24_all_ - 24.1 What form of glutamic acid would you expect...

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24.1 What form of glutamic acid would you expect to predominate in: (a) strongly acidic solution? (b) strongly basic solution? (c) at its isoelectric point ( pI 3.2)? (d) The isoelectric point of glutamine ( pI 5.7) is considerably higher than that of glutamic acid. Explain. Answer: (a) NH 3 O HO O OH (b) NH 2 O O O O (c) NH 3 O O O OH (d) Because pI =(pKa 1 +pKa 2 )/2, and obviously glutamine is much more basic than glutamic acid. It lead to the higher isoelectric point of glutamine . 24.2 The guanidino group H N C NH NH 2 of arginine is one of the most strongly basic of all organic groups. Explain. Solution: The cation form of the guanidino group has three resonance structures as follows: H N C NH 2 NH 2 H N C NH 2 NH 2 N H C NH 2 NH 2 In another word, the cation is very stable. So it is more ready to attract a proton, and is one of the most strongly basic groups.
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24.3 Starting with diethyl α -bromomalonate and potassium phthalimide and using any other necessary reagents show how you might synthesize: (a) DL -leucine, (b) DL -alanine, and (c) DL -phenylalanine. Answer: (a) NK O O + BrCH(CO 2 C 2 H 5 ) 2 (82-85%) NCH(CO 2 C 2 H 5 ) 2 O O NaOCH 2 CH 3 ClCH 2 CH(CH 3 ) 2 NC CH(CH 3 ) 2 O O CO 2 C 2 H 5 CO 2 C 2 H 5 NaOH CO 2 C O NHC CH(CH 3 ) 2 CO 2 CO 2 HCl (84-85%) CH 3 CHCH 2 CHCO 2 NH 3 CH 3 + CO 2 + COH O COH O NK O O + BrCH(CO 2 C 2 H 5 ) 2 (82-85%) NCH(CO 2 C 2 H 5 ) 2 O O NaOCH 2 CH 3 CH 3 I NC O O CO 2 C 2 H 5 CO 2 C 2 H 5 NaOH CO 2 C O NHC CO 2 CO 2 HCl (84-85%) CH 3 CHCO 2 NH 3 + CO 2 + COH O COH O (b) CH 3 CH 3
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NK O O + BrCH(CO 2 C 2 H 5 ) 2 (82-85%) NCH(CO 2 C 2 H 5 ) 2 O O NaOCH 2 CH 3 PhCH 2 Cl NC Ph O O CO 2 C 2 H 5 CO 2 C 2 H 5 NaOH CO 2 C O NHC CH 2 Ph CO 2 CO 2 HCl (84-85%) PhCH 2 CHCO 2 NH 3 + CO 2 + COH O COH O (c) 24.4 (a) Outline a Strecker synthesis of DL-phenylalanine. (b) DL-Methionine can also be synthesized by a Strecker synthesis. The required starting aldehyde can be prepared from acrolein (CH 2 =CHCHO) and methinethiol (CH 3 SH). Outline all steps in this synthesis of DL-methionine. Answer: (a) NH 3 + HCN PhCH 2 CHCN H 3 O + ,heat H 2 O + NH 2 PhCH 2 CHCOO - NH 3 + CH 2 CHO (b) CH 3 SH + H 2 C CHCHO CH 3 SCH 2 CH 2 CHO 3 HCN CH 3 SCH 2 CH 2 CHCN NH 2 H 3 O + ,heat H 2 O CH 3 SCH 2 CH 2 CHCOO - NH 3 + base 24.5: The electron-withdrawing property of the 2,4-dinitrophenyl group makes separation of the labeled amino acid very easy. Suggest how this is done. Answer: Because the nitrogen atom is conjugated with 2,4-dinitrophenyl group, the labeled amino acid no longer has a –NH 2 basic group. Therefore it is insoluble in aqueous acid 24.6 (a) Write a reaction showing how 2,4-dinitrofluorobenzene could be used to identify the N-terminal amino acid of Val . Ala . Gly. (b) What products would you expect (after hydrolysis) when Val . Lys . Gly is treated with 2,4-dinitrofluorobenzene?
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Answer: (a) F NO 2 O 2 N + NH 2 CH CH C O H N H C CH 3 C O O NHCH 2 COO H H 3 C CH 3 Val.Ala.Gly NHCHCONHCHCH 2 OONHCH 2 COOH NO 2 O 2 N CH H 3 C CH 3 CH 3 NHCHCH 2 OOH NO 2 O 2 N CH CH 3 H 3 C + NH 2 CHCOOH CH 3 labeled N-terminal amino acid + NH 2 CH 2 COOH (b) H N O 2 N NO 2 CHCOOH CH H 3 C CH 3 + O 2 N NO 2 H N CH 2 CH 2 CH 2 CH 2 CHCOOH NH 2 + NH 2 CH 2 COOH 24.7 Write the reactions involved in a sequential Edman degradation of Met.Ile.Arg.
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24_all_ - 24.1 What form of glutamic acid would you expect...

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