In this study, dissociative one‐electron transfer dissociation of [Cu^II (dien)Y(G/A)W]^•2+ [dien = diethylenetriamine; Y(G/A)W = tyrosyl (glycyl/alanyl)tryptophan] was used to generate the tripeptide radical cations [Y(G/A)W]^•+; subsequent loss of the Tyr side chain formed [G_α^•(G/A)W]⁺. The π‐centered species [YGW_π^•]⁺ generated the α‐centered species [G_α^•GW]⁺ through C_α–C_β bond cleavage, as revealed using infrared multiple photon dissociation (IRMPD) measurements and density functional theory (DFT) calculations. Comparisons of experimental and theoretical IR spectra confirmed that both the charge and spin densities of [Y(G/A)W_π^•]⁺ were delocalized initially at the tryptophan indolyl ring; subsequent formation of the final [G_α^•(G/A)W]⁺ structure gave the highest spin density at the α‐carbon atom of the N‐terminal glycine residue, with a proton solvated by the first amide oxygen atom. The IRMPD mass spectra and action spectra of the [G_α^•(G/A)W]⁺ species were all distinctly different from those of their isomeric [G(G/A)W_π^•]⁺ species. The mechanism of formation of the captodative [G_α^•(G/A)W]⁺ species—with the charge site separated from the radical site—from [Y(G/A)W_π^•]⁺ has been elucidated. DFT calculations suggested that the C_α–C_β bond cleavage of the tyrosine residue in the radical cationic [Y(G/A)W_π^•]⁺ precursor involves (a) through‐space electron transfer between the indolyl and phenolic groups; (b) formation of proton‐bound dimers through C_α–C_β cleavage of the tyrosine residue; and (c) a concerted proton rearrangement from the phenolic OH group to the carboxyl group and formation of the α‐carbon‐centered product [G_α^•(G/A)W]⁺ through hydrogen bond cleavage. The barriers for the electron transfer (a), the C_α–C_β cleavage (b), and the protonation rearrangement (c) were 12.8, 26.5, and 10.3 kcal mol⁻¹, respectively.

中文翻译：

---

Cα–Cβ酪氨酸键断裂的机理检查：光谱学研究酪氨酸（甘氨酰/丙氨酰基）色氨酸生成α-甘氨酰自由基阳离子

在这项研究中，[Cu ^II （dien）Y（G / A）W] ^{•2 +的}解离单电子转移解离[dien = diethylenetriamine; Y（G / A）W =酪氨酰（甘氨酰/丙氨酰基）色氨酸]用于产生三肽自由基阳离子[Y（G / A）W] ^•+；随后失去的Tyr侧链形成[ _Gα ^•（G / A）W] ⁺。该π为中心的物种[YGW _π ^• ] ⁺产生的α为中心的物种[G _α ^• GW] ⁺至C _α -C _β键断裂，如使用红外多光子离解（IRMPD）测量和密度泛函理论（DFT）计算所揭示。实验和理论红外光谱的比较证实，[Y（G / A）_Wπ ^• ] ⁺的电荷和自旋密度都最初在色氨酸吲哚基环上离域。随后形成的最终[ _Gα ^•（G / A）W] ⁺结构在N-末端甘氨酸残基的α-碳原子上具有最高的自旋密度，质子被第一个酰胺氧原子溶剂化。所述的IRMPD质谱和作用谱[G _α ^•（G / A）W] ⁺种类与它们的同分异构体[G（G / A）_Wπ ^• ] ⁺种类完全不同。形成推拉的机理[G _α ^•（G / A）W] ⁺物种与充电位点从自由基位点从分离[Y（G / A）W，_π ^• ] ⁺已被阐明。DFT计算表示，C _α -C _β键的酪氨酸残基在自由基阳离子裂解[Y（G / A）W，_π ^• ] ⁺前体涉及（a）吲哚基和酚基之间的空间电子转移；（B）至C质子结合的二聚体的形成_α -C _β酪氨酸残基的裂解; （c）质子从酚的OH基团重排至羧基，并通过氢键裂解形成以α碳为中心的产物[ _Gα ^•（G / A）W] ⁺。用于电子转移的障碍的（a）中，C _α -C _β裂解（b）和质子化重排（c）中分别为12.8，26.5，和10.3千卡摩尔^-1分别。