The uv spectrum for each of the nucleoside or nucleotide derivatives responds differently to changes in pH .The strong uv absorptions and the differences due to the specific structure of the base moiety provide the basis for sensitive methods in assaying these compounds bothtatively and quantitatively. For example, the deamination of cytosine nucleosides or nucleotides to the corresponding uracil derivatives causes a marked shift in from 270 to 262 nm.