Valine hydantoin. Isocyanic acid is formed when urea dissociates into cyanate and ammonium. In a carbamoylation reaction, the isocyanic acid form a stable adduct with the nitrogen in amino groups, e.g. to N-terminal valine in Hb chains. The carbamoylated N-terminal can be analysed as a valine hydantoin detached by acid hydrolysis.