Nanometer-scale smectic ordering of genetically engineered rodlike polymers: Synthesis and characterization of monodisperse derivatives of poly(gamma-benzyl alpha,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 expr ession: 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), wh ich 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 acco mplished by (i) bacterial production of a precursor polypeptide GluAsp(Glu( 17)Asp)xGluGlu (PLGA-X, X = 3-6), as a fusion to mouse dihydrofolate reduct ase (DHFR), (ii) CNBr digestion of the expressed protein to liberate PLGA-X , and (iii) side chain benzylation of PLGA-X via treatment with phenyldiazo methane. The growth rates of cultures transformed with the recombinant gene s, and the yields of protein expressed therein, depended on the number of G lu(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 e xhibit liquid crystalline (LC) behavior while each of the other polymers fo rmed lyotropic LC phases. The phase transition behavior of PBLG-X was disti nctly different from that of conventional polydisperse PBLG; when the conce ntration of the polymer solution was gradually increased, the transition fr om the isotropic to the liquid crystalline state occurred uniformly through out the sample without formation of distinct birefringent droplets. Solutio n-cast films of PBLG-4 and PBLG-5 formed smectic supramolecular architectur es with layer spacings (11.4 and 14.0 nm, respectively) precisely determine d by the length of the monodisperse rods. Smectic ordering was observed onl y in films cast from mixtures of chloroform (97%) and trifluoroacetic acid (3%). In films prepared from dioxane, both polydisperse and monodisperse PB LGs adopted columnar order without formation of smectic layers.