Surface Adsorption and Transfer of Organomercaptans to Colloidal Gold and Direct Identification by Matrix Assisted Laser Desorption/Ionization Mass Spectrometry

The adsorption characteristics of two different organomercaptan adsorbates, 5-((2-(and-3)-S-(acetylmercapto)succinoyl)amino)fluorescein (SAMSA) and the peptide Cys-Lys-Trp-Ala-Lys-Trp-Ala-Trp (CKWAKWAK), on colloidal Au were studied, and the conjugates produced were characterized by UV-vis spectroscopy, transmission electron microscopy, and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Fluorescence difference measurements of free thiols in solution were used to assemble surface adsorption isotherms on Au colloid revealing surface coverages of 1.0 x 10(14) molecules cm(-2) for SAMSA and 3.1 x 10(14) molecules cm(-2) for CKWAKWAK. The free energies of adsorption were calculated to be -48.4 kJ/mol for SAMSA and -49.2 kJ/mol for CKWAKWAK. UV-visible absorption spectroscopy and transmission electron microscopy reveal that the thiol/colloid conjugates flocculate under conditions where the net charge per colloid is small or neutral and that flocculated colloids can be resuspended by a change in pH to more basic for the acidic SAMSA/Au conjugates or to more acidic for the basic CKWAKWAK/Au conjugates. The reversible flocculation allows the conjugates to be readily separated from free adsorbate in solution and thereby prepared for further characterization. CKWAKWAK/colloid conjugates were analyzed by MALDI-MS, and the mass spectra show (M + H)(+), (M + Na)(+), and (M + K)(+) ions attributable to the peptide. The manipulations studied here constitute a powerful complement to microfluidic-based separation and analysis methods. Conjugating mass-limited analytes to Au colloids makes it possible to sequester and transfer small quantities of analytes with high efficiency.