Sodiation of Enhanced Green Fluorescent Protein (EGFP) in Basic Solution Studied by Electrospray Mass Spectrometry

ABSTRACT

In our previous work, the sodiation of melittin, cytochrome c, and ubiquitin in a 1 mM NaOH water/methanol solution was studied by electrospray mass spectrometry. It was suggested that the α‐helix is more resistant to sodiation than the β‐sheet. In this study, sodiation of enhanced green fluorescent protein (EGFP) composed of a β‐barrel was studied in 1% CH₃COOH (AcOH) or 1 mM NaOH water/methanol solution by electrospray mass spectrometry. Although EGFP was denatured in an acidic solution, it maintains a near‐native structure in a basic solution. For the 1% AcOH solution, the protonated EGFP, [EGFP + nH − mH + mNa]ⁿ⁺, with n = 14 − 36 and m = 0 was detected. For 1 mM NaOH, the number n for [EGFP + nH − mH + mNa]ⁿ⁺ was found to increase with the sodiation number m and vice versa for [EGFP + nH − mH + mNa]ⁿ⁻. Namely, Na⁺ adducts counteract the negative charges of deprotonated acidic residues. The protonated EGFP detected as major ions for basic 1 mM NaOH was ascribed to the more surface‐active H₃O⁺(aq) than OH⁻(aq).