Photophysical and structural aspects of poly-L-tryptophan: π−π stacking interaction with an excited state intermolecular proton transfer probe 3-Hydroxynaphthoic acid revealed by experiments and molecular simulation

Abstract

In biophysical studies involving proteins, the involvement of the intrinsic fluorophore Tryptophan and its energy transfer/binding interactions are already well-investigated areas. Theoretical studies have also been well corroborated with experimental findings. However, in polymeric Tryptophans (specifically homopolymers), several queries still need to be addressed – their structure, the environment of each Tryptophan and the binding preferences of the latter. This necessitated some detailed investigations on the poly-L-Tryptophan system both from experimental and theoretical standpoints. In this work, we have carried out both steady-state and time-resolved fluorescence studies along with low-temperature phosphorescence (LTP) of poly-L-tryptophan, and the nature of the emitting Tryptophan (Trp) residue in the latter has been characterized based on a comparison with the emission features of the parent monomer. The very large red-shift of the (0–0) band of phosphorescence in poly-L-Tryptophan has been explained through triplet-triplet energy transfer along with the structure of the latter which has been developed by theoretical modelling. The nature of the environment of the emitting Trp residue in poly-L-Trp has been compared with several multi-Tryptophan proteins where different Trp residues exhibit optically resolved (0–0) bands. The interaction of the excited state proton transfer (ESIPT) probe 3-hydroxynaphthoic acid (3-HNA) with poly-L-Trp has also been investigated in detail using fluorescence, LTP, and classical molecular dynamics simulations.