Characteristic negative ion fragmentations of deprotonated peptides containing post-translational modifications: mono-phosphorylated Ser, Thr and Tyr. A joint experimental and theoretical study.
Peptides and proteins may contain post-translationally modified phosphorylated amino acid residues, in particular phosphorylated serine (pSer), threonine (pThr) and tyrosine (pTyr). Following earlier work by Lehmann et al., the [M-H]- anions of peptides containing pSer and pThr functionality show loss of the elements of H3PO4. This process, illustrated for Ser (and using a model system), is CH3CONH-C(CH2OPO3H2)CONHCH(3) --> [CH3CONHC(==CH2)CONHCH3 (-OPO3H2)] (a) --> [CH3CONHC(==CH2)CONHCH3-H]- + H3PO4, a process endothermic by 83 kJ mol(-1) at the MP2/6-31++G(d,p)//HF/6-31++G(d,p) level of theory. In addition, intermediate (a) may decompose to yield CH3CONHC(==CH2)CONHCH3 + H2PO4 - in a process exothermic by 3 kJ mol(-1). The barrier to the transition state for these two processes is 49 kJ mol(-1). Characteristic cleavages of pSer and pThr are more energetically favourable than the negative ion backbone cleavages of peptides described previously. In contrast, loss of HPO3 from [M-H]- is characteristic of pTyr. The cleavage [NH2CH(CH2-C6H4-OPO3H-)CO2H] --> [NH2C(CH2-C6H4-O-)CO2H (HPO3)] (b) --> NH2CH(CH2-C6H4-O-)CO2H + HPO3 is endothermic by 318 kJ mol(-1) at the HF/6-31+G(d)//AM1 level of theory. In addition, intermediate (b) also yields NH2CH(CH2-C6H4-OH)CO2H + PO3 - (reaction endothermic by 137 kJ mol(-1)). The two negative ion cleavages of pTyr have a barrier to the transition state of 198 kJ mol(-1) (at the HF/6-31+G(d)//AM1 level of theory) comparable with those already reported for negative ion backbone cleavages.
Andreazza
Hayley J
HJ
Department of Chemistry, The University of Adelaide, South Australia, 5005.
Fitzgerald
Mark
M
Bilusich
Daniel
D
Hoffmann
Ralf
R
Hoffmann
Peter
P
Eichinger
Peter C H
PC
Bowie
John H
JH
D000595:Amino Acid Sequence; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry*; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D011522:Protons; D012694:Serine / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization; D013912:Threonine / Q000737:chemistry*; D014443:Tyrosine / Q000737:chemistry*
D010455:Peptides; D011522:Protons; D013912:Threonine; D014443:Tyrosine; D012694:Serine