Self-assembly (SA) on gold substrates of α-helical poly(γ-benzyl-L-glutamate) [(PBLG); molecular weight ∼20 000 kd] with a disulfide moiety attached at its N-terminus (PBLGSS) was investigated. The SA films were compared with control experiments using unlabeled physisorbed (PS) PBLG and Langmuir–Blodgett (LB) deposited PBLG monolayers. Characterization of the films included angle-dependent x-ray photoelectron spectroscopy, reflection–absorption Fourier transform infrared spectroscopy, ellipsometry, and contact angle measurements. For PBLGSS, a SA monolayer was formed wherein the α-helical conformation was retained and the polypeptides were preferentially bound to gold through the disulfide moiety. This terminal bond to the substrate results in a nonplanar distribution of helix axes (i.e., on average tilted up from the gold surface). This is different from the unlabeled PBLG controls, PS film and LB monolayers, wherein, respectively, less polymer was adsorbed and the helices lie in the plane of the gold substrate. Furthermore, polypeptide aggregation in the solution (e.g., in dichloromethane or 1,4-dioxane) appears to have an influence on the self-assembling behavior of these macromolecules relative to known nonaggregating solvents (N,N-dimethylformamide).

Monolayer and bilayer Langmuir–Blodgett films of poly(γ-benzyl-L-glutamate) (PBLG) were prepared on highly oriented pyrolytic graphite (HOPG), mica, silicon, and glass. The films were imaged by atomic force microscopy (AFM) in the contact and tapping modes to develop imaging strategies for ultrathin polypeptide films and to assess the effect of substrate on film microstructure. A better understanding of the local polypeptide film structure could guide future methods of device fabrication. The AFM tapping mode enables long-term nondestructive imaging of PBLG mono- and bilayers for dimensions as small as 0.2 μm. In contrast, contact mode AFM readily damages PBLG films. PBLG has the best monolayer coverage on HOPG, although film is sometimes observed on each of the other three substrates. On HOPG, the monolayer has a predominantly ‘‘lacey’’ structure whereas for the hydrophilic substrates, mica, silicon, and glass, it is ‘‘banded’’ or ‘‘solid.’’ HOPG rarely exhibits a continuous bilayer film whereas the other three substrates exhibit much better coverage as bilayers than as monolayers. The PBLG monolayer behaves as a banded template for bilayer formation in some areas of mica, and for all four substrates, the second layer appears to be oriented with respect to the first. The morphology and coverage of PBLG as a function of substrate is discussed in terms of hydrophilicity and forces of adhesion. Whether voids observed by AFM in the PBLG structures are present in the native film at the air–water interface of the Langmuir trough, or are induced by film transfer, remains undetermined.

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Synthetic homopolypeptides, such as poly(γ-benzyl-L-glutamate)(PBLG) bear oc-helical secondary structure and exhibit liquid-crystalline behavior. These properties are useful for a range of materials applications, among them nonlinear optics (NLO). The α-helical structural motif of PBLG can be exploited by regular substitution of chemical functionalities in the sidechains along the main axis. Macromolecular order can then be established in polypeptide thin films using Langmuir-Blodgett (LB) deposition. Monolayer films of PBLG have been investigated by surface spectroscopic techniques, without having an understanding of the local morphology.Atomic force microscopy (AFM) has the potential for elucidating the microstructure of LB films, providing a mechanism for relating differing morphologies to deposition parameters. Excessive contact forces exerted by an AFM tip at small size scales, however, make the technique less viable for delicate organic monolayers. By intermittently striking a sample surface with a sharp probe, the tapping mode AFM provides a means for nondestructively imaging such soft samples.