One-Dimensional Aromatic Crystals in Solution VI. Synthesis and Spectroscopic Characterization of Poly(β-9-anthrylmethyl L-aspartate)

Abstract

An aromatic poly(α-amino acid) carrying anthryl chromophores, poly(9-anthrylmethyl L-aspartate), was first synthesized in a form of a block copolymer with poly(γ-benzyl DL-glutamate) which functions as a solubilizing component. The polypeptide in trimethyl phosphate solution showed a marked exciton splitting in the circular dichroism at the 1Bb absorption band, indicating a helical arrangement of anthryl groups along the helical main chain. Electronic interactions in the ground and excited states of the aromatic polypeptide were studied by absorption, fluorescence, circular dichroism (CD), circularly polarized fluorescence (CPF), and fluorescence-detected circular dichroism (FDCD) spectroscopy. In the ground state, some portion of the anthryl groups was found to form dimers or higher aggregates. The fluorescence spectra of the polymer consisted of monomer fluorescence with a small contribution of the dimer excited state. The CPF spectrum indicated that the monomer excited state partially delocalizes over neighboring anthryl groups. The dimer emission also showed a significant circular polarization, indicating dissymmetric structure of the dimer. The FDCD spectrum, when monitored by the dimer fluorescence, showed a more intense CD signal than that of conventional absorption CD, indicating that the dimer is formed in a helical part of the polypeptide. On the basis of these observations, a scheme was proposed for the electronic states of the anthryl polypeptide.