Electrospray ionization mass spectrometry (ESI-MS) of proteins involves creating positively charged ions of the protein and separating them according to their mass-to-charge ratio (m/z).

(a) What causes the different positive charges on different particles of the protein?

(b) The amino acid composition (in numbers of residues per chain) of hen egg-white lysozyme (HEWL) is as follows:

P 2 Y 3 N 14 H 1

D 7 M 2 L 8 E 2

C 8 R 11 G 12 F 3

A 12 I 6 K 6 V 6

S 10 W 6 T 7 Q 3

What is the maximum positive charge that can be present on a HEWL ion?

Mass Spectroscopy is the technique used for characterising and sequencing proteins. For ions in the gas phase, mass spectrometry precisely detects the mass-to-charge (m/z) ratio (where m is the ion's mass and z is its charge).In Electrospray Ionization – Mass Spectrometry (ESI - MS), a solution of a macromolecule like a peptide is sprayed from a tiny capillary tube maintained at high voltage (4,000 V), generating fine, highly charged droplets from which the solvent quickly evaporates. This produces a sequence of macromolecular ions in the gas phase with ionic charges ranging from +0.5 to +2 per kilodalton.

The positive charges on different particles during ESI - MS is because of the protonation of basic side chains of amino acids i.e., Histidine, arginine, and lysine. The positive charge is also because of the protonation of N-terminal amine group of the protein chain.

The HEWL ion contains one histidine, 6 lysine, and 11 arginine residues. Also, the N-terminus contains one NH₂, so the maximum positive charge which can be present on it is 19.