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Proteins are polymers( 多聚体 ) of amino acids.

Chapter 2 Amino Acids. Proteins are polymers( 多聚体 ) of amino acids. -20 amino acids ( 氨基酸 )  millions of proteins with different properties and activities. 二、蛋白质的组成. 组成蛋白质的元素. 主要有 C 、 H 、 O 、 N 和 S 。 有些蛋白质含有少量磷或金属元素铁、铜、锌、锰、钴、钼，个别蛋白质还含有碘 。. 蛋白质元素组成的特点.

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Proteins are polymers( 多聚体 ) of amino acids.

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  1. Chapter 2 Amino Acids Proteins are polymers(多聚体) of amino acids. -20 amino acids (氨基酸)  millions of proteins with different properties and activities.

  2. 二、蛋白质的组成 • 组成蛋白质的元素 主要有C、H、O、N和 S。有些蛋白质含有少量磷或金属元素铁、铜、锌、锰、钴、钼，个别蛋白质还含有碘 。

  3. 蛋白质元素组成的特点 由于体内的含氮物质以蛋白质为主，因此，只要测定生物样品中的含氮量，就可以根据以下公式推算出蛋白质的大致含量： 各种蛋白质的含氮量很接近，平均为16％。 • 100克样品中蛋白质的含量 (g %) • = 每克样品含氮克数× 6.25×100 1/16%

  4. 二、蛋白质的水解 • 酸水解 • 碱水解 • 酶水解

  5. 三、氨基酸的一般结构 蛋白质中发现的氨基酸都是L-氨基酸（甘氨酸除外）

  6. 四、氨基酸的分类 根据氨基酸的侧链R基团： • 15种脂肪族氨基酸： 5种中性氨基酸， 4种含羟基或巯基的氨基酸， 4种酸性氨基酸以及酰胺 2种碱性氨基酸 • 3种芳香族氨基酸 • 2种杂环氨基酸

  7. (一)侧链含烃链的氨基酸属于非极性脂肪族氨基酸(一)侧链含烃链的氨基酸属于非极性脂肪族氨基酸

  8. (二)侧链有极性但不带电荷的氨基酸是极性中性氨基酸(二)侧链有极性但不带电荷的氨基酸是极性中性氨基酸

  9. (三)侧链含芳香基团的氨基酸是芳香族氨基酸

  10. (四)侧链含负性解离基团的氨基酸是酸性氨基酸(四)侧链含负性解离基团的氨基酸是酸性氨基酸

  11. (五)侧链含正性解离基团的氨基酸属于碱性氨基酸(五)侧链含正性解离基团的氨基酸属于碱性氨基酸

  12. CH CH 2 2 CHCOO CHCOO - - CH CH 2 2 NH NH + + 2 2 CH CH 2 2 • 几种特殊氨基酸 • 脯氨酸 • （亚氨基酸）

  13. - - - - OOC OOC - - CH CH - - CH CH - - SH SH HS HS - - CH CH - - CH CH - - COO COO 2 2 2 2 + NH NH NH NH + + + + 3 3 3 3 -HH - - - - OOC OOC - - CH CH - - CH CH - - S S S S - - CH CH - - CH CH - - COO COO 2 2 2 2 NH NH NH NH + + + + 3 3 3 3 二硫键  半胱氨酸 • 胱氨酸

  14. 五、20种氨基酸具有共同或特异的理化性质 （一）氨基酸具有两性解离的性质 • 两性解离及等电点 氨基酸是两性电解质，其解离程度取决于所处溶液的酸碱度。 等电点(isoelectric point, pI) 在某一pH的溶液中，氨基酸解离成阳离子和阴离子的趋势及程度相等，成为兼性离子，呈电中性。此时溶液的pH值称为该氨基酸的等电点。

  15. +H+ +OH- +H+ +OH- pH>pI pH<pI pH=pI 氨基酸的兼性离子 阳离子 阴离子 Zwitterion

  16. AA as diprotic acids（二元酸） (Handerson-Hasselbalch equation)

  17. Monoamino monocarboxylic a-amino acids (e.g., Gly, Ser, Phe with no ionizable groups，侧链不解离) all have similar two-stage titration curves. • The first stage reflects the deprotonation of the a-COOH group (pK1). • The second stage reflects the deprotonation of the a-NH 3+ group (pK2).

  18. = pH = pK1 = pH = pK2

  19. The characteristic pH at which the net electric charge is zero is called the isoelectric point or isoelectric pH, designated pI. (净电荷为零时的pH值，等电点） An amino acid does not move in an electric field at its pI The pI of monoamino monocarboxylic amino acids reflects a status at which the a-COOH group is fully deprotonated, but the a-NH3+ group has not yet started deprotonating pI = ½ (pK1+pK2)

  20. AA as triprotic acids（三元酸） A+ A0 A- A2-（很少） A+ = A- pI = ½ (pK1+pKR)

  21. A2+（很少）A+ A0 A- A+ = A- pI = ½ (pKR+pK2)

  22. The amino acids are positively charged at pH smaller than their pI values, negatively charged at pH larger than their pI values. 氨基酸所处溶液的pH值小于它们的pI时，氨基酸带正电荷； 氨基酸所处溶液的pH值大于它们的pI时，氨基酸带负电荷 溶液的pH值越偏离pI，氨基酸所带的净电荷越大

  23. （二）含共轭双键的氨基酸具有紫外吸收性质 色氨酸、酪氨酸的最大吸收峰在 280 nm附近。 大多数蛋白质含有这两种氨基酸残基，所以测定蛋白质溶液280nm的光吸收值是分析溶液中蛋白质含量的快速简便的方法。 芳香族氨基酸的紫外吸收

  24. Lambert-Beer’s law: A=Log Io/I = ecl e, molar extinction coefficient（摩尔吸光系数，L/mol·cm） c, concentration (mol/L); l, optical length (cm)

  25. （三）氨基酸的化学反应 与茚三酮反应生成蓝紫色化合物 氨基酸与茚三酮水合物共热，可生成蓝紫色化合物，其最大吸收峰在570nm处。 由于此吸收峰值与氨基酸的含量存在正比关系，因此可作为氨基酸定量分析方法。

  26. 二硫键的形成和断裂 氧化 还原

  27. 六、氨基酸混合物的分离和分析 Chromatography is a method of separating substances by allowing them to partition between two phases, one mobile, one stationary (differences in charge, size, hydrophobic interactions, specific interactions can be exploited for substance separation with chromatography). 层析：利用混合物组分的不同性质（如电荷，大小，亲水/疏水作用，特异性亲和等）在两相进行分配从而达到分离的目的 流动相 固定相 分配常数Kd

  28. ColumnChromatography (柱层析）

  29. Ion exchange chromatography（离子交换层析） • 在给定pH的溶液中，氨基酸所带的电荷不同 • When the resin (树脂)(serving as the stationary phase)is positively charged, negatively charged amino acids (or other anions) will bind, and vice versa. • The positively charged resin is called anion-exchange resin, while the negatively charged one is called cation-exchange resin. • 带正电荷的树脂称为阴离子交换树脂；带负电荷的树脂称为阳离子交换树脂 • The bound amino acids can be eluted by running a pH or salt gradient (serving as the mobile phase). • 树脂上吸附的氨基酸可以用pH或盐离子梯度溶液洗脱

  30. Cation- exchange column Basic > neutral > acidic

  31. The mobile phase can percolate through the column at low pressure or high pressure. • To operate under high pressure, specially designed resins and apparatus (the pumps and the plumbing system) are needed. • Using high pressure allows better separation in a much shorter period of time, thus named High Performance Liquid Chromatography (高效液相色谱，HPLC).

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