Spectroscopic investigation of linear and branched tryptophan-containing peptides

Abstract Intrinsic fluorescence from amino acid tryptophan (Trp) is used extensively to monitor conformational dynamics and inter/intra-molecular interactions in proteins and related systems. Peptide systems with varied number of Trp residues are desirable model compounds for understanding fluorescence behaviour of Trp under different conditions. Investigations through model peptides will provide deep insight into photophysical properties of the Trp in a complex protein environment. In this work, three aspartic acid-based peptidic molecules possessing one, two and three Trp units are designed and synthesized for their detailed photophysical and fluorescence quenching investigations within polar-protic methanol, polar-aprotic acetonitrile, and apolar ethyl acetate. From various UV–vis absorbance and steady-state/time-resolved fluorescence measurements, relevant photophysical parameters, such as, absorbance and fluorescence emission maxima, molar absorptivity, fluorescence quantum yield, steady-state anisotropy, fluorescence lifetime, rotational reorientation time, and rates of radiative and non-radiative decays were estimated that reveal not only the solvent-dependent behavior of the compounds but also the presence of interactions between Trp-Trp units on the peptidic scaffold. Quenching of fluorescence of the three Trp-containing compounds by quenching agents, acrylamide (electron/charge acceptor during quenching) and triethylamine (electron/charge donor during quenching), in the three solvents is dynamic in nature. Quenching by acrylamide is found to be significantly more efficient than that by triethylamine. Stern-Volmer quenching constant and bimolecular quenching rate constant also exhibit solvent dependence. The differences in the quenching constants for the three compounds possessing one, two, and three Trp units not only further support the presence of interactions between Trp-Trp units but also imply differential accessibility of the quencher to the Trp moiety on the peptidic scaffold.