Hb Neuilly-Sur-Marne, a New Human Hemoglobin Variant with Ser-Asp-Leu Inserted between α86(F7) Leu and α87(F8) His: Characterization by High-Energy Collision-Induced Dissociation Liquid Secondary Ion Mass Spectrometry and Low Energy Collision-Induced Dissociation Tandem Mass Spectrometry in An Ion Trap Fitted with a Nanospray Ionization Source

Abstract

Hemoglobin (Hb) Neuilly-sur-Marne is a new α-chain variant found during a systematic screening. Electrospray mass measurements showed the presence of an abnormal α-chain displaying a shift of +315 u relative to the normal value. Tryptic cleavage of this chain and molecular weight determination of the peptides indicated that the 315 u shift was located into the αT-9 peptide, the molecular weight of which is higher than 3000 Da. High-energy collision spectra of MH+ions generated by liquid secondary ion mass spectrometry from the normal and abnormal αT-9 afforded mainly amino-terminal containing ions. They indicated that these two peptides have an identical amino acid sequence from their 1st to 25th residues, the mass increase being thus located beyond this point. Too few ions were formed to establish reliably the sequence forward. It was hypothesized that this mass shift could result from a repeated sequence since the sum of the mass of the three residues—leucine, serine and aspartic acid—preceding position 25 is exactly 315 u. To get sequence information above position 25, decomposition of multicharged species was attempted. An ion trap fitted with a nanospray ionization source was used. It produced mainly triply- and quadruply-charged ions. Decomposition of the triply-charged ion afforded a series of singly-charged Y-ions in the expected region, giving a readily interpretable sequence. It confirmed the insertion of a Ser-Asp-Leu sequence above position 25. Surprisingly, decomposition of the quadruply-charged molecular ion gave too few ions to provide sequence information in the expected region. Spectra were dominated by some multicharged Y ions arising from cleavages close to the amino end. Tandem mass spectrometry experiments were performed on the abundant Y303+ion and produced again a singly-charged Y ion series in the suitable domain which confirmed the above result. In Hb Neuilly-sur Marne this insertion of the Ser-Asp-Leu residues. between positions α-86 and α-87 is very likely due to a slipped strand mispairing mechanism.