Quantitative mass spectral identification of p-bromophenylthiohydantoins from Edman degradations by tetradeuterated...

p-Bromophenylisothiocyanate is a reagent well suited for the Edman degradation of peptide chains[1-5]. The relative rate of the coupling reaction at the N-terminus is slightly more rapid than with phenyl- or methylisothiocyanate, but less rapid than with pentafluorophenylisothiocyanate. The latter reagent, however, is rapidly hydrolyzed under the conditions of the coupling reaction. The p-bromo substituent facilitates mass spectral identification of the p-bromophenylthiohydantoins obtained, due to its characteristic double peak[1-4]. Quantitation of the amount of p-bromophenylthiohydantoin obtained can easily be achieved in the parent mass spectrum by addition of a known amount of tetradeutero-p-bromophenylthiohydantoin as an internal standard. The tetradeuterated standards give rise to molecular ion peaks four mass units higher than the corresponding undeuterated probes. Standard and probe volatilize almost equally well. The composition in the vapor indicated by the H/D ratio reflects the molar concentrations over the entire range of probe evaporation. A procedure for the preparation of the tetradeutero-p-bromophenyl-isothiocyanate and the respective tetradeuterated amino acid p-bromophenylthiohydantoins is given.