COOHCOO-+deprotonatedHN-C.HH.N+- C.HprotonatedRRnonionic formzwitterionic formdipolarionExcept for proline, all 19 of the common AAs are α-amino acids; Proline is an a-imino acids.The a-carbon atom is always asymmetric or chiralcenter except in glycine, hence, all amino acids areoptical activity(旋光性) and have two stereoisomers(mirror-image forms) (D or L configuration)
COOH H2N Cα H R COO－ H3N+ Cα H R nonionic form zwitterionic form dipolar ion uExcept for proline, all 19 of the common AAs are α- amino acids; Proline is an α-imino acids. uThe α-carbon atom is always asymmetric or chiral center except in glycine, hence, all amino acids are optical activity(旋光性) and have two stereoisomers (mirror-image forms) (D or L configuration). deprotonated protonated
The two stereoisomers of each AA are designatedby D, L system according to the D- and L-glyceraldehyde1CHOCHOHO-2C-HH-C-OH3CH,0HCH,OHL-GlyceraldehydeD-GlyceraldehydeCOOCOOH-C-NH3H,N-C-HCH3CH3L-AlanineD-Alanine
u The two stereoisomers of each AA are designated by D, L system according to the D- and L- glyceraldehyde
Only the L-AAs have been found in proteinsD-isomers have been found only in small peptidesof bacteria cell walls or in some peptide antibioticsRacemate(外消旋物): An equimolar mixture of theD- and L-isomers of an optically active compound.A racemic mixture shows no optical activity
u Only the L-AAs have been found in proteins u D-isomers have been found only in small peptides of bacteria cell walls or in some peptide antibiotics u Racemate(外消旋物): An equimolar mixture of the D- and L-isomers of an optically active compound. u A racemic mixture shows no optical activity