Protonated N-benzyl- and N-(1-phenylethyl)tyrosine amides dissociate via ion/neutral complexes.
BACKGROUND
The collisional-induced dissociations (CID) of the [M+H]+ions of molecules having benzyl groups attached to N-atoms have been proposed to involve migration of the benzyl group through the intermediacy of ion/neutral complexes (INCs). We report the investigation of the mechanism of dissociation of protonated N-benzyl- and N-(1-phenylethyl)tyrosine amides by electrospray ionization (ESI) tandem mass spectrometry (MS/MS) and density functional theory (DFT) calculations.


METHODS
The amides were synthesized from the corresponding amino acids and amines. The ESI-MS/MS spectra were recorded using an Agilent QTOF 6540 mass spectrometer. The DFT calculations were performed by using Gaussian 09 software. The structures of the [M+H]+ions, intermediates, products and transition states (TS) were optimized at the B3LYP/6-31G(d,p) level of theory.


RESULTS
CID of the [M+H]+ions of N-benzyltyrosine amide yields two product ions due to rearrangements: (i) the [M+H-74]+ion (m/z 197) due to benzyl migration to the hydroxyphenyl ring and (ii) the [M+H-45]+ion (m/z 226) due to benzyl migration to the NH2group. DFT calculations suggest that the rearrangements occur through an INC in which the benzyl cation is the cation partner. The [M+H]+ion of N-(1-phenylethyl)tyrosine amide rearranges to an INC of the 1-phenylethyl cation. Subsequent elimination of styrene occurs by transfer of a proton from the 1-phenylethyl cation to the neutral partner.


CONCLUSIONS
The [M+H]+ions of both N-benzyl (1) and N-(1-phenylethyl) (2) tyrosine amide rearrange into INCs. The dissociation of [M+H]+ion of 1 yields the benzyl cation and [M+H-74]+and [M+H-45]+due to benzyl migration to the hydroxyphenyl ring and NH2group, respectively. However, the formation of the [M+H-74]+ion is not observed when the aromatic ring is deactivated. The [M+H]+ion of 2 either dissociates to form the 1-phenylethyl cation or [M+H-styrene]+. Copyright © 2015 John Wiley & Sons, Ltd.
Department of Chemistry, Sacred Heart College, Thevara, Kochi, India.|Bharathiyar University, Coimbatore, Tamilnadu, India.
Justin
J
Paulose
Department of Chemistry, Sacred Heart College, Thevara, Kochi, India.|Bharathiyar University, Coimbatore, Tamilnadu, India.
Revi P
RP
Achuthan
Department of Chemistry, Sacred Heart College, Thevara, Kochi, India.|Bharathiyar University, Coimbatore, Tamilnadu, India.
Maria P L
MP
Linsha
Department of Chemistry, Sacred Heart College, Thevara, Kochi, India.|Bharathiyar University, Coimbatore, Tamilnadu, India.
George
G
Mathai
National Institute of Pharmaceutical Education and Research, Hyderabad, India.
B
B
Prasanth
National Institute of Pharmaceutical Education and Research, Hyderabad, India.
M V N
MV
Kumar Talluri
National Centre for Mass Spectrometry, CSIR-IICT, Hyderabad, India.
Ragampeta
R
Srinivas