Nanometer-scale smectic ordering of genetically engineered rodlike polymers:: Synthesis and characterization of monodisperse derivatives of poly(γ-benzyl α,L-glutamate)

Macromolecules with precisely defined architectures on the nanometer scale are ideal building blocks for self-assembled materials and nanometer-scale devices. By utilizing recombinant DNA technology and bacterial protein expression: we have prepared a set of rodlike artificial proteins, Glu(OBzl)Asp(OBzl)[Glu(OBzl)(17)Asp(OBzl)]x-Glu(OBzl)Glu(OBxl) (PBLG-X, X = 3-6, 1), which serve as monodisperse analogues of poly(gamma-benzyl alpha,L-glutamate) (PBLG), with variation of rod length from 8.7 to 17 nm. Synthesis was accomplished by (i) bacterial production of a precursor polypeptide GluAsp(Glu(17)Asp)xGluGlu (PLGA-X, X = 3-6), as a fusion to mouse dihydrofolate reductase (DHFR), (ii) CNBr digestion of the expressed protein to liberate PLGA-X, and (iii) side chain benzylation of PLGA-X via treatment with phenyldiazomethane. The growth rates of cultures transformed with the recombinant genes, and the yields of protein expressed therein, depended on the number of Glu(17)Asp repeats appended to the fusion partner, decreasing as the number of repeats increased. PBLG-3, with the lowest aspect ratio (6.9), did not exhibit liquid crystalline (LC) behavior while each of the other polymers formed lyotropic LC phases. The phase transition behavior of PBLG-X was distinctly different from that of conventional polydisperse PBLG; when the concentration of the polymer solution was gradually increased, the transition from the isotropic to the liquid crystalline state occurred uniformly throughout the sample without formation of distinct birefringent droplets. Solution-cast films of PBLG-4 and PBLG-5 formed smectic supramolecular architectures with layer spacings (11.4 and 14.0 nm, respectively) precisely determined by the length of the monodisperse rods. Smectic ordering was observed only in films cast from mixtures of chloroform (97%) and trifluoroacetic acid (3%). In films prepared from dioxane, both polydisperse and monodisperse PBLGs adopted columnar order without formation of smectic layers.