用于后期肽修饰的温和自由基化学

在这种情况下“温和”实际上意味着什么

温和的, 肽化学, 不是营销形容词. 它编码特定的操作参数: pH 值 6–8, 温度≤ 37 ℃, 水相容溶剂体系, 和羟基耐受性, 羧基, 氨基, 硫醚, 咪唑, 和无预保护的胍侧链. 已发布的生物共轭相容性标准通常明确定义了该范围, 最近的激进化学文献越来越多地报道在其内部进行的实验.

那就是说, “温和”在下面描述的转化类别中仍然存在差异. 默认情况下，使用有机光催化剂和蓝色或绿色 LED 的光氧化还原方法往往是最具生物相容性的. 基于 HAT 的级联方法和电化学 C-H/S-H 偶联方法可能需要共溶剂 (通常是 DMSO/水混合物) 影响较长或更容易聚集的序列溶解度的比例. 定向C(sp3)–杂芳基化方法——特别是钯催化的变体——对指导基团安装和移除施加了额外的限制, 这增加了步骤和潜在的杂质源. 对于任何开发团队来说，问题不在于一种方法是否“原则上温和”，而在于它对于特定序列和下游应用是否足够温和.

光氧化还原共轭: 证据支持什么

光氧化还原催化产生了最大且最具可重复性的后期肽功能化数据. 核心机制——用可见光激发光催化剂, 在指定位置产生活性自由基中间体, 并将它们与自由基受体或伴侣偶联 - 通过激发态催化剂的电子选择性而不是通过保护基团操作提供化学选择性.

色氨酸作为主要手柄. 含色氨酸肽的 C2-烷基化和 C2-磺酰化均已在无金属和无光催化剂条件下得到证实, 后者使用芳基噻啉盐. 一个 2023 报到 有机字母 (ACS, 通过色氨酸光催化 C2 烷基化进行化学选择性后期功能化) 所描述的官能团耐受性与未受保护的天冬氨酸相容, 谷氨酸盐, 赖氨酸, 和丝氨酸侧链——这是可靠的肽后期功能化的最低标准. 一个 2024 可点击的色氨酸修饰描述于 科学进步 据报道无催化剂, 晚期 C2-磺酰化可耐受游离半胱氨酸残基, 化学选择性特别苛刻的测试.

组氨酸烷基化. 已报道使用 4-烷基-1,4-二氢吡啶作为自由基前体对含组氨酸肽进行可见光 Minisci 型 C-H 烷基化. 由于其氧化敏感性，组氨酸通常是光化学环境中的一个有问题的残留物, 因此，针对咪唑 C2 或 C5 位点的经过验证的方法添加了化学上不同的手柄.

脱氢丙氨酸 (Dha) 作为多功能自由基受体. 通过酶促或通过化学转化从丝氨酸或半胱氨酸安装的 Dha 在多种条件下充当迈克尔型自由基受体. 一个 2025 报告 通过光氧化还原催化位点选择性构建N-连接糖肽 (考研) 在可见光下使用 Dha 作为 N-糖基肟酸衍生自由基的受体, 展示吉斯型加成而不保护剩余的甘氨酸, 丙氨酸, 或存在丝氨酸残基. 该研究中的产量和非对映选择性数据——以及许多早期报告中缺乏 Dha 自由基添加的立体选择性解决方案——凸显了持续存在的差距: Dha 的彻底添加通常不具有立体保留性, 当 α-碳构型影响受体结合或蛋白酶稳定性时，这很重要.

脱羧缀合. MacMillan研究小组发现，在蓝光下，肽的C端羧基可以通过自由基脱羧选择性氧化和偶联, 利用内部和末端羧酸盐之间氧化电位的差异. This approach is mechanistically clean and avoids any side-chain preprotection, but it consumes the C-terminus — which limits its applicability unless a free C-terminus is expendable or the goal is terminal labeling.

Development note: Photoredox methods have demonstrated the broadest functional-group tolerance across the literature. The open questions for development use are reagent and photocatalyst removal, oxygen sensitivity during reaction and workup, and whether solvent systems (often DMSO/water) are compatible with downstream purification and lyophilization without introducing persistent residuals.

HAT 和 MHAT: 无需预功能化即可远程激活

Hydrogen atom transfer (帽子) chemistry occupies a different niche from photoredox. Rather than targeting a predefined redox-active handle, HAT-based methods abstract hydrogen atoms from C(sp3)–H bonds to generate carbon-centered radicals at positions that would otherwise require directing groups or prefunctionalization. In the context of peptide modification, this primarily means two things: remote functionalization via 1,5-HAT relay processes and sequential cascade reactions combining radical generation with a second catalytic bond-forming step.

多肽合成 一个 2025 paper in Organic Chemistry Frontiers (RSC, three-component alkylation of glycine derivatives and peptides via a site-selective 1,5-HAT cascade) demonstrated visible-light-mediated N-centered radical generation followed by 1,5-hydrogen atom transfer to the α-carbon of glycine residues, enabling a three-component alkylation. The selectivity arises from the geometry of the 1,5-shift rather than from electronic preference for any particular side chain, which has both advantages and limitations: the approach is broadly applicable to glycine-containing sequences but does not inherently distinguish between similarly positioned residues in longer chains.

Metal-assisted HAT, sometimes referred to in its cobalt- or iron-mediated forms as MHAT, brings additional control. The metal hydride generates a carbon radical under mild conditions — typically room temperature, hydrogen or silane as hydrogen source — and the resulting radical can be trapped by an alkene or radical acceptor in the same flask. For peptide application, the relevant question is whether the metal catalyst and hydrogen-source reagent are compatible with histidine, 蛋氨酸, and cysteine side chains, all of which are potential competing reaction partners. Published precedent exists for simple dipeptide and tripeptide substrates; validation at longer chain lengths with multiple reactive side chains remains limited in the primary literature as of mid-2026.

Development note: HAT and MHAT methods are attractive for modifying sequences where a natural radical handle (半胱氨酸, Trp, Dha) is absent or must be preserved for biological activity. The site-selectivity challenge is real and sequence-dependent — a short peptide where selectivity looks excellent may behave differently in a 20-residue sequence with multiple competing abstractable positions.

C(sp3)–H 功能化: 主干相邻编辑

定向C(sp3)–H functionalization represents the most ambitious arm of this chemistry for peptide applications, targeting internal backbone-adjacent positions without a traditional reactive handle. Two approaches have demonstrated preparative-scale potential on peptide substrates.

Side-chain-directed arylation. 一个 2024 study from Hou et al. — summarized in a 2026 RSC review (recent advances in site-specific modifications of peptides and proteins) — reported β-C(sp3)–H arylation of internal positions in both linear and cyclic peptides using S-alkyl cysteine as the directing group and 2-pyridone as an auxiliary ligand, under palladium catalysis. The method achieved arylation at the β-carbon of the residue adjacent to the cysteine-derived directing group under exceedingly mild conditions for a palladium-catalyzed C–H activation.

Backbone-inserted directing groups. 一个 2025 报到 JACS (考研, 钯(二)-catalyzed C(sp3)–H functionalization directed by a removable, backbone-inserted amidoxime ether) described amidoxime ether insertion at any amide bond in the backbone, using it as a removable directing group for subsequent C(sp3)–H arylation. The amidoxime ether is installed and removed in separate synthetic steps, adding two transformations to the route — but the positional flexibility it provides has no equivalent in current standard modification approaches.

For sequence-agnostic positioning, this class is chemically significant. The practical burden — palladium removal, directing group installation and removal, SPPS-compatible protecting group compatibility, and potential epimerization at the α-carbon under palladium catalysis — is also the heaviest among the three transformation families discussed here.

硫基自由基方法: 由数据支持的化学选择性

Cysteine remains the most analytically tractable residue for site-selective peptide modification because its sulfur nucleophilicity and redox chemistry are sufficiently distinct from other side chains to allow clean reactions with appropriate electrophilic radical sources. Several 2024–2025 reports extend this toolkit in directions relevant to conjugation programs.

Aryl radicals generated from aryl pinacol boronates under mild aqueous conditions have been shown to perform selective S-arylation of peptides and proteins (aryl radicals from aryl pinacol boronates for peptide and protein modification, TU Darmstadt/Wiley). The selectivity for sulfur over aromatic amino acid side chains under aqueous, near-neutral conditions represents a meaningful advance over aryl diazonium salt chemistry, which is far less selective.

An electrochemical approach to C–H/S–H cross-coupling for cysteine-containing peptides, including intramolecular macrocyclization, was described in Nature Communications (electro-induced C–H/S–H cross-coupling for functionalization/macrocyclization of cysteine-containing peptides, 2025). The umpolung strategy — generating a thiyl radical electrochemically and coupling it to an adjacent C–H bond — operates without external oxidant or photocatalyst and tolerates a range of side chains.

For programs involving stapled peptides, ADC-like conjugation, or fluorescence labeling at defined positions, these sulfur-selective methods represent the lowest-risk entry point because chemoselectivity data exists across the longest validated chain lengths and the most diverse side-chain environments.

兼容性, 杂质, 和必须回答的分析问题

The chemistry described above is real and reproducible at discovery scale. Translation to preparative and development scale requires honest answers to four categories of questions before any of these methods enter a formal development workflow.

兼容性评估

Before committing to a late-stage radical modification route, development teams should map the following:

杂质分布预测

Late-stage radical modification introduces a class of impurities distinct from SPPS-derived impurities. Expect and plan analytical methods for:

• Unreacted starting material: unconjugated peptide co-eluting with or near the product

• Over-modified species: if more than one reactive site exists (例如, two tryptophan residues), multiple-modification products will appear as higher-mass signals on ESI-MS or MALDI-TOF

• Regioisomeric products: for Dha radical addition or histidine alkylation, positional isomers that differ only in the site of bond formation may be analytically indistinguishable by RP-HPLC alone

• Oxidation products: radical conditions can oxidize methionine to the sulfoxide or tryptophan to the oxindole — peaks that are structurally close to the starting material on reverse-phase chromatography

• Catalyst-derived adducts: organic photocatalyst fragments, palladium residues (for C–H functionalization), or reagent by-products from HAT co-reagents that form adducts with the peptide

The regulatory and CMC implications of this impurity landscape align with the EMA’s 合成肽开发与生产指导原则, which explicitly addresses process reagents, elemental impurities, and the control strategy for synthetic intermediates.

分析要求

Standard RP-HPLC at 214 纳米, which works well for SPPS impurity profiling, is insufficient for radical late-stage modification outputs. Teams should plan for:

• LC-MS or LC-HRMS as the primary characterization tool — MS is required to distinguish unconjugated starting material from mono-modified product when they have similar chromatographic retention

• Orthogonal chromatography modes (例如, HILIC or IEX in addition to RP) when regioisomers are anticipated; regioisomers with identical mass will not resolve by mass detection alone

• 氨基酸分析 (AAA) for confirming that the amino acid composition after modification is consistent with a single addition event, not backbone cleavage or unusual side-chain loss

• Stability-indicating methods that can distinguish synthesis impurities (present in the initial batch) from degradation products that appear over time — critical when a radical modification step introduces new chromophores or functional groups that alter the oxidation or hydrolysis stability profile of the peptide

放大工程

Photoredox methods present the most technically specific scale-up challenges because photon delivery does not scale linearly. The incident light intensity at the center of a 1-liter flask is a fraction of that at the vessel surface, and lamp-to-reactor geometry determines conversion in a way that has no analogue in thermally driven chemistry. Flow chemistry reactors designed for photocatalysis address this through thin-film or flow-through irradiation, and several validated reactor designs now exist for pharmaceutical process development — but they require upfront capital and method re-optimization at each scale step.

HAT and electrochemical methods have different scale-up profiles. Electrode surface area scales more predictably than photon delivery, but electrochemical methods introduce questions about electrode materials compatibility, electrolyte removal, and current density uniformity at preparative scale.

For all transformation classes, the transition from analytical-scale HPLC purification to preparative HPLC introduces additional resolution challenges. A late-stage modification product co-eluting with mono-modified regioisomers at analytical scale will require careful re-optimization of gradient slope, mobile phase composition, and column chemistry at preparative loading — and preparative HPLC recovery from a complex mixture is the rate-limiting step in peptide scale-up more often than the synthesis itself.

评估采用可行性的框架

Rather than adopting any of these methods wholesale, development teams should run a structured feasibility assessment tied to the specific modification target. The following decision matrix reflects the compatibility and risk dimensions discussed above.

Passing the full checklist does not guarantee success — but it identifies which experiments to prioritize before committing development resources. A failed compatibility screen at the 5-milligram scale saves far more time than the same failure at 100 milligrams during a late CRO run.

现有的缀合平台可以提供帮助

Programs evaluating these newer methods alongside existing, validated modification approaches will naturally reference what their synthesis partner can execute without a custom process development phase. Peptide modification platforms that routinely handle click chemistry handles (叠氮化物, 在国际象棋中, 炔烃), glycopeptide installation, 和 fluorescent or isotope label conjugation already operate within the functional group tolerance envelope that radical late-stage methods are working to expand.

The value of the newer radical methods is not to replace these established conjugation routes — it is to access modification positions that classical chemistries cannot reach, particularly internal backbone positions and residues that lack convenient nucleophilic handles. When that capability is the bottleneck, a targeted radical functionalization method is worth the analytical and process development investment.

For everything else, 已确立的 定制肽合成 workflows with defined impurity acceptance criteria and validated analytical packages remain the faster path to a QC-released, development-ready batch.

该领域尚未解答的问题

To close with an honest inventory: the 2024–2025 literature has established that mild radical chemistry works on peptides under discovery conditions. It has not yet established, for most methods:

1. 可扩展性: how conversion and selectivity behave at 100 milligrams versus 1 gram versus 100 grams under process-realistic conditions, with proper oxygen exclusion and temperature control

2. Stereoselectivity at Dha: radical addition to dehydroalanine generates a new α-center with variable diastereomeric ratio; this is an open problem, and the recent ChemBioChem review (advances in dehydroalanine-specific modification, 2025) explicitly flags it as unresolved for most substrate classes

3. Regulatory precedent: none of these routes has, to public knowledge, been the subject of a CMC IND section covering impurity qualification, genotoxic impurity assessment of photocatalyst residues, or stability-indicating method validation for radical-modified API — the groundwork remains to be laid

4. Long-term stability of the modification bond: radical additions to Dha, Trp, and His create C–C, C–S, or C–N bonds with varying stability profiles; stability study data is sparse for most conjugates

These are not reasons to dismiss the chemistry. They are the experimental and regulatory program that must be designed alongside any decision to incorporate these methods into a clinical development workflow.

开发团队的实际后续步骤

For teams actively assessing whether mild radical late-stage functionalization belongs in their modification strategy, the most productive near-term actions are:

• Commission a small-scale feasibility run (5–10 毫克) with the exact sequence, targeting the specific residue type, under the proposed radical conditions — with LC-HRMS characterization of the crude mixture, not just HPLC purity

• Request stability data on the radical adduct bond under pH extremes, 氧化应激, and thermal stress before designing a purification method

• Map the impurity profile of the crude against what your preparative HPLC system can resolve, using spiked reference standards if available

• Identify whether regulatory precedent for the catalyst or reagent class exists in published IND CMC summaries, FDA guidance documents, or EMA assessment reports

The chemistry is moving fast. The analytical and process development infrastructure to use it responsibly in a development program takes longer to build. Starting that construction now, on a well-defined target sequence, is the right approach.

商船三井的变化 supports development teams navigating complex modification requirements across more than 300 functional group transformation types, including click chemistry conjugation, glycopeptide synthesis, and isotope-labeled peptide production within a Class 100 sterile manufacturing environment. If your program involves modification positions or conjugation targets that fall outside standard SPPS workflows, a technical feasibility discussion is a practical starting point.