Acid derivatives are best distinguished by

Less reveahng are the ir spectra of larger peptide molecules. Unless sophisticated instruments, such as high resolution Fourier-transform spectrophotometers are applied, the large amide absorption overshadows other carbonyl frequencies. In some special cases, however, ir spectra can be uniquely informative. For example esterification of the phenolic hydroxyl in tyrosine with sulfuric acid is best revealed through the characteristic ir spectrum of sulfate esters, clearly distinguishable from the spectrum of the sulfonic acid derivatives which are the potential by-products of the sulfation process. 

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