The ability of microbally produced poly(gamma,d-glutamic acid) to form stable polyelectrolyte-opposite charged surfactant complexes was investigated. A sonicated sample of polyacid with a molecular weight about 10(5) Da and a content of d enantiomer higher than 90% was used in this study. Nearly stoichiometric complexes of poly(gamma,d-glutamate) anions and alkyltrimethylammonium cations bearing linear alkyl chains with even numbers of carbon atoms from 12 up to 22 were "synthesized" by precipitation from equimolar mixtures of aqueous solutions of the two components. All complexes were found to adopt stratified supramolecular structures made of alternating layers of poly(gamma,d-glutamate) and surfactant with a periodicity increasing from 3.2 up to 4.3 nm according to the length of the alkyl side chain. No definite evidence indicative of the conformation adopted by the main chain in these complexes could be afforded. In all cases, the alkyl chains are in an extended conformation and oriented normal or nearly normal to the layer planes. Polymethylene chains with more than 16 carbon atoms were partially crystallized in the complexes in a separated paraffinic phase, whereas no crystallinity was detected for shorter lengths. The crystallized paraffinic phases were found to melt reversibly at temperatures between 40 and 70 degrees C. This process was found to happen with a concomitant expansion-contraction that amounts between 2 and 8% of the long period of the structure but without significant alteration of the layered arrangement.

Methyl esters of microbial poly(beta,L-malic acid) for conversion degrees of 25, 50, 75, and 100% were prepared by treatment of the polyacid with diazomethane. Esterification proceeded with retention of the molecular weight of the parent polyacid and the copolymers displayed a blocky microstructure consisting of short segments of malic and methyl malate sequences. The thermal stability of the copolyesters was lower than those of the parent homopolymers and all of them were fairly crystalline with melting temperatures within the range of 170-175 degrees C. They were degraded rapidly by water, the hydrolysis rate being highly dependent on the methylation degree. Microspheres with mean-average diameters in the range of 1-20 microm were prepared from the 100% methylated product by the emulsion-evaporation solvent method. Encapsulation of erythromycin was efficiently performed in these microparticles and its releasing upon incubation in simulated physiological medium was evaluated for different drug loads. Drug delivery was observed to occur by a releasing mechanism largely determined by the hydrodegradation of the host polymer and independent of the amount of loaded drug.

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The conformation of three poly(α-alkyl β-l-aspartate)s, abbreviated PAALA-n, with the alkyl group being methyl, ethyl, and propyl (n = 1, 2, and 3), was examined by X-ray diffraction, polarized infrared spectroscopy, and molecular mechanics calculations. The crystal structure was modeled and refined against diffraction data with the linked-atom least-squares methodology (LALS). Two crystal forms reminiscent of the well-known α-helix and pleated β-sheet structures typical of polypeptides were characterized for the methyl derivative. The helical form is a pseudohexagonal lattice of right-handed 17/4 helices stabilized by intramolecular hydrogen bonds. The β form is made of chains in nearly extended conformation and hydrogen bonded to each other to form sheets which stack together in a metrically orthorhombic crystal lattice. In addition to these two forms, a second folded form made of right-handed 4/1 helices arranged in a tetragonal structure was characterized for the ethyl derivative. Poly(α-propyl β-l-aspartate) was found to display a crystal dimorphism entailing the hexagonal and tetragonal helical forms whereas no signs of extended conformation were detected for this polymer. In this case, the hexagonal form was found to be composed of right-handed 13/4 helices, which is the conformation usually observed in poly(β-l-aspartate)s with linear alkyl side chains of medium and larger sizes. The structural behavior exhibited by the three compounds under study is consistent with the general pattern described for PAALA-n with n ranging from 4 to 22 which have been previously studied by us in full detail.

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