PHOTOCONTROL OF MOLECULAR-ORIENTATION OF A PHOTORESPONSIVE AMPHIPHILIC ALPHA-HELIX IN A LIPID MONOLAYER

We prepared a photoresponsive polypeptide, M(n) = 10 630, composed of two amphiphilic alpha-helical copolypeptides, poly[(gamma-methyl L-glu tamate)-co-(L-glutamic acid)], joined by an azobenzene (am.-MAzoM) as a molecular photoreceptor. Dipalmitoylphosphatidylcholine monolayers c ontaining the am.-MAzoM were formed at the air-water interface. We inv estigated photoinduced changes in the structure of the polypeptide-lip id monolayer systems. The alpha-helical rod of am.-MAzoM was laid at t he air-water interface when the monolayer was in a liquid state. While in the solid condensed monolayer, the polypeptide molecules arranged perpendicular to the membrane (the air-water interface) and formed a b undle by self-association. The bundle of am.-MAzoM could be observed a s an intramembranous particle ca. 4 nm in diameter with atomic force m icroscopy. UV light irradiation induced the destabilization of the bun dle structure in the monolayer. This behavior arose from the photoindu ced denaturation of the amphiphilic character of the polypeptide offin g to a bending of the polypeptide in the main chain by a cis-tl ans ph otoisomerization of the azobenzene moiety. After removal of the light, the am.-MAzoM in the solid condensed monolayer returned to the origin al bundle structure.