An ESR titration technique for the determination of accessible tyrosine and tryptophan residues in globular proteins

Abstract The reaction of potassium nitrosyldisulphonate (NDS) with globular proteins has been monitored by electron spin resonance. The reaction is shown to be sufficiently sensitive and reproducible to provide a rapid new method for the estimation of exposed tyrosine and tryptophan residues in these proteins at physiological pH. Under these conditions each tyrosine residue was found to consume 2 moles, and each tryptophan residue 3 moles, of NDS. The estimation of accessible groups thus obtained are in good agreement with published data obtained using other chemical probes.