ELECTRIC-FIELD-ENHANCED SELF-ASSEMBLY OF ALPHA-HELICAL POLYPEPTIDES

The rigid, rodlike polypeptide poly(gamma-benzyl L-glutamate) (PBLG) l abeled at its N-terminus with a disulfide moiety (PBLGSS) was self-ass embled (SA) on gold. In an attempt to influence PBLGSS alignment in si tu, the orientation of the dipole moment along the PBLGSS helix axis w as biased during self-assembly by applying a voltage between two gold electrodes in a solution of PBLGSS and electrolyte to generate a stron g field (the electrical double layer) at the electrode interfaces. The resulting SA films were analyzed with X-ray photoelectron spectroscop y (XPS) and reflection-absorption Fourier transform infrared spectrosc opy (RA-FTIRS). Both techniques indicated that greater polypeptide fil m coverage-more chemisorption-was present on the negative gold electro de compared with either the positive electrode or a SA control without a voltage applied. This is suggestive of helix orientation during pol ing, whereby the PBLGSS disulfide moiety is exposed preferentially to the negative electrode. However, there is minimal evidence of polypept ide order in the SA film (the poling field and solvent were removed pr ior to film analysis); the helix axes appeared to be Gaussian distribu ted about the substrate normal according to the RA-FTIRS analysis. Ele mental percentages determined with XPS on the electrically-poled films were essentially identical to the unpoled control, indicating no redo x processes had altered the PBLG molecular composition. Chemisorption was also seen on the positive electrode and is believed to he the resu lt of solvent-mediated lateral-aggregation of PBLGSS into (apolar) spe cies with disulfide moieties on both ends.