Formation, isomerization and dissociation of aromatic-containing peptide radical cations

Abstract

This thesis explores the mechanistic role of aromatic radical isomer generation in radical-mediated biochemical processes, with a focus on the formation, isomerization, and characterization of essential aromatic isomers: canonical π-, phenoxyl, α-, and β-carbon-centered, particularly the tyrosyl radical. Tyrosyl radicals are pivotal intermediates in these processes. For instance, in ribonucleotide reductase (RNR), the phenoxyl radical forms early in a complex radical transfer pathway, ensuring the precise conversion of ribonucleotides to deoxyribonucleotides, which is vital for DNA synthesis and repair. Conversely, in radical-mediated peptide tyrosine nitration, the phenoxyl radical forms late, a crucial step for efficient nitration. Understanding the role of tyrosyl radicals, whether early or late in the reaction, is essential for the successful execution of these biochemical processes. However, the exact mechanisms governing their site-specific formation under various conditions remain unclear. Chapters 3 to 6 unveil the isomeric structures and reactivities of prototypical aromatic radical isomers, especially tyrosyl residues, using a combination of isotopic labeling, tandem mass spectrometry, infrared multiple photon dissociation (IRMPD), and density functional theory (DFT) calculations. The structural and electronic properties on prototypical tyrosyl-containing tripeptides influence formation of phenoxyl radicals at different stages of a cascade radical transfer reaction, revealing favorable isomerization pathways in some cases and unexpected radical structures. Two novel isomeric structures were discovered: a new β-radical isomer originating from the phenoxyl radical and an unexpected α-carbon structure with a non-conventional N-terminal captodative structure. In Chapter 3, a detailed mechanistic study of a redox reaction is conducted by gas-phase dissociative electron transfer of prototypical zwitterionic [CuII(dien)GGW]•2+ complexes. The two final products are (i) the reduced [CuI(dien)]+ and (ii) the complementary oxidized non-zwitterionic π-centered [GGW]•+ ion. Chapter 4 broadens the scope to include the captodative α-carbon-centered [Gα•GW]+ by transitioning from π-centered [Y(G/A)Wπ•]+ via through-space electron transfer between the indolyl and phenolic groups and Cα–Cβ bond cleavage of the tyrosine residue. In Chapter 5, a new β-radical-induced N–Cα peptide bond cleavage pathway originates from the phenoxyl radical structure of GGY is discovered and systematically elucidated. This reveals a favorable transition from [GGYo•]+ to [GGYβ•]+ with a favorable isomerization barrier (25 kcal). Isotopic labeling with deuterium on two of the β-hydrogen atoms in the [GGYo•]+ analogue supports the proposed β-hydrogen atom rearrangement. In Chapter 6, two incipient glycyltyrosylglycine radical cationic isomers [GYβ•G]+ and [GYα•G]+ with the radical centers at the β- and α-carbon atoms of the tyrosyl residue, respectively, were generated, isolated, and characterized. Unlike conventional aromatic-containing peptide radical cations that primarily form canonical π- and beta-radicals, our findings revealed that a significant population of the [GYα•G]+ precursor are in an unexpected form of α-carbon [GYG]•+. A simpler prototypical system of glycylphenylalanylglycine radical cation ([GFG]•+) strongly resembles the [GYG]•+, suggesting the [GFG]•+ possess unconventional structural characters. A large population (75%) of the [GFG]•+ precursor are in an unexpected form of [GF•G]+, possessing energy levels approximately 6.8 kcal/mol higher than the other 25% conventional N-terminal captodative structures. (497 words)