Ethidium Bromide Fluorescence

Bryant Fujimoto fujimoto at u.washington.edu
Fri Jul 27 14:25:08 EST 2001


NJ <njuni at poppy.ocn.ne.jp> writes:

>in article 60805a5e.0107250646.1292beb7 at posting.google.com, Bill Atkins at
>billatkins at bigfoot.com wrote on 7/25/01 11:46 PM:

>> Why does ethidium bromide only fluoresce in the prescense of DNA?
>> 
>> Thanks,
>> Bill

>Free EtBr also emits fluorescence when irradiated with longer wave length
>(~302 and ~366 nm) of UV. This is why you get higher background fluorescence
>when you observe EtBr-stained gels under longer wave lengths of UV (in this
>case, we can recognize DNA bands just because EtBr accumulates in DNA, I
>suppose...). 

Also, the quantum efficiency for fluorescence is about 11 times larger
when the ethidium is intercalated into DNA than when it is free in solution.

Bryant Fujimoto

>In the case of EtBr-DNA complex, DNA absorbs shorter wave
>length (~254 nm) of UV, which is not absorbed by EtBr itself, and transmits
>excitation energy to intercalating EtBr, yielding greater fluorescence but
>lower background fluorescence from free EtBr. See Molecular Cloning.
>--
>NJ

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