fluorescent dye · BEGINNER LESSON
DAPI
DAPI is 4′,6-diamidino-2-phenylindole, a fluorescent dye that binds DNA, especially A/T-rich regions. When bound to double-stranded DNA, it absorbs ultraviolet light and emits blue light.
What it is and what is measured
DAPI is a fluorescent dye, also called a fluorochrome. In practice, scientists measure whether it binds to DNA and how it changes light emission: when bound to double-stranded DNA, it absorbs ultraviolet light near 358 nm and emits blue light near 461 nm. It binds strongly to adenine-thymine, or A/T-rich, DNA regions and can also bind RNA more weakly.
[2] 4',6-Diamidino-2-Phenylindole [1] Crystal structure of the DNA sequence d(CGTGAATTCACG)2 ...
How it works
Structural and microscopy studies show DAPI fits into the narrow minor groove of DNA, especially over A/T base pairs. The binding is held by hydrogen bonding and van der Waals interactions, and it can displace ordered water molecules from that groove. This helps explain why the DNA-DAPI complex becomes more hydrophobic and why the fluorescent signal appears in microscopy.
[1] Crystal structure of the DNA sequence d(CGTGAATTCACG)2 ... [2] 4',6-Diamidino-2-Phenylindole
What changes it can cause
Evidence from different experiments shows that DAPI does more than stick to DNA. It can change DNA shape, including groove structure, and under some conditions it can switch from groove binding to intercalation, which means inserting between base pairs. In long double-stranded DNA, it can also promote a compact folded or coiled DNA-dye complex, but these effects depend on conditions and are not fully understood.
[3] The fluorophore 4',6-diamidino-2-phenylindole (DAPI) induces DNA folding in long double-stranded DNA - PubMed [1] Crystal structure of the DNA sequence d(CGTGAATTCACG)2 ...
Limitations and cautions in interpretation
The binding behavior of DAPI is not a single fixed mode. The literature notes that its binding features to DNA and RNA are complicated and still not fully understood. Structural observations also depend on the particular DNA sequence and experimental method, so the same dye can show groove binding in one setting and intercalation or folding effects in another.
[3] The fluorophore 4',6-diamidino-2-phenylindole (DAPI) induces DNA folding in long double-stranded DNA - PubMed [1] Crystal structure of the DNA sequence d(CGTGAATTCACG)2 ... [2] 4',6-Diamidino-2-Phenylindole
SEE THE IDEA
How DAPI interacts with DNA
Ultraviolet light is used to excite DAPI.
DAPI binds mainly to A/T-rich DNA in the minor groove.
Bound DAPI emits blue light that can be detected in microscopy.
Binding can displace water and sometimes alter DNA shape.
Excitation leads to fluorescence, while binding to DNA also changes the local DNA structure and hydration.
DAPI is a fluorescent DNA-binding dye that both reports on DNA and can influence the DNA environment.
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.