Among all the amino acids, intrinsic fluorescence from tryptophan (Trp) has been well-known among various researchers to study conformational dynamics in several proteins and protein-related peptidomimetics. To understand the fluorescence behaviour of Trp buried within different motifs of protein, we generally use model compounds with a variable number of Trp residues. The study of these prototypes might help us providing deep insight into the photophysical properties of the Trp in a complex protein environment. In our work, we designed and synthesized three aspartic acid-based peptidic molecules possessing one, two, and three Trp units, respectively. Detailed photophysical and fluorescence quenching investigations of these model compounds have been performed 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 nonradiative decays were estimated which reveal the solvent-dependent behavior of the compounds as well as the presence of Trp-Trp interactions on the peptidic scaffold. Popular quenching agents like acrylamide (quenching by electron/charge acceptor) and triethylamine (quenching by electron/charge donor) have been used as a dynamic quencher in the aforementioned three model compounds. Quenching by acrylamide is found to be significantly more efficient than that by triethylamine. Significant solvent dependency has been found by Stern-Volmer quenching constant and bimolecular quenching rate constant. The differences in the quenching constants for the three model compounds support the presence of Trp-Trp interactions and suggest differential accessibility of the quencher to the Trp moiety on the three peptidic scaffolds. Investigations through such model peptides will help to understand the properties of the Trp in a complex protein environment.

Photophysical study of linear and branched tryptophan based peptidomimetics

ACS Spring Meeting 2021

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