有机化学 英文 讲义 chapter 25.pptVIP

Chapter 25 Amino Acids, Peptides and  Proteins * 25.2 Amino acids Definition: Amino acids are carboxylic acids that contain amino groups. General molecule of a-amino acids: Except the simplest a-amino acid— glycine(甘氨酸), the a-carbons in all of a-amino acids are chiral carbons. Only L-enantiomers of a-amino acids are found in nature. L-a-amino acids According to the position of amino group, amino acids can be divided into: a, b, g ... w amino acids According to the numbers of amino groups and carboxylic groups, amino acids can be divided into: neutral amino acids— the number of the NH2 = the number of COOH acidic amino acids— the number of the NH2 the number of COOH basic amino acids— the number of the NH2 the number of COOH Category: 25.3, 25.4 Chemical reactions of amino acids a. Acid-base chemistry of amino acids An intramolecular acid-base reaction to form a dipolar ion, also called as a zwitterion. A zwitterion can react either with an acid to form a cation or with a base to form an anion. The concentration of each form depends on the pH value of the solution. In a basic solution, amino acids mainly exist in an anionic form, and in an acidic solution, amino acids mainly exist in a cationic form. When the pH value of the solution is equal to the average of pKa1 and pKa2 , the concentration of the zwitterion is at its maximum value, and this pH value is called the isoelectric point (pI, 等电点). For neutral amino acids: pI = 5.6 ~ 6.3 For acidic amino acids: pI = 2.8 ~ 3.2 For basic amino acids: pI = 7.6 ~ 10.8 Write the major existing form of glycine (molecular formula: H2NCH2COOH) in aqueous solution at the following pH values. pH = 11 pH = 6 pH = 2 H2NCH2COO- H3N+CH2COO- H3N+CH2COOH b. Esterification of the COOH group and amidation of the NH2 group c. Dehydration and deammoniation of amino acids i. a-amino acids (intermolecular dehydration): i