Formation of Peptide Bonds and Structure of Peptides | 肽键的形成与肽的结构

📚 Formation of Peptide Bonds and Structure of Peptides | 肽键的形成与肽的结构

Peptides and proteins are fundamental biomolecules formed from amino acids linked by peptide bonds. Understanding the formation of peptide bonds and the structural hierarchy of peptides is essential for CIE A-Level Chemistry, as it connects organic chemistry, biochemistry, and the thermodynamics of condensation reactions. This article covers everything you need for exam success: from the condensation mechanism to the four levels of protein structure, and from hydrolysis to the Biuret test.

肽和蛋白质是由氨基酸通过肽键连接而成的基本生物分子。理解肽键的形成和肽的结构层次对CIE A-Level化学至关重要，它将有机化学、生物化学以及缩合反应的热力学联系起来。本文涵盖你在考试中需要的一切：从缩合机理到蛋白质的四个结构层次，从水解到双缩脲试验。

1. Amino Acids: The Building Blocks | 氨基酸：肽的构建单元

Amino acids are organic compounds that contain two functional groups: a basic amino group (-NH₂) and an acidic carboxyl group (-COOH). Both groups are attached to the same carbon atom, known as the α-carbon. The general formula of an α-amino acid is:

氨基酸是含有两个官能团的有机化合物：一个碱性的氨基（-NH₂）和一个酸性的羧基（-COOH）。这两个基团连接在同一个碳原子——称为α-碳原子上。α-氨基酸的通式为：

H₂N-CHR-COOH

where R represents the variable side chain. In naturally occurring proteins there are 20 different standard R groups, each giving the amino acid distinct chemical properties. For example, glycine has R = H, alanine has R = CH₃, and cysteine has R = CH₂SH.

其中R代表可变的侧链。在天然蛋白质中，存在20种不同的标准R基团，每一种都赋予氨基酸独特的化学性质。例如，甘氨酸的R = H，丙氨酸的R = CH₃，半胱氨酸的R = CH₂SH。

In aqueous solution near neutral pH, amino acids exist as zwitterions, with both a positive charge on the amino group (-NH₃⁺) and a negative charge on the carboxylate group (-COO⁻). This dipolar nature explains their relatively high melting points and solubility in water.

在中性pH附近的水溶液中，氨基酸以两性离子形式存在，氨基带正电荷（-NH₃⁺），羧酸根带负电荷（-COO⁻）。这种偶极性质解释了其相对较高的熔点和在水中的溶解性。

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2. Formation of the Peptide Bond | 肽键的形成

The peptide bond is formed via a condensation reaction between the carboxyl group (-COOH) of one amino acid and the amino group (-NH₂) of another amino acid. During this reaction, a molecule of water is eliminated. The reaction can be represented as follows:

肽键是通过一个氨基酸的羧基（-COOH）与另一个氨基酸的氨基（-NH₂）之间的缩合反应形成的。在该反应过程中消除一分子水。该反应可表示如下：

H₂N-CHR₁-COOH + H₂N-CHR₂-COOH → H₂N-CHR₁-CO-NH-CHR₂-COOH + H₂O

The newly formed -CO-NH- linkage is called a peptide bond (also known as an amide bond). When extended to many amino acids, this process is an example of condensation polymerisation. In living cells, this reaction is catalysed by ribosomes during protein synthesis, but in the laboratory it typically requires activating agents to drive the thermodynamically unfavourable condensation forward.

新形成的-CO-NH-连接键被称为肽键（也称为酰胺键）。当该反应扩展到大量氨基酸时，即为缩合聚合反应的实例。在活细胞中，该反应由核糖体在蛋白质合成过程中催化；而在实验室中，通常需要活化试剂推动热力学上不利的缩合反应正向进行。

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3. Dipeptides and Tripeptides | 二肽和三肽

When two amino acids join together, the product is a dipeptide, which contains exactly one peptide bond. When three amino acids join in sequence, a tripeptide is formed, containing two peptide bonds. In general, a chain of n amino acid residues contains (n – 1) peptide bonds.

当两个氨基酸连接在一起时，产物是二肽，恰好含有一个肽键。当三个氨基酸依次连接时，形成三肽，含有两个肽键。一般来说，由n个氨基酸残基组成的链含有（n – 1）个肽键。

The two ends of a peptide chain are structurally different:

肽链的两端在结构上是不同的：

• The end with a free amino group is called the N-terminus (amino terminus).
• The end with a free carboxyl group is called the C-terminus (carboxyl terminus).

• 带有游离氨基的一端称为N端（氨基末端）。
• 带有游离羧基的一端称为C端（羧基末端）。

The sequence of amino acids in a dipeptide matters. Glycine-alanine (Gly-Ala) is structurally different from alanine-glycine (Ala-Gly), because the N-terminal and C-terminal amino acids are swapped. This directionality is a crucial concept for understanding peptide structure and function.

二肽中氨基酸的序列非常重要。甘氨酰丙氨酸（Gly-Ala）与丙氨酰甘氨酸（Ala-Gly）结构不同，因为N端和C端的氨基酸互换了位置。这种方向性是理解肽结构和功能的关键概念。

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4. Polypeptides and Proteins | 多肽与蛋白质

A polypeptide is a long chain of amino acids linked by peptide bonds. When a polypeptide chain contains more than approximately 50 amino acid residues, it is generally classified as a protein. Proteins are therefore natural condensation polymers, with amino acids serving as the monomer units.

多肽是由肽键连接的氨基酸长链。当一条多肽链含有约50个以上氨基酸残基时，通常被归类为蛋白质。因此，蛋白质是天然的缩合聚合物，氨基酸作为单体单元。

Example	Number of residues	Number of chains	Function
Insulin	51	2	Blood sugar regulation
Haemoglobin	~574	4	Oxygen transport
Lysozyme	129	1	Bacterial cell wall digestion

例如，胰岛素含有51个氨基酸残基（两条链），血红蛋白约有574个残基（四条链）。注意，单个氨基酸残基是指已参与肽键形成后失去一分子水的氨基酸部分。

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5. Primary Structure | 一级结构

The primary structure of a peptide or protein is the linear sequence of amino acids in the polypeptide chain, held together by covalent peptide bonds. It is the most fundamental level of structural organisation, and it is determined directly by the genetic code: each three-nucleotide codon on DNA specifies one amino acid.

肽或蛋白质的一级结构是多肽链中氨基酸的线性序列，由共价肽键连接。这是最基本的结构组织层次，直接由遗传密码决定：DNA上每三个核苷酸组成的密码子指定一个氨基酸。

For example, the primary structure of a tripeptide with sequence Gly-Ser-Ala means that glycine is at the N-terminus, linked to serine, which is in turn linked to alanine at the C-terminus. Even a single amino acid substitution can affect biological function dramatically. A well-known case is sickle-cell anaemia, where a single change of glutamic acid to valine at position 6 of the β-chain of haemoglobin causes the red blood cells to adopt a sickle shape under low-oxygen conditions.

例如，序列为Gly-Ser-Ala的三肽的一级结构意味着甘氨酸位于N端，连接到丝氨酸，丝氨酸再连接到C端的丙氨酸。即使仅有一个氨基酸发生替换，也能极大影响生物学功能。镰状细胞贫血是著名的例子：血红蛋白β链第6位的谷氨酸被缬氨酸替换，导致红细胞在低氧条件下呈镰刀状。

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6. Secondary Structure | 二级结构

The secondary structure refers to the regular, recurring local conformations of the polypeptide backbone, stabilised primarily by hydrogen bonds between backbone -C=O and -N-H groups. The two principal types are the α-helix and the

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