Ch 26 notes - Chemistry 140C Spring 2009(K Albizati Chapter...

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Chemistry 140C Spring 2009 (K. Albizati) 1 Chapter 26 – Amino Acids, Peptides, Proteins and Nucleic Acids Many important classes of biomolecules contain amine groups, carboxyl groups, heterocycles and carbohydrate moieties --- the topics of Chapters 19, 21, 24 and 25. α -Amino Acids About 500 α -amino acids (hereafter just called amino acids or AA’s) exist in nature but only about 20 constitute the vast majority of the amino acids found in cells either as independent molecules or as parts of proteins and other biomolecules. All 20 of these possess the common substructure: which differs only in the identity of the R group, which tends to be fairly simple in structure. Almost without exception, AA’s possess an S- configured chiral carbon and in nature are found as single enantiomers. The structures of these 20 AA’s are found on pages 1193 – 1194 in V&S. Some comments about the 20: These should be memorized by categorizing the R group and by relating structures to one another. So……. - Chirality: only Gly is achiral; Ile and Thr have an additional chiral carbon - Pro is the only 2 o amine; there are no 3 o amines - There is a methyl group for R (Ala), an isopropyl group (Val) and an isobutyl (Leu) but no ethyl group as R - Ser and Cys are heteroatom analogs - The chiral carbon of Cys is R because sulfur takes priority over oxygen in assigning R and S, but the configurations are the same - Asp and Glu contain carboxyl groups and differ only in the length of the chain - Gln is the 1 o amide of Glu and Asn is the 1 o amide of Asp
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Chemistry 140C Spring 2009 (K. Albizati) 2 AA’s Containing Only C and H in the R Group H O OH NH 2 H O OH NH 2 H O OH NH 2 H O OH NH 2 H O OH NH 2 H O OH NH 2 H S O OH NH H Glycine (Gly; G) Alanine (Ala; A) Phenylalanine (Phe; F) Valine (Val; V) Leucine (Leu; L) Isoleucine (Ile; I) Proline (Pro; P)
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Chemistry 140C Spring 2009 (K. Albizati) 3 AA’s Containing Nitrogen in the R Group
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Chemistry 140C Spring 2009 (K. Albizati) 4 AA’s Containing Oxygen and Sulfur in the R Group
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Spring 2009 (K. Albizati) 5 Acidity and Basicity of AA’s Because AA’s possess both amino and carboxyl groups they are both acidic and basic--- that is, they are amphoteric. The carboxyl groups in AA’s are more acidic than a normal carboxylic acid (pKa = 2 – 3 in AA’s vs. about 5 normally). In the solid state and in solution near pH 7, the carboxyl group is deprotonated by the base to form a zwitterion : R O O NH 3 H Being very polar substances, AA’s are generally very soluble in water and relatively insoluble in organic solvents. The state of protonation in water solution is highly dependent on the pH of the solution. R O O NH 3 H R O OH NH 3 H R O O NH 2 H Predominant at pH > 13 Predominant at pH about 6 or 7 Predominant at pH < 1 The amount of each species at a given pH can be quantified if you have knowledge of the pKa’s of the individual carboxyl and amino groups. The situation becomes more complicated when other acidic or basic functional groups are present such as in Lys, Asp, Glu, Arg or His. The His imidazole ring contains a basic nitrogen atom.
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This note was uploaded on 08/04/2009 for the course CHEM CHEM 140C taught by Professor Finn during the Spring '08 term at UCSD.

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Ch 26 notes - Chemistry 140C Spring 2009(K Albizati Chapter...

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