POLY(ALPHA-BUTYL BETA-L-ASPARTATE) - A 2ND ALKOXYCARBONYL NYLON-3 DERIVATIVE IN HELICAL CONFORMATION

A structural study was carried out on poly(alpha-butyl beta-L-aspartat e), which is a stereoregular nylon-3 derivative with an alkoxycarbonyl group attached to the beta-carbon of the main-chain repeating unit. T wo crystal forms, namely hexagonal and tetragonal, made of 13/4 and 4/ 1 helices, respectively, and bearing a structural resemblance to the p olypeptide alpha-helix, were characterized by X-ray diffraction. Thoug h these forms are basically identical to those previously reported for poly(alpha-isobutyl beta-L-aspartate), the crystal transition from he xagonal to tetragonal known to take place in this polymer by effect of heating, was not observed in the present case. Modeling calculations revealed that although similar conformations are favoured for the two polymers in both crystal forms, the relative stability of the 13/4 hel ix to the 4/1 helix was found to be significantly reduced in the case of the butyl derivative. The presence of helical conformation in solut ion has been evidenced by both H-1 NMR and circular dichroism and the helix-coil transition promoted by acids was investigated by means of t hese techniques.