Peptide‐metal ion interactions in solution: Detection by laser desorption time‐of‐flight mass spectrometry and electrospray ionization mass spectrometry

The specific interaction of Cu(II) ions with metal‐binding peptides in solution has been investigated by two different methods of soft ionization mass spectrometry, namely matrix‐assisted ultraviolet laser desorption time‐of‐flight mass spectrometry (LDTOF) and electrospray ionization mass spectrometry (ES). The metal‐binding peptide selected for these investigations is a 26‐residue sequence found on the surface of the human plasma metal‐transport protein histidine‐rich glycoprotein. The peptide, (GHHPH)₅ G, was synthesized and evaluated by ES and LDTOF before and after the addition of Cu(II) or Mn(II) ions in solution. In the absence of added metal ions, the peptide was observed to have a mass equal to within 0.5 Da of its calculated mass (2903.0 Da) by both LDTOF and ES. In the presence of Cu(II), up to five additional peaks were observed at mass increments of approximately 63.9 Da (LDTOF) or 62.3 Da (ES); Mn was not bound to the peptide under identical experimental conditions. By both LDTOF and ES, the maximum Cu‐binding capacity observed (i.e., 5 g‐atoms mol⁻¹) demonstrated that up to 1 Cu could be bound per (GHHPH) internal repeat unit. This peptide‐metal ion interaction stoichiometry was verified by direct titration in solution and, with immobilized peptide, by quantitative metal ion affinity chromatography. Thus, the ability to detect stable peptide‐metal complexes did not appear to be differentially affected by the two different volatilization/ionization methods needed to generate charged intact molecular ions. The quantity and stoichiometry of bound Cu atoms was affected, however, by experimental conditions such as LDTOF matrix and ES solution composition. These results demonstrate the ability to verify directly the solution‐phase binding capacity of metal‐binding peptides by LDTOF and by ES. We conclude from these studies that other metallo‐organic interactions may also be amenable to investigation by these rapid and sensitive techniques.