No abstract

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.

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

A recent investigation of (Pro-Pro-Gly), (n = 10,15) by 'H-nmr a t 220 MHz pointed out that the triple-helix + random-coil transition is accompanied by cis-trans isomerization a t the proline peptide b o n d~. l -~ The proline C"H resonance was a single peak a t low temperature and was converted into two peaks of equal areas at high temperature. This effect was attributed to the existence of all-trans Gly-Pro and Pro-Pro C-N bonds a t low temperature and to their conversion to 50% cis at high temperature, following the conformational transition.

Vicinal 13C-15N coupling constants for three 15N-labelled peptide models having Cp-Ca-CISN dihedral angles constrained to 110 or 160' are smaller than 1 Hz, which minimizes their potential utility in peptide conformation studies.

The electron spin resonance (e.s.r.) study of the ethyleneimine in polycrystalline and glassy state shows CH,-XH, that the radical \N/ is formed by y-irradiation at 77 O K . The features of the e.s.r. spectrum of this radical are discussed in terms of the isotropic and anisotropic hyperfine coupling constants of the hydrogen and nitrogen nuclei. The effects of bleaching the y-irradiated sample with visible and ultraviolet light are also reported. -and ultraviolet (u-v.) The field sweep calibration was carried out by a Mn2+ irradiation in the solid state (1-4). The low doped sample of MgO (5). melting points of these compounds make single crystal studies difficult. The published spectra Results have been obtained from irradiated polyciysta~-Anhydrous ethyleneirmne frozen at -196 oC line samples. The anisotropic effects give rise to gives an opaque crystalline matrix which after poorly spectra which are generally y-irradiation shows a weak blue color. The jnterpret Only in a product containing about 20% of water gives a few cases are fairly clear spectra obtained.transparent glass with an intense blue color after In the present investigation the of y-irradiation. The e.s,r. spectrum, recorded at which gives a we' ' -196 OC, is practically the same in both cases and spectrum, is reported.does not change from -196 to about -90 "C. ExperimentalEthyleneimine (Fluka reagent) was purified by distillation and dried with a sodium mirror. Ethyleneimine-dl was prepared by exchange of ethyleneimine with D,O. Its isotopic purity, checked by nuclear magnetic resonance, was better than 95 %. The e.s.r. spectra were recorded at 9.2 GHz using a Hilger and Watts Microspin spectrometer with a TED,, cylindrical cavity.The measurements were performed at -196 "C by keeping the sample tubes in a quartz Dewar filled with liquid nitrogen, or at higher temperatures by using a variable nitrogen flow system. 'This work was supported by Consiglio .Nazionale delle Ricerche.

SynopsisThe helix-coil transition of poly-NS-(3-hydroxypropyl)-~..-glutamine in methanol-water solutions has been observed using 13C nuclear magnetic resonance. Two signals appear in the a-CH region in the course of the transition; this is in contrast with previous proton magnetic resonance experiments carried out on this polymer in the same solvent system. 765

SynopsisThe helix-coil transition of poly-N~-(3-hydroxypropyl>~-glutamine (PHPLG) has been studied in methanol-water by CD and cmr spectroscopy. For polydisperse PHPLG, two separate peaks arising from residues in helical and random-coil conformations are observed during the transition for both main-chain carbons. These results are discussed and compared to those observed in the case of a polymer sample obtained by racemization of PHPLG in 0.1 M NaOH and of PHPLG samples of controlled molecular weight and dispersity. The dominant influence of the molecular-weight heterogeneity on the double-peak phenomenon has been verified. The linewidths and chemical shift of the 13C resonances are discussed in terms of side-chain-main-chain interactions and side-chain solvation.