Improved chromatographic identification of coloured amino acid thiohydantoins.

A new N-terminal reagent for peptides and proteins, 4-N,N-dimethylamino-naphthylazobenzene-4'-isothiocyanate, is described which gives purple thiohydantoin derivatives; chromatographic separation of 24 amino acid thiohydantoins is reported. Such standard purple derivatives can be used as markers in the separation of the red 4-N,N-dimethylaminoazobenzene-4'-thiohydantoins. Conversely, standard red thiohydantoins can be used as markers in the separation of unknown purple amino acid thiohydantoins. In two-dimensional thin-layer chromatography, the precision of identifying the unknowns can be greatly improved by running markers with a colour different from the unknowns on the same side of the sheet.