ~o~~( c I , /~-D , L -A s~)~ that the nature of peptide bonds may be unequivocally characterized by 13C-nmr spectroscopy. We report here the results obtained by application of this method to several samples of poly(Asp) prepared by usual techniques. Biopolymers, Vol. 20, 1615-1623(1981
MATERIALS AND METHODSAll chemicals were purchased from Fluka unless stated otherwise and were of p,a. quality. L-Aspartic acid, p.a., was from Loba-Chemie. Chemical Company ( M = 3 X lo4) and from the Biophysics-Laboratory, Portsmouth ( M = 6.5 X lo4), were used. The ir and uv spectra of the three samples were found to be identical.
Poly
Poly(L-aspartic acid)Poly(Asp) samples were prepared by debenzylation of poly [Asp(OBzl)] (synthesized by us) as described in Table I. The other two samples of poly[Asp(OBzl)] were debenzylated by the method applied for the preparation of poly(Asp)I. Samples I, 111, V-IX, and XI were dissolved in water by adding 1M NaOH to give a solution with pH 9-10, filtered, dialyzed for 2 days against running water, acidified with 2M HC1 to pH about 1.5, and kept overnight at 4°C. The precipitate (except sample I) was decanted with water, methanol, ether, filtered off, and dried at atmospheric pressure over H2SO4 to constant weight. Sample I did not precipitate, but was lyophilized. Excessive drying was avoided for reasons explained in t...
