📚 A-Level Chemistry: Structural Features of Natural Biochemical Polymers | A-Level 化学：天然生化聚合物的结构特性
Natural biochemical polymers such as carbohydrates, proteins and nucleic acids are fundamental to life. Their structures determine their highly specific biological functions, and A-Level Chemistry requires a clear understanding of how monomer units link together to form these macromolecules.
天然生化聚合物，如碳水化合物、蛋白质和核酸，是生命的基础。它们的结构决定了其高度特异的生物学功能，A-Level 化学要求我们清楚地理解单体单元如何连接形成这些大分子。
1. Monomers and Condensation Reactions | 单体与缩合反应
All natural biochemical polymers are formed from small repeating units called monomers. The key reaction is condensation polymerisation, where monomers join together with the elimination of a small molecule, usually water.
所有天然生化聚合物都由称为单体的小重复单元构成。关键反应是缩合聚合，即单体连接在一起并消除一个小分子，通常是水。
For example, glucose monomers form polysaccharides, amino acids form proteins, and nucleotides form DNA or RNA. In each case, a hydroxyl group from one monomer reacts with a hydrogen atom or hydroxyl group from another monomer, releasing water.
例如，葡萄糖单体形成多糖，氨基酸形成蛋白质，核苷酸形成 DNA 或 RNA。在每种情况下，一个单体的羟基与另一个单体的氢原子或羟基反应，释放出水。
HO–A–H + HO–B–OH → A–B + H₂O
The bond formed between monomers is called a glycosidic bond (carbohydrates), a peptide bond (proteins), or a phosphodiester bond (nucleic acids). These bonds require energy to form and are broken by hydrolysis reactions.
单体之间形成的键分别称为糖苷键（碳水化合物）、肽键（蛋白质）和磷酸二酯键（核酸）。这些键的形成需要能量，并通过水解反应断裂。
2. Carbohydrates: Monosaccharides | 碳水化合物：单糖
Monosaccharides are the simplest sugars, with the general formula Cₙ(H₂O)ₙ. They contain a carbonyl group and multiple hydroxyl groups. Common examples include glucose, fructose and galactose.
单糖是最简单的糖，通式为 Cₙ(H₂O)ₙ。它们含有一个羰基和多个羟基。常见例子包括葡萄糖、果糖和半乳糖。
Glucose is a hexose sugar with six carbon atoms. In aqueous solution, it exists predominantly in a ring form. The linear aldehyde form cyclises to produce a six-membered ring (pyranose) containing five carbon atoms and one oxygen atom.
葡萄糖是一种含六个碳原子的己糖。在水溶液中，它主要以环状形式存在。线性醛式环化后产生一个六元环（吡喃糖），包含五个碳原子和一个氧原子。
The ring structure of glucose can be represented as α-glucose or β-glucose, depending on the position of the –OH group on carbon-1 (C1). This difference is crucial because it determines the type of polysaccharide formed.
葡萄糖的环状结构可分为 α-葡萄糖或 β-葡萄糖，取决于碳-1（C1）上 –OH 基团的位置。这一差异至关重要，因为它决定了形成的多糖类型。
- α-glucose: –OH on C1 points down (opposite to the CH₂OH group).
- β-glucose: –OH on C1 points up (same side as the CH₂OH group).
- α-葡萄糖：C1 上的 –OH 指向下方（与 CH₂OH 基团相反）。
- β-葡萄糖：C1 上的 –OH 指向上方（与 CH₂OH 基团同侧）。
3. Disaccharides | 二糖
A disaccharide forms when two monosaccharides join via a condensation reaction. The bond formed is a glycosidic bond, which links the anomeric carbon of one monosaccharide to a hydroxyl group of another.
当两个单糖通过缩合反应连接时，形成二糖。所生成的键是糖苷键，它将一个单糖的异头碳与另一个单糖的羟基连接起来。
Common disaccharides include:
常见的二糖包括：
| Disaccharide | Monomers | Glycosidic Link |
| Maltose | α-glucose + α-glucose | α(1→4) |
| Cellobiose | β-glucose + β-glucose | β(1→4) |
| Sucrose | α-glucose + fructose | α(1→2) |
| Lactose | β-galactose + glucose | β(1→4) |
Maltose is produced during the digestion of starch and is further hydrolysed into two glucose molecules. Sucrose, common table sugar, is a non-reducing sugar because both anomeric carbons are involved in the glycosidic bond.
麦芽糖在淀粉消化过程中产生，并进一步水解为两个葡萄糖分子。蔗糖是普通食糖，是一种非还原性糖，因为两个异头碳都参与了糖苷键的形成。
4. Polysaccharides: Starch and Glycogen | 多糖：淀粉与糖原
Starch is a storage polysaccharide in plants, composed entirely of α-glucose monomers. It exists in two forms: amylose (linear) and amylopectin (branched).
淀粉是植物中的储存多糖，完全由 α-葡萄糖单体组成。它以两种形式存在：直链淀粉（线性）和支链淀粉（有分支）。
Amylose consists of α(1→4) glycosidic bonds, forming a long unbranched chain that coils into a helical structure. This compact shape makes it ideal for energy storage.
直链淀粉由 α(1→4) 糖苷键组成，形成一条长的不分支链，并卷曲成螺旋结构。这种紧凑的形状使其非常适合能量储存。
Amylopectin contains both α(1→4) bonds and α(1→6) bonds at branch points, typically every 24–30 residues. The branching allows enzymes to access glucose units rapidly.
支链淀粉含有 α(1→4) 键和分支点处的 α(1→6) 键，通常每 24–30 个残基出现一次分支。分支使酶能够快速接触葡萄糖单元。
Glycogen is the animal equivalent of starch. It has a similar structure to amylopectin but is more extensively branched, with branch points every 8–12 glucose units. This high degree of branching provides quick glucose release for energy demand.
糖原是动物中与淀粉对应的储存多糖。它的结构类似于支链淀粉，但分支程度更高，每 8–12 个葡萄糖单元就有一个分支点。这种高分支程度能够快速释放葡萄糖以满足能量需求。
5. Polysaccharides: Cellulose | 多糖：纤维素
Cellulose is a structural polysaccharide found in plant cell walls. It is composed of β-glucose monomers joined by β(1→4) glycosidic bonds.
纤维素是植物细胞壁中的结构多糖。它由 β-葡萄糖单体通过 β(1→4) 糖苷键连接而成。
Each β-glucose residue is rotated 180° relative to its neighbour. This alternating arrangement allows extensive intermolecular hydrogen bonding between parallel chains.
每个 β-葡萄糖残基相对于其相邻残基旋转 180°。这种交替排列使得平行链之间能够形成广泛的分子间氢键。
These hydrogen bonds bundle cellulose chains into microfibrils, which are extremely strong and resistant to hydrolysis. Humans cannot digest cellulose because we lack the enzyme cellulase, which is needed to break β(1→4) bonds.
这些氢键将纤维素链捆绑成微纤维，使其极其坚固且抗水解。人类无法消化纤维素，因为我们缺乏分解 β(1→4) 键所需的纤维素酶。
The key comparison between starch and cellulose is the stereochemistry of glucose. α-glucose gives tightly coiled, digestible polymers, while β-glucose gives straight, strong, indigestible fibres.
淀粉与纤维素的关键区别在于葡萄糖的立体化学。α-葡萄糖形成紧密卷曲、可消化的聚合物，而 β-葡萄糖形成笔直、坚固、不可消化的纤维。
6. Proteins: Amino Acid Monomers | 蛋白质：氨基酸单体
Proteins are polymers of amino acids. Each amino acid has the general structure containing an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable R group, all attached to the central α-carbon.
蛋白质是氨基酸的聚合物。每种氨基酸具有通用结构，包含氨基（–NH₂）、羧基（–COOH）、一个氢原子和一个可变的 R 基团，均连接在中心 α-碳上。
At physiological pH, amino acids exist as zwitterions, with the amino group protonated (–NH₃⁺) and the carboxyl group deprotonated (–COO⁻). This dipolar ion form gives amino acids their amphoteric behaviour and relatively high melting points.
在生理 pH 下，氨基酸以两性离子形式存在，氨基质子化为 –NH₃⁺，羧基去质子化为 –COO⁻。这种偶极离子形式赋予氨基酸两性行为和相对较高的熔点。
The R group determines the identity and properties of the amino acid. It can be non-polar, polar, acidic, or basic, which affects the folding and function of the protein.
R 基团决定了氨基酸的身份和性质。它可以是非极性、极性、酸性或碱性的，这会影响蛋白质的折叠和功能。
7. Peptide Bond Formation and Primary Structure | 肽键形成与一级结构
Amino acids join together by condensation reactions between the carboxyl group of one amino acid and the amino group of another. The resulting C–N bond is called a peptide bond.
氨基酸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连接。所形成的 C–N 键称为肽键。
For example, two amino acids form a dipeptide with the release of one water molecule:
例如，两个氨基酸形成二肽并释放一个水分子：
H₂N–CHR₁–COOH + H₂N–CHR₂–COOH → H₂N–CHR₁–CO–NH–CHR₂–COOH + H₂O
The sequence of amino acids in a polypeptide chain is the primary structure. This sequence is determined by the DNA and is unique for each protein. Even a single amino acid change can lead to a different protein, as seen in sickle cell anaemia.
多肽链中氨基酸的序列是一级结构。该序列由 DNA 决定，每种蛋白质都是独特的。即使单个氨基酸的改变也可能导致不同的蛋白质，如镰状细胞贫血症所见。
8. Secondary Structure: α-Helix and β-Sheet | 二级结构：α-螺旋与β-折叠
The secondary structure arises from hydrogen bonding between the backbone N–H group and the C=O group of nearby amino acids. The two main patterns are the α-helix and the β-pleated sheet.
二级结构源于骨架 N–H 基团与附近氨基酸 C=O 基团之间的氢键。两种主要模式是 α-螺旋和 β-折叠。
In the α-helix, the polypeptide chain coils clockwise, with hydrogen bonds formed between every fourth peptide bond. The R groups extend outward from the helix, avoiding steric clashes.
在 α-螺旋中，多肽链顺时针盘绕，每第四个肽键之间形成氢键。R 基团从螺旋向外伸展，避免空间位阻。
In the β-pleated sheet, adjacent polypeptide chains lie side by side, held together by hydrogen bonds perpendicular to the direction of the chains. The R groups alternate above and below the plane of the sheet.
在 β-折叠中，相邻多肽链并排排列，由垂直于链方向的氢键连接。R 基团在折叠平面上下交替排列。
Keratin is rich in α-helices, while silk fibroin is rich in β-sheets. The difference in secondary structure directly affects the mechanical properties of these proteins.
角蛋白富含 α-螺旋，而丝心蛋白富含 β-折叠。二级结构的差异直接影响这些蛋白质的机械性能。
9. Tertiary and Quaternary Structure | 三级与四级结构
Tertiary structure is the overall three-dimensional shape of a single polypeptide, stabilised by interactions between R groups. These include hydrogen bonds, ionic bonds, hydrophobic interactions and disulfide bridges.
三级结构是单条多肽的整体三维形状，由 R 基团之间的相互作用稳定。这些包括氢键、离子键、疏水相互作用和二硫键。
Disulfide bridges are covalent bonds formed between two cysteine residues. They are the strongest of the stabilising interactions and cannot be broken by changes in pH alone. Ionic bonds form between oppositely charged R groups, while hydrophobic interactions occur between non-polar R groups in the protein core.
二硫键是两个半胱氨酸残基之间形成的共价键。它们是稳定相互作用中最强的，仅靠 pH 变化无法断裂。离子键在带相反电荷的 R 基团之间形成，而疏水相互作用发生在蛋白质核心中的非极性 R 基团之间。
Quaternary structure refers to the assembly of two or more polypeptide chains into one functional protein. Examples include haemoglobin (four subunits) and collagen (three helical chains twisted together).
四级结构指两条或多条多肽链组装成一个功能性蛋白质。例如血红蛋白（四个亚基）和胶原蛋白（三条螺旋链缠绕在一起）。
10. Nucleic Acids: Nucleotide Structure | 核酸：核苷酸结构
Nucleic acids, DNA and RNA, are polymers of nucleotides. Each nucleotide consists of three components: a pentose sugar, a phosphate group, and a nitrogen-containing base.
核酸 DNA 和 RNA 是核苷酸的聚合物。每个核苷酸由三个组成部分：戊糖、磷酸基团和含氮碱基。
The pentose sugar in DNA is deoxyribose, which lacks an –OH group at the 2′ carbon. In RNA, the sugar is ribose, which has an –OH group at the 2′ carbon. This single difference has major consequences for stability and structure.
DNA 中的戊糖是脱氧核糖，在 2′ 碳上缺少 –OH 基团。在 RNA 中，糖是核糖，在 2′ 碳上有一个 –OH 基团。这一单一差异对稳定性和结构有重大影响。
There are two types of nitrogenous bases: purines (adenine and guanine), which have a double-ring structure, and pyrimidines (cytosine, thymine and uracil), which have a single-ring structure.
有两类含氮碱基：嘌呤（腺嘌呤和鸟嘌呤），具有双环结构；嘧啶（胞嘧啶、胸腺嘧啶和尿嘧啶），具有单环结构。
11. Phosphodiester Bonds and Polynucleotide Chains | 磷酸二酯键与多核苷酸链
Nucleotides join together via condensation reactions between the phosphate group of one nucleotide and the 3′ –OH group of the pentose sugar of another nucleotide. This forms a phosphodiester bond.
核苷酸通过一个核苷酸的磷酸基团与另一个核苷酸戊糖的 3′ –OH 基团之间的缩合反应连接。这形成磷酸二酯键。
The repeating sugar-phosphate backbone forms along the 5′ to 3′ direction. The bases project outward from the backbone and are available for hydrogen bonding with complementary bases on another strand.
重复的糖-磷酸骨架沿 5′ 到 3′ 方向延伸。碱基从骨架向外伸出，并可与另一条链上的互补碱基形成氢键。
In DNA, adenine pairs with thymine via two hydrogen bonds, and guanine pairs with cytosine via three hydrogen bonds. This complementary base pairing is essential for accurate DNA replication and transcription.
在 DNA 中，腺嘌呤与胸腺嘧啶通过两个氢键配对，鸟嘌呤与胞嘧啶通过三个氢键配对。这种互补碱基配对对于 DNA 的准确复制和转录至关重要。
12. DNA Double Helix vs RNA Structure | DNA 双螺旋与 RNA 结构
DNA exists as a double helix, where two antiparallel polynucleotide chains wind around each other. The sugar-phosphate backbones are on the outside, while the nitrogenous bases are stacked in the interior, perpendicular to the helical axis.
DNA 以双螺旋形式存在，两条反向平行的多核苷酸链相互缠绕。糖-磷酸骨架位于外部，而含氮碱基堆叠在内部，垂直于螺旋轴。
The double helix is stabilised by hydrogen bonds between complementary bases and by base stacking interactions, which exclude water and hold the structure together. The diameter of the helix is constant because a purine always pairs with a pyrimidine.
双螺旋由互补碱基之间的氢键和碱基堆叠相互作用稳定，这些相互作用排除水并将结构固定在一起。由于嘌呤总是与嘧啶配对，因此螺旋的直径恒定。
RNA, in contrast, is usually single-stranded. It contains uracil instead of thymine, and the presence of the 2′ –OH group makes RNA more chemically reactive and less stable than DNA. RNA can fold into complex shapes through intramolecular base pairing, enabling it to carry out catalytic and regulatory roles.
相比之下，RNA 通常是单链的。它含有尿嘧啶而非胸腺嘧啶，并且 2′ –OH 基团的存在使 RNA 更具化学反应活性，稳定性低于 DNA。RNA 可以通过分子内碱基配对折叠成复杂形状，从而执行催化和调控功能。
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