Biotinylated peptides/proteins.  I. Determination of stoichiometry of derivatization. 
A method is described for the determination of the stoichiometry of biotinylation of peptides and proteins after reaction with an N-hydroxysuccinimide ester of biotin containing the extended spacer arm 6-aminohexanoic acid (NHS-epsilon Ahx-biotin).  The method of analysis, based on the quantification of phenylthiocarbamyl derivatives of 6-aminohexanoic acid, is able to measure low picomolar amounts of biotinyl derivative.  Analyses were performed using an automated on-line hydrolyzer-derivatizer followed by high-performance liquid chromatography.  Compositional analyses determined for known peptides were in excellent agreement with analyses obtained by mass spectrometry.  Procedures are also described for the production of biotinylated protein probes that can be labeled reproducibly to contain specific amounts of biotin.  The analytical advantage and steric freedom provided by the 6-aminohexanoic acid spacer arm argue strongly for the NHS-epsilon Ahx-biotin reagent to be a reagent of choice for the biotinylation of peptides and proteins.