📚 Amino Acids, Proteins and DNA | 氨基酸、蛋白质与 DNA
This topic focuses on amino acids as bifunctional monomers, how they polymerise to form proteins, and how DNA stores genetic information through a sequence of nucleotide bases. It brings together ideas from organic chemistry, transition metal chemistry and hydrogen bonding to explain the structure and function of biological macromolecules.
本主题重点介绍氨基酸作为双官能团单体，如何通过聚合形成蛋白质，以及 DNA 如何通过核苷酸碱基序列储存遗传信息。它将有机化学、过渡金属化学和氢键等知识结合起来，解释生物大分子的结构与功能。
1. Amino Acid Structure and Chirality | 氨基酸结构与手性
An α-amino acid has an amine group and a carboxylic acid group attached to the same carbon atom, called the α-carbon. The α-carbon also carries a hydrogen atom and a variable side chain, R, so the general formula is H₂N–CHR–COOH.
α-氨基酸的氨基和羧基连接在同一个碳原子上，该碳原子称为 α-碳。α-碳还连接一个氢原子和可变的侧链 R，因此通式为 H₂N–CHR–COOH。
All α-amino acids except glycine have four different groups on the α-carbon, so the α-carbon is chiral. This produces two optical isomers, or enantiomers, which are non-superimposable mirror images.
除甘氨酸外，所有 α-氨基酸的 α-碳上都连有四个不同的基团，因此 α-碳具有手性。这会形成两种光学异构体，即对映异构体，它们互为不可重叠的镜像。
2. Zwitterions and Isoelectric Point | 两性离子与等电点
In aqueous solution, an amino acid exists mainly as a zwitterion. The amine group accepts a proton to become –NH₃⁺, and the carboxylic acid group donates a proton to become –COO⁻. The zwitterion is written H₃N⁺–CHR–COO⁻ and has no net charge overall.
在水溶液中，氨基酸主要以两性离子形式存在。氨基接受一个质子形成 –NH₃⁺，羧基给出一个质子形成 –COO⁻。两性离子写作 H₃N⁺–CHR–COO⁻，整体不带净电荷。
In acidic solution, the carboxylate ion is protonated, so the amino acid forms a positive ion. In alkaline solution, the ammonium group loses a proton, so the amino acid forms a negative ion. The pH at which the amino acid has no net electrical charge is its isoelectric point.
在酸性溶液中，羧酸根离子被质子化，氨基酸形成带正电的离子。在碱性溶液中，铵基失去质子，氨基酸形成带负电的离子。氨基酸不带净电荷时的 pH 就是它的等电点。
3. Peptide Bond Formation and Hydrolysis | 肽键形成与水解
Amino acids polymerise by condensation reactions. The carboxylic acid group of one amino acid reacts with the amine group of another amino acid, eliminating one water molecule and forming a peptide bond, –CONH–.
氨基酸通过缩合反应聚合。一个氨基酸的羧基与另一个氨基酸的氨基反应，脱去一分子水，形成肽键 –CONH–。
Two amino acids joined together form a dipeptide; many amino acid residues form a polypeptide. The reaction can be represented as:
两个氨基酸连接在一起形成二肽；许多氨基酸残基形成多肽。该反应可表示为：
H₂N–CHR–COOH + H₂N–CHR'–COOH → H₂N–CHR–CONH–CHR'–COOH + H₂O
The reverse reaction, hydrolysis, breaks peptide bonds. In the laboratory, proteins are hydrolysed by heating under reflux with 6 mol dm⁻³ hydrochloric acid for about 24 hours, or by aqueous alkali. In living organisms, enzymes catalyse hydrolysis under mild conditions.
逆反应是水解，可断裂肽键。在实验室中，蛋白质通常与 6 mol dm⁻³ 盐酸加热回流约 24 小时，或用碱溶液水解。在生物体内，酶在温和条件下催化水解。
4. Polypeptides and Primary Structure | 多肽与蛋白质一级结构
The primary structure of a protein is the specific sequence of amino acid residues in the polypeptide chain. This sequence is determined by the base sequence in DNA and is held together by covalent peptide bonds.
蛋白质的一级结构是多肽链中氨基酸残基的特定排列顺序。该顺序由 DNA 中的碱基序列决定，并通过共价肽键连接在一起。
Even a small change in the primary structure can alter the protein's final shape and function, as seen in sickle cell anaemia where one amino acid is replaced in haemoglobin.
即使一级结构发生很小的改变，也可能改变蛋白质的最终形状和功能。例如镰状细胞贫血中，血红蛋白里有一个氨基酸被替换。
5. Secondary Structure: α-Helix and β-Pleated Sheet | 二级结构：α-螺旋与 β-折叠片
se patterns are stabilised by hydrogen bonds between the C=O of one peptide bond and the N–H of another peptide bond.
二级结构描述多肽主链有规律地折叠成 α-螺旋和 β-折叠片等结构。这些结构通过一个肽键的 C=O 与另一个肽键的 N–H 之间的氢键保持稳定。
The R groups are not directly involved in forming the secondary structure hydrogen bonds; they point away from the main pattern and influence later folding.
R 基团不直接参与形成二级结构的氢键；它们指向主链结构的外侧，并影响后续的折叠。
6. Tertiary Structure and Bonding | 三级结构与作用力
Tertiary structure is the overall three-dimensional shape of a single polypeptide chain. It is held together by interactions between R groups, including hydrogen bonds, ionic bonds between charged side chains, disulfide bridges formed between cysteine residues, and hydrophobic interactions between non-polar side chains.
三级结构是一条多肽链整体的三维形状。它由 R 基团之间的相互作用维持，包括氢键、带电侧链之间的离子键、半胱氨酸残基之间形成的二硫桥，以及非极性侧链之间的疏水作用。
Disulfide bridges are covalent –S–S– bonds and are much stronger than hydrogen or ionic bonds. Changes in pH or high temperature can disrupt hydrogen and ionic bonds, causing the protein to denature and lose its tertiary structure.
二硫桥是共价的 –S–S– 键，比氢键和离子键强得多。pH
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