Peptides: Structure, Formation and Hydrolysis | 多肽：结构、形成与水解

📚 Peptides: Structure, Formation and Hydrolysis | 多肽：结构、形成与水解

Peptides are short chains of amino acids linked by peptide bonds. In A-level Chemistry, you need to know how these bonds form through condensation, how to draw and name dipeptides, how hydrolysis breaks them apart, and how peptide structure relates to proteins.

多肽是由肽键连接的氨基酸短链。在 A-level 化学中，你需要掌握肽键如何通过缩合反应形成、如何绘制和命名二肽、水解如何将其断开，以及多肽结构如何与蛋白质相关。

1. Amino Acids as Building Blocks | 氨基酸是多肽的基本单元

Amino acids contain both an amine group and a carboxylic acid group. The general formula of an α-amino acid is H₂N–CHR–COOH, where R is a variable side chain.

氨基酸同时含有氨基和羧基官能团。α-氨基酸的通式为 H₂N–CHR–COOH，其中 R 是可变的侧链。

At the central α-carbon, four different groups are usually attached: –NH₂, –COOH, –H and –R. This makes most amino acids chiral, except glycine where R = H.

中心 α-碳上通常连接四个不同基团：–NH₂、–COOH、–H 和 –R。这使得大多数氨基酸具有手性，但甘氨酸（R = H）除外。

H₂N–CHR–COOH

2. Zwitterions and Acid-Base Behaviour | 两性离子与酸碱行为

In aqueous solution, amino acids exist mainly as zwitterions because the –NH₂ group accepts a proton and the –COOH group donates one.

在水溶液中，氨基酸主要以两性离子形式存在，因为 –NH₂ 基团接受一个质子，而 –COOH 基团释放一个质子。

H₃N⁺–CHR–COO⁻

The zwitterion can act as both an acid and a base, which is why amino acids are described as amphoteric.

两性离子既可以作为酸也可以作为碱，因此氨基酸具有两性。

At the isoelectric point (pI), the amino acid has no overall charge and does not move in an electric field.

在等电点（pI）时，氨基酸净电荷为零，在电场中不移动。

3. Peptide Bond Formation: Condensation | 肽键形成：缩合反应

When two amino acids react, the –COOH group of one and the –NH₂ group of the other form an amide link called a peptide bond, with the elimination of a water molecule.

当两个氨基酸反应时，一个氨基酸的 –COOH 基团与另一个的 –NH₂ 基团形成酰胺键，称为肽键，同时脱去一分子水。

H₂N–CHR–COOH + H₂N–CHR’–COOH → H₂N–CHR–CO–NH–CHR’–COOH + H₂O

The product with two amino acid residues is a dipeptide. The new bond is shown as –CONH– or –CO–NH–.

含有两个氨基酸残基的产物称为二肽。新键表示为 –CONH– 或 –CO–NH–。

Condensation can continue at either end, so a polypeptide chain grows from the N-terminus to the C-terminus.

缩合反应可以在两端继续进行，因此多肽链从 N 端向 C 端延伸。

The peptide bond has partial double-bond character because the lone pair on nitrogen delocalises into the adjacent C=O group, restricting rotation around the C–N bond.

肽键具有部分双键性质，因为氮上的孤对电子离域到相邻的 C=O 基团，限制了 C–N 键的旋转。

4. Drawing and Naming Dipeptides | 二肽的绘制与命名

A dipeptide is named by listing the N-terminal amino acid first, changing its ending from -ine to -yl, followed by the full name of the C-terminal amino acid.

二肽命名时先列出 N 端氨基酸，将其词尾从 -ine 改为 -yl，然后接 C 端氨基酸的全名。

For example, glycine (Gly) and alanine (Ala) can form two different dipeptides: glycylalanine (Gly-Ala) and alanylglycine (Ala-Gly). In glycylalanine, glycine provides the N-terminus and alanine provides the C-terminus.

例如，甘氨酸（Gly）和丙氨酸（Ala）可以形成两种不同的二肽：甘氨酰丙氨酸（Gly-Ala）和丙氨酰甘氨酸（Ala-Gly）。在甘氨酰丙氨酸中，甘氨酸提供 N 端，丙氨酸提供 C 端。

When drawing, keep the N-terminal amino group on the left and the C-terminal carboxyl group on the right.

绘制时，将 N 端氨基放在左侧，C 端羧基放在右侧。

The dipeptide formed from glycine and alanine has the structure H₂N–CH₂–CO–NH–CH(CH₃)–COOH.

由甘氨酸和丙氨酸形成的二肽结构为 H₂N–CH₂–CO–NH–CH(CH₃)–COOH。

5. Polypeptides and Proteins | 多肽与蛋白质

Polypeptides are chains of many amino acid residues joined by peptide bonds. Proteins contain one or more polypeptide chains folded into specific three-dimensional shapes.

多肽是由许多氨基酸残基通过肽键连接而成的长链。蛋白质含有一条或多条多肽链，折叠成特定的三维形状。

The sequence of amino acids is the primary structure. Hydrogen bonding between peptide groups gives secondary structures such as the α-helix and β-pleated sheet.

氨基酸序列是一级结构。肽基团之间的氢键形成二级结构，如 α-螺旋和 β-折叠片。

Further folding due to side-chain interactions gives tertiary structure, and multiple polypeptide chains can associate to form quaternary structure.

侧链相互作用进一步折叠形成三级结构，多条多肽链可以结合形成四级结构。

Primary structure	Linear sequence of amino acids
Secondary structure	α-helix and β-pleated sheet held by hydrogen bonds
Tertiary structure	Overall 3D folding due to side-chain interactions
Quaternary structure	Association of multiple polypeptide chains

6. Hydrolysis of Peptides | 多肽的水解

Peptide bonds can be broken by hydrolysis. Under acidic or alkaline conditions, water adds across the C–N bond and regenerates the original amino acids.

肽键可以通过水解断裂。在酸性或碱性条件下，水加成到 C–N 键上，重新生成原来的氨基酸。

H₂N–CHR–CO–NH–CHR’–COOH + H₂O → H₂N–CHR–COOH + H₂N–CHR’–COOH

Acid hydrolysis usually uses 6 mol dm⁻³ HCl and gives protonated amino acid salts; alkaline hydrolysis uses NaOH and gives deprotonated forms.

酸水解通常使用 6 mol dm⁻³ HCl，得到质子化的氨基酸盐；碱水解使用 NaOH，得到去质子化形式。

Enzymatic hydrolysis is used in digestion, where proteases break proteins into peptides and amino acids.

酶水解用于消化过程，蛋白酶将蛋白质分解为多肽和氨基酸。

7. Identification and Separation of Amino Acids and Peptides | 氨基酸与多肽的鉴别与分离

Thin-layer chromatography (TLC) can separate amino acids after hydrolysis. Ninhydrin is a locating agent that reacts with amino acids to give purple spots.

薄层色谱（TLC）可以分离水解后的氨基酸。茚三酮是一种显色剂，与氨基酸反应生成紫色斑点。

The Rf value is calculated and compared with standards to identify the amino acid residues present in a peptide.

计算 Rf 值并与标准品比较，可以鉴定多肽中存在的氨基酸残基。

Rf = distance moved by amino acid ÷ distance moved by solvent front

8. Stereochemistry and Optical Isomerism | 立体化学与光学异构

Except glycine, all α-amino acids are optically active because the α-carbon is a chiral centre with four different attached groups.

除甘氨酸外，所有 α-氨基酸都有光学活性，因为 α-碳是带有四个不同基团的手性中心。

Naturally occurring amino acids in proteins are mainly L-isomers. In a Fischer projection, the L form has the –NH₂ group on the left.

蛋白质中的天然氨基酸主要为 L-异构体。在 Fischer 投影式中，L 型的 –NH₂ 基团位于左侧。

Peptide formation does not remove chiral centres, so dipeptides and longer peptides are also optically active.

肽键形成不会消除手性中心，因此二肽和更长的多肽也具有光学活性。

9. Acid-Base Titration of Amino Acids | 氨基酸的酸碱滴定

Amino acids show characteristic titration curves because both the –COOH and –NH₃⁺ groups can release protons.

氨基酸具有特征的滴定曲线，因为 –COOH 和 –NH₃⁺ 基团都能释放质子。

The isoelectric point lies between the two pKa values, and at this pH the amino acid is least soluble.

等电点位于两个 pKa 值之间，在该 pH 下氨基酸溶解度最低。

This acid-base behaviour matters when peptides are separated by electrophoresis or when their biological activity depends on pH.

当多肽通过电泳分离，或当它们的生物活性依赖于 pH 时，这种酸碱行为非常重要。

10. Common Examples and Biological Importance | 常见示例与生物学意义

Important peptides include glutathione, an antioxidant; insulin, a hormone made of two polypeptide chains linked by disulfide bonds; and enkephalins, natural painkillers.

重要的多肽包括谷胱甘肽（一种抗氧化剂）、胰岛素（由两条多肽链通过二硫键连接而成的激素）和脑啡肽（天然镇痛剂）。

Peptides are used in medicine and research, but their amide bonds are susceptible to hydrolysis, so oral delivery can be challenging.

多肽在医学和研究中广泛应用，但其酰胺键易水解，因此口服递送可能具有挑战性。

11. Exam Tips and Common Misconceptions | 考试提示与常见误区

Do not confuse peptide bond formation with esterification: an amide bond forms, not an ester. Water is eliminated from the –OH of the carboxyl group and the –H of the amine group.

不要将肽键形成与酯化混淆：形成的是酰胺键而不是酯键。水由羧基的 –OH 和氨基的 –H 脱去。

When naming a dipeptide, the N-terminal residue always ends in -yl; the C-terminal keeps its full amino acid name.

命名二肽时，N 端残基总是以 -yl 结尾；C 端保留

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