Helix orientation at interfaces is one of the key factors to fabricate biofunctional synthetic polymer systems that mimic natural polymers such as proteins. The orientation of segments in membrane proteins is known to govern functions such as molecular recognition and electrochemical communication, including electron transfer. In this context, there has been considerable interest in constructing perpendicularly oriented polypeptide monolayers on aqueous or solid substrates. It has been indicated that a helices prefer to lie flat on every possible interface. [1] Efforts have been made to orient a helices perpendicular to the surface of the water or solid substrates, [2±5] by incorporation of functional groups onto one terminus of polypeptide helices. Such groups under consideration include quaternary ammonium groups [6,7] to interact with anionic templates, a crown ether moiety, [8±10] or a sulfur atom [11±14] to attach to a gold surface.
In this communication, we describe the reversible variation of the orientation of a disulfide-modified poly(l-glutamic acid) (PLGA-SS) a helix assemblies attached to a gold surface and their enhanced molecular packing based on the interaction of helix macroscopic dipoles (ªmacrodipolesº). We (45 mL) at 23 8C. The clear yellow reaction mixture was stirred for 4 h, and a precipitate formed. Removal of all volatile materials under vacuum and subsequent washing of the yellow residue with n-pentane (2 Â 30 mL) yielded, after drying, powdery white 4 (1.32 g, 1.74 mmol, 83 %); m.p. 190 8C (decomp.). Elemental analysis calcd for C 18 H 30 MgN 2 O 3 S: C 57. 07, H 7.98, N 7.40; found: C 57.35, H 7.77, N 7.59; 1 H NMR ([D 8 ]THF, 23 8C): 7.23 (br, 2 H, o-H), 6.86 (m, 2 H, m-H), 6.33 (m, 1 H, p-H), 3.58 (m, 8 H, (CH 2 ) 2 (CH 2 ) 2 O), 1.74 (m, 8 H, (CH 2 ) 2 (CH 2 ) 2 O), 1.34 (s, 9 H, C(CH 3 ) 3 ); 13 C{ 1 H} NMR ([D 8 ]THF, 23 8C): 154.0 (s, ipso-C), 128.4 (s, o-C), 115.0 (s, m-C), 96.8 (s, p-C), 67.9 (s, (CH 2 ) 2 (CH 2 ) 2 O), 52.4 (s, C(CH 3 ) 3 ), 34.3 (s, C(CH 3 ) 3 ), 26.4 (s, (CH 2 ) 2 (CH 2 ) 2 O).