Document Type

Article

Publication Date

1993

Abstract

In efforts to utilize the a-helical structural motif as a basic building block in the formation of supramolecular assemblies, a study of the self-assembly of a-helical poly(glutamate)s from dilute solution onto a solid support was initiated. In particular, poly(y-benzyl-L-glutamate) [PBLG], which has an unusually robust a-helical conformation, was functionalized at the amino terminus with a disulfide-containing moiety (lipoic acid), which in turn has a specific and strong chemisorptive interaction with gold surfaces. The same end group functionalization was used with the /i-alkyl side chain analogs of PBLG. The resulting self-assembled monolayers were characterized by grazing angle reflection infrared (GIR-IR) spectroscopy, angle dependent X-ray photoelectron spectroscopy, ellipsometry, atomic force microscopy, and wettability measurements. The self-assembled films were also compared with control monolayers prepared by the Langmuir-Blodgett deposition technique. It was found that the chemisorptive end group effectively changes the manner of adsorption from dilute solutions of low molecular weight lipoic-acid labelled PBLG [PBLGSS] compared with the non-functionalized PBLG of the same MW. This difference was especially reflected in the average orientation of the helices in the monolayer formed on gold: for the self-assembled PBLGSS monolayer, the average orientation of the helix axis appears to be random; in physisorbed PBLG, the helices lie in the plane of the gold surface. For higher MW polypeptides, the effect of the chemisorptive end group is significanly diminished and the polypeptides lie in the plane of the gold surface regardless of the disulfide functionality. This was explained in terms of the enhancement in the aggregate strength of the side chain-gold interaction as the MW is increased. For the poly(alkylglutamate)s, PCnLG, the chemisorptive end group did not appear to influence the orientation of the helices on gold; rather, the length of the side chains had a marked effect on the self-assembly process. The low stability of the a-helical conformation in the low MW PClgLG, coupled with the propensity for the side chains to crystallize, resulted in denaturation of the polypeptide on adsorption to gold—the (3-form was observed in the adsorbed films on gold. The high MW PC18LG adsorbs on gold with the a-helix mainly intact (with occasional p-form component or fraction) and the polymers lie in the plane of the substrate surface due mainly to the length of the backbone. The physisorbed and self-assembled films of polypeptides with n = 6 and 12 also show a parallel orientation of the helices on gold; again attributed to the high MW of these polymers.

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