coo C00 H H NHa C00 COO C00 C00 HaN- C-H H-GNH, HN-C-H H-C-NH CH CHa CHa CHa CH CHa L-Alanine D-Alanine L-Alanine D-Alanine L-Alanine DAlanine (a) (b) (c) Because the resulting amino acid contains one positive and one negative charge,it is a neutral molecule called a zwitterion.Amino acids are also chiral molecules.With four different groups attached to it,the a-carbon is said to be asymmetric.The two possible configurations for the a- carbon constitute nonidentical mirror image isomers or enantiomers(对映体)，• Because the resulting amino acid contains one positive and one negative charge, it is a neutral molecule called a zwitterion. Amino acids are also chiral molecules. With four different groups attached to it, the a-carbon is said to be asymmetric. The two possible configurations for the a￾carbon constitute nonidentical mirror image isomers or enantiomers(对映体)