Aliphatic amines ( ε -amino group of Lys) are moderately nucleophilic when deprotonated (beyond pH 9 for the free amino acid, but often much lower in proteins); on the other hand, the inactivation of the labelling reagent by water increases with pH. The α -amino group has a pKa of 7. 6–8. 5, thus the N-terminus of a protein can be labelled specifically around pH 7. Aromatic amines require very reactive probes (thiocyanate, sulphonyl chloride), but can be labelled at any pH > 4. The nitrogen of peptide bonds and Gln, Asn, Arg and His side-chains, and of guanosine and adenosine are almost unreactive. Amino-sugars in glycoproteins may also react.