An NIH shift is a chemical rearrangement where a hydrogen atom on an aromatic ring undergoes an intramolecular migration primarily during a hydroxylation reaction. This process is also known as a 1,2-hydride shift. These shifts are often studied and observed by isotopic labeling. An example of an NIH shift is shown below:

Example of NIH shift

In this example, a hydrogen atom has been isotopically labeled using deuterium (shown in red). As the hydroxylase adds a hydroxyl (the -OH group), the labeled site shifts one position around the aromatic ring relative to the stationary methyl group (-CH₃).

Several hydroxylase enzymes are believed to incorporate an NIH shift in their mechanism, including 4-hydroxyphenylpyruvate dioxygenase and the tetrahydrobiopterin dependent hydroxylases. The name NIH shift arises from the U.S. National Institutes of Health from where studies first reported observing this transformation.

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