Problem 726
In quite alkaline solution, an amino acid contains two basic groups, \(-\mathrm{NH}_{2}\) and \(-\mathrm{COO}^{-}\). Which is the more basic? To which group will a proton preferentially go as acid is added to the solution? What will the product be?
Problem 730
Suggest a way to separate a mixture of amino acids into three fractions: monoamino monocarboxylic acids, monoamino dicarboxylic acids (the acidic amino acids), and diamino monocarboxylic acids (the basic amino acids).
Problem 731
Show how the following amino acids can be prepared from the indicated starting materials. (a) leucine from isobuty1 alcohol (b) lysine from 1,4 -dibromobutane (c) proline from adipic acid (d) glutamic acid from \(\alpha\) -ketoglutaric acid
Problem 732
It is thought that the first amino acids were synthesized from formaldehyde, hydrogen cyanide, ammonia and water in the primitive atmosphere. A possible synthesis involves a series of nucleophilic attacks and proton transfers. Propose a mechanism for the synthesis of glycine using the above mentioned compounds.
Problem 733
Many years before the Hofmann degradation of optically active a-phenylpropionamide was studied, the following observations were made: when the cyclopentane derivative, \(\mathrm{I}\), in which the \(-\mathrm{COOH}\) and \(-\mathrm{CONH}_{2}\) groups are cis to each other, was treated with hypobromite, compound II was obtained; compound II could be converted by heat into the amide III (called a lactam). What do these results show about the mechanism of the rearrangement? (Use models.)
Problem 735
(a) What contributing structure(s) would account for the double-bond character of the carbon-nitrogen bond in amides? (b) What does this resonance mean in terms of orbitals?
Problem 736
Of what importance are proteins to biological svstems?
Problem 737
Hemoglobin, the protein responsible for carrying oxygen from the lungs to the body tissues, contains \(0.355\) per cent iron. Hydrolysis of \(100 \mathrm{~g}\) of hemoglobin gives \(1.48 \mathrm{~g}\) of tryptphan; calculate the minimum molecular weight of hemoglobin which is consistent with these results.
Problem 738
A pentapeptide on complete hydrolysis yields 3 moles of glycine, 1 mole of alanine, and 1 mole of phenylalanine. Among the products of partial hydrolysis are found \(\mathrm{H} \cdot \mathrm{Ala} \cdot \mathrm{Gly} \cdot \mathrm{OH}\) and \(\mathrm{H} \cdot \mathrm{Gly} \cdot \mathrm{Ala} \cdot \mathrm{OH}\). What structures are possible for this substance on the basis of its giving no nitrogen in the Van Slyke determination?
Problem 739
The complete structure of Gramicidin \(\mathrm{S}\), a polypeptide with antibiotic properties, has been worked out as follows: (a) Analysis of the hydrolysis products gave an empirical formula of Leu, Orn, Phe, Pro, Val. (Ornithine, Orn, is a rare amino acid of formula \(\left.{ }^{+} \mathrm{H}_{3} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}\left(\mathrm{NH}_{2}\right) \mathrm{COO}^{-} .\right)\) It is interesting that the phenylalanine has the unusual D-configuration. Measurement of the molecular weight gave an approximate value of \(1300 .\) On this basis, what is the molecular formula of Gramicidin \(\mathrm{S} ?\) (b) Analysis for the C-terminal residue was negative; analysis for the N-terminal residue using DNFB yielded only DNP-NHCH \(_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}\left(\mathrm{N}^{+} \mathrm{H}_{3}\right) \mathrm{COO}^{-}\). What structural feature must the peptide chain possess? (c) Partial hydrolysis of Gramicidin \(\mathrm{S}\) gave the following di- and tripeptides: Leu.Phe Phe.Pro Phe.Pro.Val Val.Orn.Leu \(\begin{array}{lll}\text { Orn.Leu } & \text { Val.Orn } & \text { Pro. Val.Orn }\end{array}\) What is the structure of Gramicidin \(\mathrm{S} ?\)
