Secondary Structure and Side-Chain Orientation of Poly(L-glutamines) Having Naphthalene Chrornophores

Abstract

Four poly(L-glutamines), i. e., poly[N5-[(R, S, and RS)-1-(1-naphthyl) ethy1]-L-glutamine](P(R)NELG, P(S)NELG, and P(RS)NELG) and poly[N5-(1-naphthylmethyl)-L-glutamine](PNMLG), were prepared from poly (L-glutamic acid) and corresponding amines using dicyclohexylcarbodiimide and 1-hydroxybenzotriazole as condensing agents. Their secondary structure and side-chain orientation were examined in solution as well as in film. P(R)NELG, P(S)NELG, and P(RS)NELG in dioxane solution gave strong circular dichroism (CD) signals in the '13, absorption band of the naphthalene chromophore. P(R)NELG, which gives the strongest CD signal, exhibited little excimer emission in its fluorescence spectrum, suggesting that the side-chain naphthyl chromophores in P(R)NELG are rigidly oriented with respect to each other along the peptide main chain. In film, P(S)NELG and PNMLG, while showing macroscopic anisotropy, gave strong CD singnals that split into positive and negative peaks in the 'Li, absorption band, suggesting that during film formation the naphthalene chromophores are so oriented that their 'L a transition moments undergo exciton interactions. The Amax values of excimer emission from P(R)NELG, P(S)NELG, and P(RS)NELG films were all around 360 nm, which is ca.20 nm shorter in wavelength than that of PNMLG film, i. e., 380 nm. These results suggest that chromophore orientati on can be regulated in solution and in film when chromophores are introduced to side -chains of poly (L-glutamic acid) through amide bonds.