Protein structure information from mass spectrometry? Selective titration of arginine residues by sulfonates

Noncovalently bound complexes between basic sites of peptides/proteins and sulfonates are studied using Matrix Assisted Laser Desorption/Ionization (M ALDI) Mass Spectrometry. Reactive sulfonate dyes such as Cibacron Blue F3G- A are known to bind to protonated amino groups on the exterior of a protein . In this work, we examine a wide range of other sulfonates with distinctly simpler structure and more predictable reactivity. Naphthalene-sulfonic ac id derivatives were found to bind to arginine only, as opposed to expected binding to all basic sites (Arg, Lys and His). Detailed control experiments were designed to unambigously confirm this selectivity and to rule out non specific adduct formation in the gas phase. The data show that the number o f complex adducts found equals the number of accessible arginine sites on t he surface of folded peptides and proteins, plus the N-terminus. Lys and Hi s are not complexed nor are buried residues with hindered access. MALDI-MS can therefore provide fast information related to the exposed surface of th ese biomolecules. Additional titration experiments with 1-anilino-naphthale ne-8-sulfonic acid (ANS) revealed that this fluorescent dye, which was ofte n hypothesized to bind to so-called molten globule states of proteins, beha ved exactly like all other naphthalene-sulfonic acids. ANS binding thus occ urs largely through the sulfonate group. (C) 2001 American Society for Mass Spectrometry.