Cα–Cβ Tyrosyl Bond Cleavage: Theoretical and Spectroscopic Investigation of the Generation of α-Glycyl Radical Cations from Tyrosylglycylglycine

Mechanistic study of peptide radical cation has gained lot of attention due to its key intermediate’s role in some neurodegenerative disorders. Herein, detailed exploration of single electron transfer dissociation of [Cu^II(dien)(YGG)]^•2+ complex [dien = diethylenetriamine, YGG = Tyrosylglycylglycine] was accomplished. The Cu^II is reduced to Cu^I and formed (YGG)^•+ radical cation. The comprehensive analyses of the formation of (YGG)^•+ and its fragmentation pattern were carried out via mass spectrometry, infrared multiple photon dissociation (IRMPD) with the conjunction of density functional theory (DFT). Geometry optimizations and harmonic frequency analyses were performed at UB3LYP/6-311++G(d,p) level. Relative enthalpies at 0 K (ΔH0) were evaluated with hybrid meta exchange-correlation functional at UM06- 2X/6-311++G(d,p) level, which was also used for natural population analyses (NPAs) to obtain charge and spin densities. The MS/MS spectrum of [Cu^II(dien)(YGG)]^•2+ showed that it can dissociate into two major products, odd electron peptide radical cation [YGG]^•+ at m/z 295 and reduced copper complex ion [Cu^I(dien)]⁺ at m/z 166. Furthermore, the product ion [YGG]^•+ undergoes dissociation. So, the m/z 278 [Z3-H] ^•+ (breakage of N-Cα bond) appears which indicates the loss of NH3. The m/z at 189 is arisen dueto the loss of p-quinomethide by forming [G•GG]⁺ radical cation. Comparison between experimental and theoretical spectrum indicating that the experimental IRMPD spectrum is a mixture of at least three different conformers namely A3 (linear) A5 (tyrosine ring O is H bonded with C-terminal) and A6 (tyrosine ring O is attached with Cu-complex). Theoretical investigation shows that after dissociation of Cu-complex the lowest energy structure is α- structure the [Yα •GG]⁺ (having unpair electron located on α-carbon and charge on 1^st amide oxygen atom).