Investigating the Role of pH and Counterions in the Intrinsic Fluorescence of Solid-State L-Lysine

TL;DR

This study explores how pH and counterions influence the intrinsic fluorescence of solid-state L-Lysine, combining experiments and simulations to understand the molecular mechanisms behind pH-dependent optical properties.

Contribution

It provides new insights into how pH and counterions affect fluorescence in non-aromatic amino acid assemblies through combined experimental and theoretical approaches.

Findings

Fluorescence increases from acidic to basic pH.

Protonation state affects non-radiative decay pathways.

Counterion type influences aggregate morphology and fluorescence.

Abstract

There is currently a growing interest in understanding the origins of intrinsic fluorescence as a way to design non-invasive probes for biophysical processes. In this regard, understanding how pH influences fluorescence in non-aromatic biomolecular assemblies is key to controlling their optical properties in realistic cellular conditions. Here, we combine experiments and theory to investigate the pH-dependent emission of solid-state L-Lysine (Lys). Lys aggregates prepared at different pH values using HCl and H$_2$SO$_4$ exhibit protonation- and counterion-dependent morphology and fluorescence, as shown by microscopy and steady-state measurements. We find an enhancement in the fluorescence moving from acidic to basic conditions. To uncover the molecular origin of these trends, we performed non-adiabatic molecular dynamics simulations on three Lys crystal models representing distinct protonation states. Our simulations indicate that enhanced protonation under acidic conditions facilitates non-radiative decay via proton transfer, whereas basic conditions favor radiative decay. Our combined experimental-theoretical work highlights pH and counterion identity as key factors tuning fluorescence in Lys assemblies, offering insights for designing pH responsive optical materials based on non-aromatic amino acids.