6-Methylisoxanthopterin (6MI) is a base analog for the nucleotide guanine. It is useful as a fluorescent indicator because unlike most other base analogs, quenching does not occur when it is incorporated into a double helix. In fact, it exhibits a 3 to 4-fold increase in quantum yield when it is incorporated into a duplex formation. This allows 6MI to be used to probe the dynamics of DNA or RNA helices using a technique such as fluorescence polarization anisotropy.
See also 5-Bromouracil
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