📚 Amino Acids | 氨基酸
Amino acids are the fundamental building blocks of proteins and play a central role in biological chemistry. Each amino acid contains both an amine (-NH₂) and a carboxylic acid (-COOH) functional group, making them amphoteric species that can act as both acids and bases. At A-Level, understanding their structure, acid-base behaviour, optical activity and the formation of peptide bonds is essential for grasping protein chemistry and metabolism.
氨基酸是蛋白质的基本构建单元，在生物化学中扮演核心角色。每个氨基酸分子同时含有胺基（-NH₂）和羧酸基（-COOH）两种官能团，这使得它们成为两性物种，既能作酸也能作碱。在 A-Level 阶段，理解氨基酸的结构、酸碱行为、光学活性和肽键的形成，是掌握蛋白质化学和代谢过程的关键。
1. Introduction to Amino Acids | 氨基酸简介
Over 500 amino acids are known in nature, but only about 20 are commonly found in proteins. These are called proteinogenic amino acids. All have a common core structure, differing only in the side chain 'R'. In the body, amino acids are used to synthesise proteins, enzymes, hormones and neurotransmitters. Their sequence in a protein determines its shape and function.
自然界已知的氨基酸超过 500 种，但蛋白质中常见的仅约 20 种，称为蛋白质氨基酸。它们都具有共同的核心结构，区别只在于侧链 "R"。在体内，氨基酸用于合成蛋白质、酶、激素和神经递质。氨基酸在蛋白质中的序列决定了蛋白质的形状和功能。
2. General Structure of α-Amino Acids | α-氨基酸的通式结构
attached to the same carbon atom, the α-carbon. The general formula can be written as H₂N–CHR– on is therefore chiral (except in glycine, where R = H), giving rise to optical isomerism.
所有蛋白质氨基酸（脯氨酸除外）都是 α-氨基酸：胺基和羧基连接在同一个碳原子（α-碳）上。通式可写作 H₂N–CHR–COOH，R 为可变的侧链。因此 α-碳是手性中心（甘氨酸除外，其 R = H），从而产生光学异构现象。
H₂N–CHR–COOH
H₂N–CHR–COOH
The side chain R can be non-polar (e.g. -CH₃ in alanine), polar (e.g. -CH₂OH in serine), acidic (e.g. -CH₂COOH in aspartic acid) or basic (e.g. -(CH₂)₄NH₂ in lysine). The nature of the R group determines the physical and chemical properties of the amino acid.
侧链 R 可以是非极性的（如丙氨酸中的 -CH₃）、极性的（如丝氨酸中的 -CH₂OH）、酸性的（如天冬氨酸中的 -CH₂COOH）或碱性的（如赖氨酸中的 -(CH₂)₄NH₂）。R 基团的性质决定了氨基酸的物理和化学性质。
3. The Zwitterion and Isoelectric Point | 两性离子与等电点
In aqueous solution, amino acids do not exist mainly as H₂N–CHR–COOH. Instead, an internal acid-base reaction occurs: the carboxyl group donates a proton to the amine group, forming a dipolar ion called a zwitterion, H₃N⁺–CHR–COO⁻. This doubly charged species has no net charge.
在水溶液中，氨基酸并不主要以 H₂N–CHR–COOH 形式存在。相反，会发生内部酸碱反应：羧基向胺基提供一个质子，生成双极性离子——两性离子 H₃N⁺–CHR–COO⁻。这个带双电荷的物种整体净电荷为零。
H₃N⁺–CHR–COO⁻ (zwitterion)
H₃N⁺–CHR–COO⁻ （两性离子）
The pH at which the amino acid carries no net electrical charge is the isoelectric point (pI). At the pI, the amino acid has minimum solubility in water and does not migrate in an electric field. Each amino acid has a characteristic pI value that depends on the nature of the R group.
氨基酸不带净电荷时的 pH 值称为等电点（pI）。在等电点时，氨基酸在水中的溶解度最小，且在电场中不发生迁移。每种氨基酸都有特征的 pI 值，取决于 R 基团的性质。
| Amino Acid | Side Chain Charge at pH 7 | Approximate pI |
| Glycine (R = H) | Neutral | 5.97 |
| Alanine (R = CH₃) | Neutral | 6.02 |
| Aspartic acid (R = CH₂COOH) | Negative | 2.85 |
| Lysine (R = (CH₂)₄NH₂) | Positive | 9.74 |
| 氨基酸 | pH 7 时侧链电荷 | 大致 pI |
| 甘氨酸 (R = H) | 中性 | 5.97 |
| 丙氨酸 (R = CH₃) | 中性 | 6.02 |
| 天冬氨酸 (R = CH₂COOH) | 负电 | 2.85 |
| 赖氨酸 (R = (CH₂)₄NH₂) | 正电 | 9.74 |
4. Acid-Base Behaviour | 酸碱行为
Amino acids are amphoteric: they can react with both acids and bases. In strongly acidic solution (low pH), the carboxylate group acts as a base and accepts a proton, giving the cation H₃N⁺–CHR–COOH. In strongly basic solution (high pH), the ammonium group loses a proton, producing the anion H₂N–CHR–COO⁻.
氨基酸是两性的：既能与酸反应，也能与碱反应。在强酸性溶液中（低 pH），羧酸根作为碱接受质子，生成阳离子 H₃N⁺–CHR–COOH。在强碱性溶液中（高 pH），铵基失去质子，生成阴离子 H₂N–CHR–COO⁻。
This ability to gain or lose protons allows amino acids to act as buffers in biological systems. For example, amino acid residues in proteins help maintain the pH of blood and cells against small additions of acid or alkali.
这种得失质子的能力使氨基酸能在生物体系中充当缓冲剂。例如，蛋白质中的氨基酸残基帮助维持血液和细胞内的 pH，抵抗少量酸或碱的加入。
5. Titration Curves and pI Calculation | 滴定曲线与等电点计算
no acid like glycine, the pI can be calculated as the average of the two pKₐ values: pI = (pKₐ₁ + pKₐ₂)/2. For amino acids with acidic or basic side chains, the pI involves the ionisable R group.
氨基酸的滴定曲线显示出两个（对于有可电离侧链的，有三个）缓冲区域和等电点。对于像甘氨酸这样的简单氨基酸，pI 可以通过两个 pKₐ 值的平均值来计算：pI = (pKₐ₁ + pKₐ₂)/2。对于侧链带酸性或碱性的氨基酸，pI 的计算涉及可电离的 R 基团。
pI = (pKₐ₁ + pKₐ₂) / 2
pI = (pKₐ₁ + pKₐ₂) / 2
H₂), giving pI ≈ 5.97. At this pH, the zwitterion predominates and the net charge is zero.
对于甘氨酸，pKₐ₁ ≈ 2.34（COOH → COO⁻），pKₐ₂ ≈ 9.60（NH₃⁺ → NH₂），因此 pI ≈ 5.97。在此 pH 下，两性离子占优势，净电荷为零。
6. Formation of Peptide Bonds | 肽键的形成
Two amino acids can join by a condensation reaction between the carboxyl group of one and the amine group of another, releasing a water molecule. The resulting amide linkage (-CONH-) is called a peptide bond. The product is a dipeptide. The peptide bond is planar and has partial double-bond character due to resonance, restricting rotation.
两个氨基酸可通过一个氨基酸的羧基与另一个氨基酸的胺基之间的缩合反应连接，脱去一分子水。生成的酰胺键（-CONH-）称为肽键，产物是二肽。肽键具有平面结构，且由于共振作用具有部分双键性质，限制了旋转。
H₂N–CHR–COOH + H₂N–CHR'–COOH → H₂N–CHR–CO–NH–CHR'–COOH + H₂O
H₂N–CHR–COOH + H₂N–CHR'–COOH → H₂N–CHR–CO–NH–CHR'–COOH + H₂O
By convention, the amino acid with a free -NH₂ group is at the N-terminus, and the one with a free -COOH group is at the C-terminus. The chain can grow further to form oligopeptides, polypeptides and finally proteins.
按惯例，带有游离 -NH₂ 基团的氨基酸端称为 N 端，带有游离 -COOH 基团的一端为 C 端。链可继续延伸形成寡肽、多肽，最终成为蛋白质。
7. Hydrolysis of Peptides | 肽的水解
Peptide bonds can be broken by hydrolysis in the laboratory or in digestion. Acid hydrolysis (e.g. with 6 mol dm⁻³ HCl, heat) breaks peptides into constituent amino acids. Alkaline hydrolysis (NaOH) can also be used. Enzymatic hydrolysis, carried out by proteases such as pepsin and trypsin, is highly specific and occurs under mild conditions in the body.
肽键可以通过水解反应断裂，既可在实验室中进行，也可在消化过程中发生。酸水解（如用 6 mol dm⁻³ HCl 加热）将肽分解为构成氨基酸。碱水解（NaOH）亦可使用。酶促水解由蛋白酶（如胃蛋白酶和胰蛋白酶）催化，具有高度专一性，并在体内温和条件下进行。
Hydrolysis is the reverse of peptide bond formation and is essential for amino acid recycling, digestion and protein turnover.
水解是肽键形成的逆反应，对于氨基酸的回收、消化和蛋白质周转至关重要。
8. Protein Structure Overview | 蛋白质结构概述
The sequence of amino acids in a polypeptide chain is the primary structure. Secondary structures are local folded shapes such as α-helices and β-pleated sheets, stabilised by hydrogen bonds between backbone amide and carbonyl groups. The tertiary structure is the overall 3D shape of one polypeptide, held together by hydrogen bonds, ionic interactions, hydrophobic packing and disulfide bridges (covalent S-S between cysteine residues). Some proteins have quaternary structure, involving the assembly of multiple polypeptide subunits.
多肽链中氨基酸的序列构成一级结构。二级结构是局部折叠形状，如 α-螺旋和 β-折叠层，由主链酰胺和羰基间的氢键维持稳定。三级结构是一条多肽链的整体三维形状，通过氢键、离子相互作用、疏水堆积和二硫桥（半胱氨酸残基之间的共价 S-S 键）维系。某些蛋白质具有四级结构，涉及多个多肽亚基的组装。
Changes in conditions (pH, temperature) can disrupt these interactions, leading to denaturation and loss of function.
环境条件改变（pH、温度）会破坏这些相互作用，导致变性并丧失功能。
9. Chromatography of Amino Acids | 氨基酸的色谱法
Mixtures of amino acids can be separated and identified by paper chromatography or thin-layer chromatography (TLC). A small spot of the mixture is placed near the bottom of the stationary phase (paper or silica), and a suitable solvent (e.g. butan-1-ol / ethanoic acid / water) rises by capillary action. Different amino acids move at different rates depending on their solubility in the solvent and their affinity for the stationary phase.
氨基酸混合物可通过纸色谱或薄层色谱（TLC）进行分离和鉴定。将一小点混合物点样在固定相（纸或硅胶板）底端附近，合适的溶剂（如丁醇/乙酸/水）通过毛细管作用上升。不同的氨基酸因其在溶剂中的溶解度以及对固定相的亲和力不同，以不同速率移动。
The retardation factor (Rf) is calculated as: distance moved by the spot divided by distance moved by the solvent front. Amino acids are usually invisible, so after development the plate is sprayed with ninhydrin and heated to produce purple or brown spots.
比移值（Rf）计算为：斑点移动距离除以溶剂前沿移动的距离。氨基酸通常不可见，因此展开后需用茚三酮溶液喷雾并加热，产生紫色或棕色斑点。
Rf = (distance moved by amino acid) / (distance moved by solvent front)
Rf = (氨基酸移动距离) / (溶剂前沿移动距离)
10. Optical Isomerism | 光学异构
With the exception of glycine, all α-amino acids have a chiral centre at the α-carbon, which is bonded to four different groups: -H, -NH₂, -COOH and -R. Therefore they exist as a pair of non-superimposable mirror images called enantiomers. Enantiomers rotate plane-polarised light in opposite directions, but have identical physical properties otherwise.
除甘氨酸外，所有 α-氨基酸的 α-碳都是手性中心，连接着四个不同的基团：-H、-NH₂、-COOH 和 -R。因此，它们以一对不能重叠的镜像异构体存在，称为对映体。对映体使平面偏振光向相反方向旋转，但其它物理性质相同。
Living organisms use almost exclusively L-amino acids (L from laevo, but not necessarily laevorotatory!). The L configuration relates to the arrangement around the chiral carbon using the Fisher projection. This homochirality is a key feature of biochemistry.
生物体几乎只利用 L-氨基酸（L 源自 laevo，但并不一定代表左旋）。L 构型是根据 Fisher 投影式中手性碳周围的排列确定的。这种同一手性是生物化学的一个关键特征。
11. Biological Significance and Nutrition | 生物学意义与营养
tters (e.g. tryptophan → serotonin, tyrosine → dopamine), hormones, nucleotides and porphyrins. In humans, about half of the 20 proteinogenic amino acids cannot be synthesised and must be supplied in the diet; these are called essential amino acids. Examples include valine, leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan and histidine (for children).
氨基酸不仅是蛋白质的单体，也是许多生物分子的前体。它们参与合成神经递质（如色氨酸 → 血清素，酪氨酸 → 多巴胺）、激素、核苷酸和卟啉类化合物。在人体中，20 种蛋白质氨基酸中约有一半无法合成，必须通过膳食供给，称为必需氨基酸。例子包括缬氨酸、亮氨酸、异亮氨酸、赖氨酸、甲硫氨酸、苯丙氨酸、苏氨酸、色氨酸和组氨酸（儿童期）。
A deficiency in any essential amino acid impairs protein synthesis and can cause illness. Complete protein sources such as meat, fish, eggs and soy provide all essential amino acids in adequate amounts.
任何一种必需氨基酸的缺乏都会损害蛋白质合成，并可能引发疾病。完全蛋白质来源，如肉类、鱼类、鸡蛋和大豆，可提供充足的必需氨基酸。
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