Long living excited state of protonated adenosine unveiled by ultrafast fluorescence spectroscopy and density functional theoretical study

Enhancement of intrinsic guanine fluorescence by protonation in DNA of various structures

Understanding the diversity of DNA structure and functions in biology requires tools to study this biomolecule selectively and thoroughly. Fluorescence methods are powerful technique for non-invasive research. Due to the low quantum yield, the intrinsic fluorescence of nucleotides has not been considered for use in the detection and differentiation of nucleic acid bases. Here, we have studied the influence of protonation of nucleotides on their fluorescence properties. We show that protonation of ATP and GTP leads to enhanced intrinsic fluorescence. Fluorescence enhancement at acidic pH has been observed for double-stranded DNA and single-stranded oligonucleotides. The formation of G4 secondary structures apparently protected certain nucleotides from protonation, resulting in less pronounced fluorescence enhancement. Furthermore, acid-induced depurination under protonation was less noticeable in G4 structures than in double-stranded and single-stranded DNA. We show that changes in the intrinsic fluorescence of guanine can be used as a sensitive sensor for changes in the structure of the DNA and for the protonation of specific nucleotides.

Non-Aromatic Fluorescence in Biological Matter: The Exception or the Rule?

While in the vast majority of cases fluorescence in biological matter has been attributed to aromatic or conjugated groups, peptides associated with neurodegenerative diseases, such as Alzheimer's, Parkinson's, or Huntington's, have been recently shown to display an intrinsic visible fluorescence even in the absence of aromatic residues. This has called the attention of researchers from many different fields, trying to understand the origin of this peculiar behavior and, at the same time, motivating the search for novel strategies to control the optical properties of new biophotonic materials. Today, after nearly 15 years of its discovery, there is a growing consensus about the mechanism underlying this phenomenon, namely, that electronic interactions between non-optically active molecules can result in supramolecular assemblies that are fluorescent. Despite this progress, many aspects of this phenomenon remain uncharted territory. In this Perspective, we lay down the state-of-the-art in the field highlighting the open questions from both experimental and theoretical fronts in this fascinating emerging area of non-aromatic fluorescence.