The solid-state structure of comblike poly(alpha-n-alkyl gamma-glutamate)s derived from bacterial poly(gamma-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 C-13 CP/MAS NMR. In all cases and within the whole examined range of temperature, the polypeptide main chain was arranged in an alpha-helix-like conformation. A first-order transition occurring at a temperature T-1 between 20 and 80 degreesC and interconverting two well-defined phases, A and B, was characterized for PAAG-n with n greater than or equal to 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 degreesC, was characterized for PAAG-n with n greater than or equal to 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.