Electrospray ionization tandem mass spectrometry of protonated and alkali-cationized Boc-N-protected hybrid peptides containing repeats of D-Ala-APyC and APyC-D-Ala: formation of [b(n-1) + OCH3 + Na]+ and [b(n-1) + OH + Na]+ ions.
BACKGROUND
Differentiation and structural characterization of positional isomers of non-natural amino acid hybrid peptides by using electrospray ionization tandem mass spectrometry (ESI-MS(n) ) is desirable because of their fundamental importance from the view point of peptide mass spectrometry and also of their increasing importance in the area of research towards biomedical and material applications; hence, the present study is undertaken.


METHODS
Electrospray ionization ion-trap tandem mass spectrometry (ESI-MS(n)) was used to characterize and differentiate three pairs of positional isomers of Boc-N-protected hybrid peptides containing repeats of D-Ala-APyC and APyC-D-Ala (D-Ala = D-alanine and APyC = trans-3-aminopyran-2-carboxylic acid).


RESULTS
ESI-MS(n) spectra of protonated and alkali-cationized positional isomeric peptides display characteristic fragmentation involving the peptide backbone, the Boc group, and the side chain. It is observed that abundant rearrangement ions [b(n-1) + OCH(3) + Na](+) or [b(n-1) + OH + Na](+) are formed when D-Ala is present at C-terminus and the presence of APyC at the C-terminus inhibits the formation of rearrangement ions. In addition, abundant b(n-1)(+) ions are formed, presumably with stable oxazolone structures, when the C-terminus of b(n-1) (+) ions possessed D-Ala.


CONCLUSIONS
The present study demonstrates that ESI tandem mass spectrometry is very useful for differentiating positional isomers of hybrid peptides containing D-Ala and APyC amino acids. While the protonated peptides give rise to characteristic sequencing ions, the cationized peptides produce additional rearrangement ions ([b(n-1) + OCH(3) + Na](+) and [b(n-1) + OH + Na](+)) which helps distinguish between the presence of D-Ala and APyC amino acids at the C-terminus.
National Centre for Mass Spectrometry, CSIR - Indian Institute of Chemical Technology, Hyderabad 500 007, India.
G
G
Raju
C
C
Purna Chander
K
K
Srinivas Reddy
R
R
Srinivas
G V M
GV
Sharma
D000409:Alanine / Q000737:chemistry; D002264:Carboxylic Acids / Q000737:chemistry; D007477:Ions / Q000737:chemistry; D007536:Isomerism; D008958:Models, Molecular; D010455:Peptides / Q000737:chemistry*; D011714:Pyrans / Q000737:chemistry; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D053719:Tandem Mass Spectrometry / Q000379:methods
C569129:3-aminopyran-2-carboxylic acid; D002264:Carboxylic Acids; D007477:Ions; D010455:Peptides; D011714:Pyrans; D000409:Alanine