A cautionary (spectral) tail: red-shifted fluorescence by DAPI-DAPI interactions.

Area of Science:

• Biochemistry
• Molecular Biology
• Spectroscopy

Background:

• The fluorescent dye DAPI typically emits at ~460 nm upon binding to double-stranded DNA (dsDNA).
• Higher concentrations of DAPI can induce a red-shifted emission (~520-550 nm), previously associated with polyphosphate (polyP), RNA, polyadenylic acid (polyA), inositol phosphates (IPs), and heparin.
• This spectral shift is attributed to DAPI-DAPI electrostatic interactions facilitated by highly negatively charged molecules, increasing local dye concentration and molecular proximity.

Purpose of the Study:

• To investigate whether amorphous calcium phosphate (ACP) exhibits red-shifted DAPI emission.
• To explore the mechanism behind DAPI spectral shifts at high dye concentrations and in the presence of various negatively charged molecules.

Main Methods:

• Utilized DAPI staining at high concentrations on amorphous calcium phosphate (ACP).
• Analyzed fluorescence emission spectra of DAPI in the presence of ACP and other negatively charged substrates.
• Examined the effect of increasing dye/phosphate ratio on DAPI emission.
• Excited dry DAPI at ~360 nm to observe emission characteristics.

Main Results:

• Amorphous calcium phosphate (ACP) demonstrated red-shifted DAPI emission at high DAPI concentrations.
• The observed spectral shift is consistent with DAPI-DAPI electrostatic interactions driven by high negative charge density.
• Excitation of dry DAPI also confirmed red-shifted emission, supporting the role of molecular proximity.

Conclusions:

• Amorphous calcium phosphate (ACP) can induce red-shifted DAPI fluorescence, similar to polyphosphate and other polyanions.
• The DAPI spectral shift is a sensitive indicator of molecules with high negative charge density that promote DAPI aggregation.
• Researchers should employ multiple detection methods beyond DAPI spectral shifts for definitive substrate identification.