Comblike Alkyl Esters of Biosynthetic Poly(γ-glutamic acid). 2. Supramolecular Structure and Thermal Transitions

Abstract

The solid-state structure of comblike poly(R-n-alkyl γ-glutamate)s derived from bacterial poly(γ-glutamic acid) (PAAG-n, n being the number of carbons contained in the linear alkyl side chain) was investigated for even values of n between 12 and 22. Two series, one with a D:L enantiomeric ratio of approximately 9:1 and the other with a nearly racemic composition, were examined. The thermal transitions and the structures involved therein were characterized by DSC, polarizing infrared, X-ray and electron diffraction, and 13 C CP/MAS NMR. In all cases and within the whole examined range of temperature, the polypeptide main chain was arranged in an R-helix-like conformation. A first-order transition occurring at a temperature T1 between 20 and 80 °C and interconverting two well-defined phases, A and B, was characterized for PAAG-n with n g 14. Phase A is a biphasic structure consisting of layers of polypeptide helices separated by a paraffinic pool of alkyl side chains. The layer periodicity was found to increase steadily from 2.7 to 3.7 nm according to a structure in which the alkyl side groups are partially crystallized, oriented nearly normal to the layers, and extensively interdigitized. Phase B retains the layered structure present in phase A but with side chains being in a disordered state. A second first-order transition, leading to a third phase C at a temperature T 2 near 100 °C, was characterized for PAAG-n with n g 18. Phase C is thought to be a nematic structure consisting of a quasi-hexagonal packing of helical polypeptide rods lacking axial register. Although no clear differences in the structure of any of the three phases were detected between the two investigated series, the response to the thermal treatment was significantly affected by the stereochemistry of the PAAG-n. Crystallization from the melt was favored in the DL series, the dimensional changes involved in the A-B transition showed opposite sign for D and DL polymers, and the B-C transition was much more noticeable in the DL series.

Order–disorder transition in eicosylated polyethyleneimine comblike polymers

“…Furthermore, the methyl peak at 15 ppm vanished with temperature increasing, which might be induced by the enhanced mobility of the alkyl side chains, resulting in the entire side chain entering into the molten state. The transition behavior of PEI20C described by NMR is similar to that of comblike poly(a-alkyl b,L-aspartate)s [5,52,53] and poly(a-alkyl g-glutamate)s [8,9,54]. The obtained temperature transition region from 13 C CP/MAS NMR spectra is in good agreement with that of DSC, WAXD, and FTIR, thus further confirming the existence of the ODT in PEI20C comblike polymer.…”

Comblike Complexes of Poly(aspartic acid) and Alkyltrimethylamonium Cationic Surfactants

“…[2,3] Such type of systems are unique in displaying a combination of properties that includes a high solubility in organic solvents and the ability to generate periodically layered structures at the nanometric scale. Very recently, similar structures have been shown to occur also in poly(a-alkyl b,L-aspartate)s [4,5] and poly(a-alkyl g-glutamate)s, [6,7] two families of unconventional polypeptides that are able to take up regularly folded conformations of a-helix type.…”

Comblike Ionic Complexes of Hyaluronic Acid and Alkanoylcholine Surfactants as a Platform for Drug Delivery Systems

“…It is worthy to note that the water degradation observed for nACh·HyA complexes is much faster than that for their analogues made of PGGA, a difference that is reasonable provided the greater capacity of hyaluronic acid to swell which makes easier the uptake of water by the complex. 43,44 The FTIR spectra recorded from the residual samples collected at increasing incubation times are compared in Figure S9 of the SI file. Strikingly, the chemical composition remained essentially unchanged and no band indicative of hydrolysis of the choline stearate was detected.…”