One-dimensional aromatic crystals in solution. 4. Ground- and excited-state interactions of poly(L-1-pyrenylalanine) studied by chiroptical spectroscopy including circularly polarized fluorescence and fluorescence-detected circular dichroism

Abstract

A novel aromatic poly(a-amino acid), poly(l-1-pyrenylalanine), was synthesized in the form of a block copolymer with poly(7-benzyl DL-glutamate) as a solubilizing unit. The polymer solution in trimethyl phosphate and dimethylformamide was subjected to spectroscopic measurements including such chiroptical methods as circular dichroism (CD), circularly polarized fluorescence (CPF), and fluorescence-detected circular dichroism (FDCD). A significant hypochromicity in the absorption spectrum and strong CD Cotton bands indicated the ground-state interaction between pyrenyl groups which are kept in a helical arrangement along the polypeptide chain. The fluorescence spectra showed monomer and excimer emissions. The excimer/ monomer intensity ratio of the L polymer was significantly lower than that of the corresponding random copolymer of d-and L-pyrenylalanines. The CPF spectra of the l polymer showed no dissymmetry in the monomer fluorescence but showed negative and positive dissymmetries in the excimer region, indicating the presence of two kinds of excimer species. One component with the negative CPF dissymmetry around 460 nm increased its contribution at higher temperatures, whereas the other with the positive dissymmetry at longer wavelengths than 500 nm was significant at lower temperatures. The FDCD spectrum of the L polymer monitored at \em > 520 nm was more intense than that monitored at 420-480 nm. It was therefore concluded that the excimer at the shorter wavelength originates from the excited pyrenyl groups situated in a randomly coiled or partially unfolded part of the polypeptide chain and the excimer fluorescing at longer wavelengths originates from a slightly perturbed region of the helix. The CPF spectrum of the l polymer was compared with those of intermolecular excimers of N-acetyl-L-1-pyrenylalanine methyl ester in concentrated solutions of dimethylformamide and toluene. The results suggested that the excimer of the polymer at shorter wavelength is nonpolar and has a definite geometry, whereas that at longer wavelength is polar and favored in dimethylformamide solution. The fluorescence rise and decay analysis supported the above assignments.

Aromatic and Cystine Side-Chain Circular Dichroism in Proteins

“…This group has synthesized and studied polymers in which the phenyl group of poly(Phe) is replaced by naphthyl (Sisido et al, 1983a,b) and pyrenyl (Egusa et al, 1985) groups. Molecular mechanics calculations of conformer energies and matrix method (Bayley et al, 1969) calculations of rotational strengths have given reasonable agreement with the experimentally determined CD spectra (Sisido et al, 1983a,b;Sisido and Imanishi, 1985).…”

Conformational Effects on the Electron‐Transfer Efficiency in Peptide Foldamers Based on α,α‐Disubstituted Glycyl Residues

“…4, exhibit an intense negative maximum around 202 -205 nm, typical of peptides attaining a right-handed 3 10 -helical conformation [29] [30]. In the 230 -250-nm range some differences among the three peptides can be noted, probably due to an induced CD resulting from electronic transitions of the aromatic side chains (pyrene and azulene), caused by the nearest-neighbor peptide groups [31] [32].…”

Structural properties of DNO investigated with pyrene excimer formation

“…It has been observed that the intensity of the excimer emission in pyrene-labelled polypeptides is dependent on the chirality of the 1-pyrenylalanine employed in their synthesis [23]. Circular polarized fluorescence spectra (CPF), of pyrene-labelled polypeptides, show negative and positive dissymmetries in the pyrene excimer emission wavelength range [24].…”

“…Dissymmetric CPF spectra are diagnostic of the presence of the more than one emissive species [25]. Egusa et al [23,26] attributed the dissymmetry observed in the CPF spectra of pyrenelabelled polyalanines to the presence of two kinds of excimer species: a non-polar excimer (u E =460 nm) of well-defined geometry and a polar excimer emitting at longer wavelengths (u E =520 nm), which may exist in a variety of configurations and where the interchromophoric distance is longer than for the non-polar excimer.…”

“…But static excimer formation is only observed in the case of DNO-1 and to a small extent for DNO-2. Some examples have been reported in the literature of strongly interacting pyrene moieties showing no excimer emission [23,27,28]. Oishi et al [29] have reported on the photophysical properties of a peptide carrying two pyrene residues which formed a sandwich-type close pair in the ground state as evidenced by CD spectroscopy.…”