Topological diversity of artificial beta-barrels in water

Rigid-rod beta -barrels are composed of interdigitating, short, amphiphilic peptide strands flanked by stabilizing rigid-rod "staves". We here report studies on the topological diversity of these recently devised artificial b eta -barrels with regard to their length. For this purpose, homologous p-oc tiphenyl, p-sexiphenyl, and p-quarterphenyl rods were equipped with complem entary tripeptide strands based on the sequences Lys-Leu-Lys and Glu-Leu-Gl u. The stability of rigid-rod beta -barrels of different length was determi ned by denaturation with guanidinium chloride. Free energies of DeltaG(H2O) = -5.2 kcalmol(-1), DeltaG(H2O) = -2.9 kcalmol(-1), and DeltaG(H2O) < -0.3 kcalmol(-1) found for homologous p-octiphenyl, p-sexiphenyl, and p-quarter phenyl <beta>-barrels demonstrated strong dependence of beta -barrel stabil ity on beta -barrel length. These results revealed a very qualitative minim al (similar to 23 Angstrom) and an "ideal" beta -barrel length (similar to 34 Angstrom), synergistic formation (alpha = 1.4) and remarkable stability for "ideal" p-octiphenyl beta -barrels exceeding that of several proteins a nd most synthetic models. Rigid-rod beta -barrels with p-oligophenyl "stave s" longer than similar to 34 Angstrom will be very difficult to make and st udy because of rapidly decreasing rod solubilities. However, a strategy to bypass this apparent upper limitation of beta -barrel length is introduced: supramolecular matching of mismatched rods yielded elongated beta -barrels (61 Angstrom) of acceptable stability (DeltaG(H2O) = 2.2 - 3.1 kcalmol(-1) ). Chirality 13:170-176, 2001. (C) 2001 Wiley-Liss, Inc.