📚 Peptides | 多肽
Peptides are short chains of amino acids linked by peptide bonds. They are fundamental biological molecules that make up proteins and carry out essential signalling and structural roles. In A-Level Cambridge Chemistry, understanding peptides involves grasping the structure of amino acids, how peptide bonds form, the behaviour of zwitterions, and the higher-level organisation of proteins. This article provides a systematic breakdown of peptides for your revision.
多肽是由氨基酸通过肽键连接而成的短链分子。它们是构成蛋白质的基本生物分子，并承担关键的信号传导与结构功能。在剑桥 A-Level 化学课程中，理解多肽需要掌握氨基酸的结构、肽键如何形成、两性离子的行为以及蛋白质的高级组织形式。本文将为你的复习提供多肽知识的系统梳理。
1. Amino Acids as Peptide Monomers | 氨基酸作为多肽的单体
All peptides are built from α-amino acids, which share a general structure: a central carbon (the α-carbon) bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable side chain (R group). The R group determines the amino acid's identity and properties. In A-Level chemistry, you must recognise the general formula as H₂N–CHR–COOH, where R can be H (glycine), CH₃ (alanine), or more complex groups like CH₂SH (cysteine).
所有多肽都由 α-氨基酸构成。α-氨基酸的通式为：中心碳（α-碳）分别连接氨基（–NH₂）、羧基（–COOH）、氢原子和可变的侧链（R 基）。R 基决定了氨基酸的身份和性质。在 A-Level 化学中，你必须能识别通式 H₂N–CHR–COOH，其中 R 可以是 H（甘氨酸）、CH₃（丙氨酸）或更复杂的基团如 CH₂SH（半胱氨酸）。
2. Zwitterions and Isoelectric Point | 两性离子与等电点
In aqueous solution, amino acids exist predominantly as zwitterions, where the carboxyl group loses a proton and the amino group gains a proton, giving a dipolar ion with no net charge: ⁺H₃N–CHR–COO⁻. The pH at which the amino acid exists as a neutral zwitterion is called the isoelectric point (pI). At this pH, the amino acid does not migrate in an electric field. Every amino acid has a characteristic pI, depending on its R group.
在水溶液中，氨基酸主要以两性离子形式存在：羧基失去一个质子，氨基结合一个质子，形成净电荷为零的偶极离子：⁺H₃N–CHR–COO⁻。两性离子呈电中性的 pH 值称为等电点（pI）。在该 pH 下，氨基酸在电场中不发生迁移。每种氨基酸根据其 R 基都有特定的 pI 值。
3. Formation of the Peptide Bond | 肽键的形成
of another, with the elimination of a water molecule. The resulting linkage is an amide group –CONH– , the dipeptide glycylalanine can form, releasing H₂O. The reaction can be represented as:
H₂N–CH₂–COOH + H₂N–CH(CH₃)–COOH → H₂N–CH₂–CO–NH–CH(CH₃)–COOH + H₂O
肽键是通过一个氨基酸的羧基与另一个氨基酸的氨基之间的缩合反应形成的，同时脱去一分子水。形成的连接是酰胺基团 –CONH–，称为肽键。例如，甘氨酸（Gly, R=H）与丙氨酸（Ala, R=CH₃）反应可生成二肽甘氨酰丙氨酸，同时释放 H₂O。该反应可表示为上述方程式。
4. Peptide Bond Planarity and Geometry | 肽键的平面性和几何结构
The peptide bond has partial double‑bond character due to resonance between the carbonyl group and the nitrogen's lone pair. This restricts rotation around the C–N bond, making the six atoms (Cα–CO–NH–Cα) lie in a rigid planar unit. The trans configuration is more stable and predominates in most peptide bonds, except before proline. Understanding planarity is key to grasping protein folding.
肽键因羰基与氮孤对电子之间的共振而具有部分双键性质。这限制了 C–N 键的旋转，使得六个原子（Cα–CO–NH–Cα）处于一个刚性平面内。反式构象更稳定，在大多数肽键中占主导地位，脯氨酸之前的肽键除外。理解平面性是掌握蛋白质折叠的关键。
5. Amino Acid Residues and Peptide Notation | 氨基酸残基与多肽表示法
Once incorporated into a peptide, amino acids are called residues. The free –NH₂ end is the N‑terminus, and the free –COOH end is the C‑terminus. By convention, peptides are written and named from the N‑terminus to the C‑terminus. A tripeptide composed of glycine, alanine, and serine (Ser) would be written as Gly–Ala–Ser, or H₂N–Gly–Ala–Ser–COOH. The three‑letter abbreviations are standard in A‑Level exams.
氨基酸一旦融入多肽链，就称为残基。游离的 –NH₂ 端是 N 端，游离的 –COOH 端是 C 端。按照惯例，多肽的书写和命名从 N 端开始到 C 端结束。由甘氨酸、丙氨酸和丝氨酸（Ser）构成的三肽可写作 Gly–Ala–Ser，或 H₂N–Gly–Ala–Ser–COOH。三字母缩写是 A-Level 考试中的标准用法。
6. Number of Possible Dipeptides | 可能形成的二肽数量
When two different amino acids (A and B) react, four dipeptides can theoretically form: A–A, A–B, B–A, and B–B. This arises because both amino acids can supply the N‑terminus or C‑terminus. However, if the amino acids have identical R groups, only one dipeptide is possible. Exam questions frequently ask you to predict the number of dipeptides from a given mixture.
当两种不同氨基酸（A 和 B）反应时，理论上可形成四种二肽：A–A、A–B、B–A 和 B–B。这是因为两种氨基酸都可以提供 N 端或 C 端。但如果氨基酸的 R 基相同，则只可能形成一种二肽。考题经常要求你预测给定混合物能产生多少种二肽。
7. Hydrolysis of Peptides and Proteins | 多肽与蛋白质的水解
Peptide bonds can be cleaved by hydrolysis, the reverse of condensation. This requires heating with aqueous acid (e.g., 6 mol dm⁻³ HCl) or aqueous alkali (e.g., NaOH), or using enzymes (proteases). Complete hydrolysis breaks all peptide bonds to yield a mixture of free amino acids. Partial hydrolysis gives smaller peptides, which can be useful for sequence determination.
肽键可通过水解断裂，即缩合反应的逆过程。水解需要与酸（如 6 mol dm⁻³ HCl）或碱（如 NaOH）一起加热，或使用酶（蛋白酶）。完全水解会断开所有肽键，得到游离氨基酸混合物。部分水解则产生较短肽段，这对于测定氨基酸序列很有用。
8. Chromatographic Separation of Amino Acids | 氨基酸的色谱分离
After hydrolysis, amino acids can be separated and identified by thin‑layer chromatography (TLC) or paper chromatography. A spot of the hydrolysate is placed on the stationary phase and a suitable solvent (mobile phase) rises by capillary action. The separated spots are visualised with ninhydrin, which reacts to give purple spots. Rf values are calculated and compared with known standards. A typical Rf value table is:
水解后，氨基酸可通过薄层色谱（TLC）或纸色谱进行分离和鉴定。将水解产物点在固定相上，合适的溶剂（流动相）通过毛细作用上升。分离后的斑点用茚三酮显色，呈紫色。计算 Rf 值并与已知标准品比较。典型的 Rf 值表如下：
| Amino Acid | Rf value |
|---|---|
| Glycine | 0.26 |
| Alanine | 0.38 |
| Leucine | 0.73 |
| Lysine | 0.14 |
These Rf values depend on the solvent system; the principle that polar amino acids have lower Rf in non‑polar solvents is important for analysis.
Rf 值取决于溶剂体系；掌握极性氨基酸在非极性溶剂中 Rf 值较低这一原理对分析很重要。
9. Primary Structure of Proteins | 蛋白质的一级结构
The primary structure of a protein is the unique sequence of amino acid residues linked by peptide bonds. This sequence is determined by the genetic code and dictates all higher levels of folding. Even a single amino acid substitution can dramatically alter protein function, as seen in sickle cell anaemia where glutamic acid is replaced by valine in haemoglobin.
蛋白质的一级结构是由肽键连接的氨基酸残基的独特序列。该序列由遗传密码决定，并决定了所有更高级别的折叠。即使单个氨基酸的替换也可能极大地改变蛋白质功能，例如镰刀型细胞贫血症中，血红蛋白的谷氨酸被缬氨酸取代。
10. Secondary Structure: α-Helix and β-Pleated Sheet | 二级结构：α-螺旋和β-折叠
fers to local folding patterns maintained by hydrogen bonds between the backbone C=O and N–H groups. The α-helix is a right‑ f extended polypeptide strands lying side by side, with hydrogen bonds between adjacent strands, which can be parallel or antiparallel.
二级结构是指由主链 C=O 与 N–H 基团之间的氢键维持的局部折叠模式。α-螺旋是一种右手螺旋，其中第 i 个残基的羰基氧与第 i+4 个残基的酰胺氢之间形成氢键。β-折叠片由并排伸展的多肽链组成，相邻链间存在氢键，这些链可以是平行的或反平行的。
11. Tertiary Structure and Key Interactions | 三级结构与关键相互作用
Tertiary structure is the overall three‑dimensional folding of a single polypeptide chain, stabilised by interactions between side chains. These include:
- Disulfide bonds: covalent –S–S– bridges formed by oxidation of two cysteine thiol (–SH) groups.
- Ionic bonds (salt bridges): electrostatic attractions between charged side chains, e.g., –COO⁻ of aspartate and –NH₃⁺ of lysine.
- Hydrogen bonds: between polar R groups.
- Hydrophobic interactions: non‑polar side chains cluster away from water.
三级结构是单个多肽链的整体三维折叠，由侧链间的相互作用稳定。这些相互作用包括：
- 二硫键：由两个半胱氨酸的巯基（–SH）氧化形成的共价 –S–S– 桥。
- 离子键（盐桥）：带电侧链之间的静电吸引，例如天冬氨酸的 –COO⁻ 与赖氨酸的 –NH₃⁺。
- 氢键：极性 R 基之间的氢键。
- 疏水相互作用：非极性侧链远离水分子而聚集。
12. Quaternary Structure and Protein Function | 四级结构与蛋白质功能
Some proteins contain more than one polypeptide chain (subunit); the arrangement of these subunits is the quaternary structure. Haemoglobin, consisting of two α and two β chains, is a classic example. The same non‑covalent interactions that stabilise tertiary structure also hold subunits together, in addition to disulfide bridges in some proteins like immunoglobulins. Disruption of quaternary structure can lead to loss of function without complete unfolding of individual subunits.
一些蛋白质含有多于一条多肽链（亚基）；这些亚基的排列方式就是四级结构。血红蛋白由两条 α 链和两条 β 链组成，是典型的例子。稳定三级结构的非共价相互作用也同样维持亚基之间的结合，此外某些蛋白质如免疫球蛋白还依靠二硫键。四级结构的破坏可能导致功能丧失，而单个亚基未必完全去折叠。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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