《aminoacidsandpeptides(ch5)》-公开课件讲义.ppt

6.2 Other reagents also react with the a-amino group, keeping the R group part of the products 6.2.1 1-Fluoro-2,4-dinitrobenzene and dabsyl chloride react to form colored derivatives that are stable under harsh conditions (heating in 6N HCl at 110?C for 24 hours!) 6.2.2 Fluorescent derivatives, permitting the detection of nanogram amount of amino acids. 6.2.3 Identity of amino acids can be revealed by comparing with a standard. 7. Amino acids covalently join one another to form peptides 7.1 The a-carboxyl group of one amino acid joins with the a-amino group of another amino acid by a peptide bond (actually an amide bond) 7.1.1 This is a condensation reaction where a water molecule is liberated or eliminated. 7.1.2 ?G of the condensation reaction is about 5 kcal/mol, not being able to occur spontaneously (an endergonic reaction). 7.1.3 The condensation reaction can occur repeatedly to form oligopeptides (with less than 50 aa), polypeptides (bwt 50-100 aa), and proteins (longer). 7.2 The peptide chain is directional. 7.2.1 An amino acid unit in a peptide chain is called a residue. 7.2.2 The end having a free a-amino group is called amino-terminal or N-terminal. 7.2.3 The end having a free a-carboxyl group is called carboxyl-terminal or C-terminal. 7.2.4 By convention, the N-terminal is taken as the beginning of the peptide chain, and put at the left (C-terminal at the right). Biosynthesis starts from the N-terminal. 7.2.5 The peptide chain consist of a regularly repeating main chain (or backbone) and the variable side chains of the residues. 7.2.6 Amino acid residues have an order or sequence on a peptide. 7.2.7 The 20 amino acids are analogous to the 26 letters in English; the number of different peptides made of them is unlimited. 7.3 The size of a peptide can be described by its total number of residues (e.g., a pentapeptide, a octapeptide) or relative molecular mass (molecular weight). 7.3.1 The mean molecular weight of an amino acid residue in a p