N-Terminal Decarboxylation as a Probe for Intramolecular Contact Formation in γ-Glu-(Pro)n-Met Peptides.,The Journal of Physical Chemistry B

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N-Terminal Decarboxylation as a Probe for Intramolecular Contact Formation in γ-Glu-(Pro)n-Met Peptides.
The Journal of Physical Chemistry B ( IF 2.8 ) Pub Date : 2020-08-19 , DOI: 10.1021/acs.jpcb.0c04371
Piotr Filipiak _{1,

2} , Krzysztof Bobrowski _{3,

4} , Gordon L Hug _{1,

4} , Christian Schöneich ₅ , Bronislaw Marciniak _{1,

2}

Affiliation

The kinetics of intramolecular-contact formation between remote functional groups in peptides with restricted conformational flexibility were examined using designed peptides with variable-length proline bridges. As probes for this motion, free radicals were produced using the ^•OH-induced oxidation at the C-terminal methionine residue of γ-Glu-(Pro)_n-Met peptides (n = 0–3). The progress of the radicals’ motion along the proline bridges was monitored as the radicals underwent reactions along the peptides’ backbones. Of particular interest was the reaction between the sulfur atom located in the side chain of the oxidized Met residue and the unprotonated amino group of the glutamic acid moiety. Interactions between them were probed by the radiation-chemical yields (expressed as G values) of the formation of C-centered, α-aminoalkyl radicals (αN) on the Glu residue. These radicals were monitored directly or via their reaction with p-nitroacetophenone (PNAP) to generate the optically detected PNAP^•– radical anions. The yields of these αN radicals were found to be linearly dependent on the number of Pro residues. A constant decrease by 0.09 μM J^–1 per spacing Pro residue of the radiation-chemical yields of G(αN) was observed. Previous reports support the conclusion that the αN radicals in these cases would have to result from (S∴N)⁺-bonded cyclic radical cations that arose as a result from direct contact between the ends of the peptides. Furthermore, by analogy with the rate constants for the formation of intramolecularly (S∴S)⁺-bonded radical cations in Met-(Pro)_n-Met peptides ( J. Phys. Chem. B 2016, 120, 9732), the rate constants for the formation of intramolecularly (S∴N)⁺-bonded radical cations are activated to the same extent for all of the γ-Glu-(Pro)_n-Met peptides. Thus, the continuous decrease of G(αN) with the number of Pro residues (from 0 to 3) suggests that the formation of a contact between the S-atom in the C-terminal Met residue and the N-atom of a deprotonated N-terminal amino group of Glu is controlled in peptides with 0 to 3 Pro residues by the relative diffusion of the S^•+ and unoxidized N-atom. The overall rate constants of cyclization to form the (S∴N)-bonded radical cations were estimated to be 3.8 × 10⁶, 1.8 × 10⁶, and 8.1 × 10⁵ s^–1 for peptides with n = 0, 1, and 2 Pro residues, respectively. If activation is the same for all of the peptides, then these rate constants are a direct indication for the end-to-end dynamics along the chain.

中文翻译：

N末端脱羧作为γ-Glu-（Pro）n-Met肽中分子内接触形成的探针。

使用设计的具有可变长度脯氨酸桥的肽，检查了具有受限构象柔韧性的肽中远端官能团之间的分子内接触动力学。作为该运动的探针，使用^• OH诱导的γ-Glu-（Pro）_n -Met肽C末端甲硫氨酸残基氧化产生自由基（n= 0–3）。当自由基沿着肽的骨架进行反应时，监测自由基沿着脯氨酸桥的运动进程。特别令人感兴趣的是位于氧化的Met残基的侧链上的硫原子与谷氨酸部分的未质子化的氨基之间的反应。通过在Glu残基上形成以C为中心的α-氨基烷基自由基（αN）的放射化学产率（表示为G值）来探究它们之间的相互作用。这些自由基可以直接监测，也可以通过与对硝基苯乙酮（PNAP）的反应进行监测，以生成光学检测的PNAP ^。自由基阴离子。发现这些αN基团的产率线性依赖于Pro残基的数目。观察到每间隔Pro残基的G（αN）放射化学产率恒定减少0.09μMJ ^–1。先前的报道支持以下结论：在这些情况下，αN自由基必须归因于由于肽末端之间的直接接触而产生的（S∴N）⁺键合的环状自由基阳离子。此外，通过与所述速率常数为形成分子内（S∴S）的类比⁺键合的游离基阳离子在MET-（Pro）的_Ñ -Met肽（J.物理学。化学式乙2016，120 ，（9732），对于所有γ-Glu-（Pro）_n -Met肽，都以相同的程度激活了形成分子内（S∴N）⁺键合的自由基阳离子的速率常数。因此，G（αN）随Pro残基数量（从0到3）的连续减少，表明在C端Met残基中的S原子与去质子化的N的N原子之间形成了接触。通过S ^•+和未氧化的N原子的相对扩散，可控制具有0至3个Pro残基的肽中Glu的末端氨基。环化反应的总速率常数，以形成（S∴N）键合的自由基阳离子估计为3.8×10 ⁶，1.8×10 ⁶，和8.1×10 ⁵小号对于n = 0、1，和2个Pro残基的肽，分别为^–1。如果所有肽段的激活均相同，则这些速率常数将直接指示沿链的端到端动力学。

更新日期：2020-09-18

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