Amino Acids: Structure, Properties and Peptide Chemistry | 氨基酸：结构、性质与肽化学

📚 Amino Acids: Structure, Properties and Peptide Chemistry | 氨基酸：结构、性质与肽化学

Amino acids are the monomer units from which proteins are built. In A-Level Chemistry, you need to understand their general structure, acid-base behaviour, the formation and hydrolysis of peptide bonds, and how side chains determine the properties of proteins.

氨基酸是构成蛋白质的单体单元。在 A-Level 化学中，你需要掌握它们的通式结构、酸碱行为、肽键的形成与水解，以及侧链如何决定蛋白质的性质。

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1. General Structure of α-Amino Acids | α-氨基酸的通式结构

All standard amino acids found in proteins are α-amino acids. The α-carbon atom is bonded to four different groups: a basic amino group (–NH₂), an acidic carboxyl group (–COOH), a hydrogen atom, and a variable R group called the side chain.

蛋白质中所有标准氨基酸都是 α-氨基酸。α-碳原子连接四个不同基团：一个碱性氨基（–NH₂）、一个酸性羧基（–COOH）、一个氢原子和一个可变的 R 基（称为侧链）。

H₂N–CHR–COOH

At physiological pH, the amino group is protonated and the carboxyl group is deprotonated, giving the zwitterionic form H₃N⁺–CHR–COO⁻.

在生理 pH 下，氨基被质子化，羧基去质子化，形成两性离子形式 H₃N⁺–CHR–COO⁻。

The simplest α-amino acid is glycine, where R = H. Systematic naming treats amino acids as substituted carboxylic acids; for example, alanine (R = CH₃) is 2-aminopropanoic acid.

最简单的 α-氨基酸是甘氨酸，其 R = H。系统命名法将氨基酸视为取代羧酸；例如丙氨酸（R = CH₃）称为 2-氨基丙酸。

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2. Classification of R Groups | R 基的分类

The side chain R determines the unique properties of each amino acid. R groups can be classified by their polarity and acid-base character.

侧链 R 决定了每种氨基酸的独特性质。R 基可按极性和酸碱性质分类。

Class	Nature of R group	Examples
Non-polar	Hydrocarbon chains or rings; hydrophobic	Gly, Ala, Val, Leu, Phe
Polar uncharged	Contain –OH, –SH, –CONH₂ groups	Ser, Thr, Cys, Asn, Gln
Acidic	Extra –COOH in side chain	Asp, Glu
Basic	Extra –NH₂ or –NH– group in side chain	Lys, Arg, His

Non-polar side chains tend to cluster in the interior of proteins to avoid water, while polar and charged side chains are usually found on the surface, where they interact with water and form hydrogen bonds or ionic interactions.

非极性侧链倾向于聚集在蛋白质内部以避开水，而极性和带电侧链通常位于表面，与水相互作用并形成氢键或离子相互作用。

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3. Zwitterions and Amphoteric Behaviour | 两性离子与两性性质

In aqueous solution, the carboxylic acid group can donate a proton and the amino group can accept a proton. This internal proton transfer produces a zwitterion, which carries both a positive and a negative charge but has no overall charge.

在水溶液中，羧酸基团可以给出质子，氨基可以接受质子。这种内部质子转移产生两性离子，它同时带有一个正电荷和一个负电荷，但净电荷为零。

H₂N–CHR–COOH ⇌ H₃N⁺–CHR–COO⁻

Because amino acids contain both acidic and basic groups, they are amphoteric. They react with acids by accepting H⁺ at the –COO⁻ group, and react with bases by losing H⁺ from the –NH₃⁺ group.

由于氨基酸同时含有酸性和碱性基团，它们具有两性。它们与酸反应时在 –COO⁻ 处接受 H⁺，与碱反应时从 –NH₃⁺ 失去 H⁺。

This amphoteric behaviour means amino acids can act as buffers in biological systems, resisting small changes in pH around their pKₐ values.

这种两性行为意味着氨基酸可以在生物体系中充当缓冲剂，在其 pKₐ 值附近抵抗 pH 的微小变化。

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4. Isoelectric Point (pI) | 等电点（pI）

The isoelectric point is the pH at which the amino acid exists predominantly as a zwitterion and has no net electrical charge. At pH values below the pI, the species is positively charged; at pH values above the pI, it is negatively charged.

等电点是氨基酸主要以两性离子形式存在且净电荷为零时的 pH。在低于 pI 的 pH 下，物种带正电荷；在高于 pI 的 pH 下，带负电荷。

For neutral amino acids with two acid dissociation constants, the pI is the average of the two pKₐ values: pI = ½ (pKₐ₁ + pKₐ₂).

对于具有两个酸解离常数的中性氨基酸，pI 是两个 pKₐ 值的平均值：pI = ½ (pKₐ₁ + pKₐ₂)。

For example, glycine has pKₐ₁ = 2.34 for –COOH and pKₐ₂ = 9.60 for –NH₃⁺, so its pI = ½ (2.34 + 9.60) = 5.97.

例如，甘氨酸的 –COOH 的 pKₐ₁ = 2.34，–NH₃⁺ 的 pKₐ₂ = 9.60，因此其 pI = ½ (2.34 + 9.60) = 5.97。

For acidic amino acids, the side-chain carboxyl group contributes a lower pKₐ, so the pI is calculated using the two carboxyl pKₐ values. For basic amino acids, the pI is calculated using the two amino group pKₐ values and is therefore higher.

对于酸性氨基酸，侧链羧基贡献较低的 pKₐ，因此 pI 使用两个羧基的 pKₐ 值计算。对于碱性氨基酸，pI 使用两个氨基的 pKₐ 值计算，因此偏高。

At the pI, amino acids have minimum solubility and do not migrate in an electric field, which is exploited in electrophoresis.

在等电点时，氨基酸溶解度最低，并且在电场中不发生迁移，这一性质被用于电泳。

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5. Acid-Base Titration Curves | 酸碱滴定曲线

A neutral amino acid behaves as a diprotic acid when titrated with a strong base. The titration curve shows two equivalence points and two buffer regions corresponding to the loss of H⁺ from –COOH and then from –NH₃⁺.

中性氨基酸在用强碱滴定时表现为二元酸。滴定曲线呈现两个等当点和两个缓冲区域，分别对应 –COOH 失去 H⁺ 和随后 –NH₃⁺ 失去 H⁺。

The pKₐ values can be read from the half-equivalence points. The pH at which the zwitterion is the dominant species lies midway between the two pKₐ values and is equal to the pI.

pKₐ 值可从半等当点读取。两性离子为主要物种时的 pH 位于两个 pKₐ 值的中间，等于 pI。

Acidic amino acids such as glutamic acid show three pKₐ values and three buffer regions, whereas basic amino acids such as lysine also show three distinct protonation states.

谷氨酸等酸性氨基酸显示三个 pKₐ 值和三个缓冲区域，而赖氨酸等碱性氨基酸也显示三种不同的质子化状态。

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