AminoAcidWorkshopcorrrectedkey

AminoAcidWorkshopcorrrectedkey - Amino Acid Workshop 1. The...

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Amino Acid Workshop 1. The pK a ’s of the amino acid phenylalanine (I) are 2.58 and 9.24. The zwitterionic form of phenylalanine is given below (I). (I) (a) Write the reactions that describe the acid dissociation equilibria of phenylalanine. 1 H C O _ O H 2 C NH 3 + H C O _ O H 2 C NH 3 + H C OH O H 2 C NH 3 + H C O _ O H 2 C NH 3 + H C O _ O H 2 C NH 2 K 1 K 2 + + H + H +
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2. Use the following table to answer the questions about amino acids Selected Amino Acids Found in Proteins Amino acid Structure pK1 (COOH) pK2 (NH2) pK R Group Glycine HC(NH 2 )COOH 2.4 9.6 Serine HOCH 2 CH(NH 2 )COOH 2.2 9.2 Alanine CH 3 CH(NH 2 )COOH 2.4 9.9 Cysteine HSCH 2 CH(NH 2 )COOH 1.9 10.8 8.3 Glutamic acid HOOCCH 2 CH 2 CH(NH 2 )COOH 2.1 9.5 4.1 Lysine H 2 N(CH 2 ) 4 CH(NH 2 )COOH 2.2 9.2 10.8 Phenylalanine (C 6 H 5 )CH 2 CH(NH 2 )COOH 2.6 9.2 (a) What is the pH of a 1.0 M solution of glycine? pH = 2.4 + 9.6 2 = 6.1 (b) What forms of alanine are present at a pH of 2.4? Since this is the pH that corresponds to pK 1 , there are equal amounts of the protonated species and the zwitterions. (c) What forms of cysteine are present at pH 8.3 ? Since this is the pH that corresponds to the pK of the R group where you have an acidic proton on the sulfur, the ratio HS to S- is 1 The forms of the amino acid present are -SCH 2 CH(NH 3 + )COO- and HSCH 2 CH(NH 3 + )COO- in equal amounts. (d) What forms of aspartic acid are present at pH 9.9?
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This note was uploaded on 07/13/2008 for the course CHEM 14CL taught by Professor Henary during the Winter '08 term at UC Davis.

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AminoAcidWorkshopcorrrectedkey - Amino Acid Workshop 1. The...

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