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 \(\mathrm{S} ?\)

Step by step solution

01

Find the molecular formula based on empirical formula and molecular weight

First, we have to calculate the molecular weight of the empirical formula: Leu, Orn, Phe, Pro, and Val. Then we will multiply the empirical formula with an integer to get a molecular weight close to the given value of 1300. Molecular weight of Leu (Leucine) = 131 Molecular weight of Orn (Ornithine) = \(HNCH_2 CH_2 CH_2 CH(NH_2) COO^{-}\) = 132 (ignoring charges) Molecular weight of Phe (Phenylalanine) = 165 Molecular weight of Pro (Proline) = 115 Molecular weight of Val (Valine) = 117 Total molecular weight of empirical formula = 131 + 132 + 165 + 115 + 117 = 660. Now, we will multiply this empirical formula with an integer such that the resulting molecular weight is close to 1300. In this case, multiplying the empirical formula by 2 will give us a molecular weight of 660 × 2 = 1320, which is close to 1300. So, the molecular formula of Gramicidin S is 2(Leu, Orn, Phe, Pro, Val).

02

Analyze the structural feature based on the N-terminal residue result

The given information about the N-terminal residue analysis states that it only yielded the derivatized Ornithine (Orn). This indicates that the polypeptide chain does not have any free N-terminus, which is possible only if the chain is circular. Therefore, the polypeptide chain of Gramicidin S must be cyclic.

03

Determine the structure of Gramicidin S using partial hydrolysis data

Using the information about dipeptides and tripeptides obtained from partial hydrolysis, we will construct the cyclic structure of Gramicidin S. From the given information, we have the following peptide sequence: Leu.Phe, Phe.Pro, Phe.Pro.Val, Val.Orn.Leu, Orn.Leu, Val.Orn, Pro.Val.Orn Combining these sequences, we get the order of amino acids in the cyclic polypeptide chain as: Leu-Phe-Pro-Val-Orn-Leu-Val-Orn-Phe-Pro Hence, the structure of Gramicidin S is a cyclic polypeptide chain with the amino acid sequence: Leu-Phe-Pro-Val-Orn-Leu-Val-Orn-Phe-Pro

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