SynopsisThe heat of solution of a series of three polyglutamntes as a function of solvent composition was measured. The abrupt increase in heat of solution a t the solvent composition of the helix-coil transition (as evidenced by optical rotation data) allows the estimation of the transition enthalpy change. The difference of side chain in the three polyglutamates has no appreciable effect on the transition enthalpy, although it affects the helix stability, as judged from the solvent composition at the transition points. These facts are discussed on the basis of existing models of the transition.
The heal of solution of Poly‐ε‐carbobenzoxy‐L‐lysine and of a series of its copolymers with phenylalanine was measured as a function of solvent composition. The enthalpy change of the helix‐to‐coil transition was estimated for the various cases. The previous findings that side chains do not greatly affect the transition enthalpy is confirmed also in cases having much larger differences in helix stability.
SynopsisThe lack of the positive band a t around 226 nm in the CD spectra of poly(proly1-azetidine-2-carbonyl-proline) in trifluoroethanol and of poly(azetidine-2-carbonyl-prolyl-azetidine-2-carboxylic) acid in F3EtOH and water, the hyperchromism of the absorption maximum a t about 202 nm, and the extremely small intensity of the CP-Pro, CY-Pro, and C@-Aze signals for the cis peptide bonds in the 13C nmr spectrum of poly(Pro-Aze-Pro) in FZEtOH indicate that both polyproline analogs exist as disordered chains in this solvent, the trans peptide group being maintained. The disordering of the chains is attributed to an increase in the accessible range of $ due to the reduced dimensions of the square ring of ~-azetidine-2-carboxylic acid residue relative to the pyrrolidine ring of proline and to strong interactions of the haloalcohol with the peptide groups of the chains.Significant conformational differences between polyproline and poly(azetidine-2-carboxylic acid) in water and other polar solvents were recently In water, poly(Aze) does not assume the form-I1 helix of polyproline because cis peptide bonds, randomly distributed along the chain predominantly in the trans configuration, preclude the existence of long sequences of residues in this ordered conformation.2 A tightly wound helical conformation similar to polyproline I is stabilized in EtOH-rich EtOH/water mixtures wherein favorable intra-chain interactions supplant less favorable polymer/solvent interaction. Thus, structural alterations of poly(Aze) chains relative to polyproline seem to result largely from features not simply expressible as stereochemical differences between the two residues, but also by stronger solvent interactions with the peptide groups of the backbone. To elucidate further this point we have studied the conformational properties in solution of the sequential polymers poly(Pro-Aze-Pro) (A?,. = 23,000) and poly(Aze-Pro-Aze) (A?T = 19,000), whose synthesis has been described in detail.3The common and outstanding feature of the CD spectra of these Azecontaining polymers in FBEtOH is the lack of the positive component of
The proton and magnetic resonance spectra of poly(~-azetidine-2-carboxylic acid) were determined in water and in formic acid. The 100-MHz 'H nmr spectra do not give evidence of cis-trans isomerism in the two solvents. On the other hand, the 220-MHz 'H nmr spectrum in water shows two peaks for the aCH proton. 13C nmr spectra show two separate peaks for each carbon atom, .which are assigned to cis and trans isomers of the amide bond. Some model compounds have been examined to aid this assignment. The percentage of the cis isomer decreases from water to formic acid and increases upon addition of CaC12 to the water solution. High yields of high molecular weight poly(~-azetidine-2-carboxylic acid) (mol wt = 23,000) have been obtained by the polymeric selfcondensation of the (Aze)s pentachlorophenyl ester trifluoroacetate.
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