SynopsisThe conformation of poly(a-L-aspartic acid) was investigated on a sample in which @-bonds were not detected. CD and ir spectroscopy showed that poly(a-L-aspartic acid) passes through a conformational change induced by changes of the degree of ionization that is accompanied by precipitation; the precipitate is probably highly helical. The change was also detected by potentiometric titration.
The conformation of several samples of poly(α,β‐L‐Asp) with a molar fraction of β‐bonds ranging from 0.1 to 0.55 was investigated by means of ir and CD spectroscopy and potentiometric titration and compared with the results obtained previously with poly(α‐L‐Asp). All samples investigated underwent a conformational change induced by changes in their degree of ionization: unpronounced ir absorption of amide V at 650 cm−1 was shifted to 620 cm−1 and substantially increased on deionization; CD spectra changed with the degree of ionization, passing through an isosbestic point; and the pattern of the titration curves was more complex than that of a simple polyelectrolyte. The conformation developing with the decreasing degree of ionization may be considered to be α‐helix, as deduced according to the analogous behavior of other polypeptides. The extent of the conformational change in the individual samples depends on the molar fraction of β‐bonds: the higher it is, the lower is the helix‐forming ability of the sample.
Conformational transitions of basic sequential polytripeptides (Lys-Ala-Leu)n, (Arg-Ala-Ala)n, (Arg-Leu-Ala)n, and (Arg-Ala-Leu)n, induced by elevated salt concentrations and/or temperatures in aqueous solutions, were investigated by CD, sedimentation equilibrium, and viscometry. The behavior of (Lys-Ala-Leu)n was compared with that of the sequential isomer (Lys-Leu-Ala)n, studied previously. It was found that both polypeptides are highly helical with a tendency to aggregate in high salt solutions. Although the hydrophobic interactions between Lys and Leu residues play an important role in both cases, the final effect on helix stabilization and aggregation is different. The Arg-containing polypeptides were found to assume the alpha-helical conformation. Compared to the Lys-containing polypeptides (Lys-Ala-Leu)n and (Lys-Leu-Ala)n, a very low tendency to aggregate was observed.
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