Question 21.2: Draw the structure of the amino acid glycine at each pH: (a)...

General, organic, & biological chemistry

Draw the structure of the amino acid glycine at each pH: (a) 6; (b) 2; (c) 11.

Analysis

A neutral amino acid exists in its zwitterionic form (no net charge) at its isoelectric point, which is pH ≈ 6. The zwitterionic forms of neutral amino acids appear in Table 21.2 . At low pH ( ≤ 2), the carboxylate anion is protonated and the amino acid has a net positive (+1) charge. At high pH ( ≥ 10), the ammonium cation loses a proton and the amino acid has a net negative (−1) charge.

Table 21.2 The 20 Common Naturally Occurring Amino Acids
Neutral Amino Acids
Name	Structure	Abbreviations	Name	Structure	Abbreviations
Alanine		Ala A	Phenylalanine*		Phe F
Asparagine		Asn N	Proline		Pro P
Cysteine		Cys C	Serine		Ser S
Glutamine		Gln Q	Threonine*		Thr T
Glycine		Gly G	Tryptophan*		Trp W
Isoleucine*		Ile I	Tyrosine		Tyr Y
Leucine*		Leu L	Valine*		Val V
Methionine*		Met M	Arginine*		Arg R
Aspartic acid		Asp D	Histidine*		His H
Glutamic acid		Glu E	Lysine*		Lys K
Essential amino acids are labeled with an asterisk (*).

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