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Consider the dipeptide with the following structural formula: H H O H | | || | H2N-C---C---C---C---OH | | / | H CH3 CH2SH a - VCE - SSCE Chemistry - Question 8 - 2004 - Paper 1
Question 8
Consider the dipeptide with the following structural formula: H H O H | | || | H2N-C---C---C---C---OH | | / | H CH3 CH2SH a. On the ab... show full transcript
Worked Solution & Example Answer:Consider the dipeptide with the following structural formula: H H O H | | || | H2N-C---C---C---C---OH | | / | H CH3 CH2SH a - VCE - SSCE Chemistry - Question 8 - 2004 - Paper 1
Step 1
a. On the above formula, circle the peptide link.
Answer
The peptide link in the dipeptide structure is formed between the carboxyl group (C=O) of one amino acid and the amino group (NH2) of another amino acid. You would circle the bond that connects these two groups, which is a covalent bond called the peptide bond or linkage.
Step 2
b. Draw a structural formula for one of these amino acids as it would exist in a solution of pH 2.
Answer
At pH 2, the amino acid will be in its protonated form. A typical amino acid, for example, Alanine, would have the following structural formula:
H3N⁺
|
H2C—C—COOH
|
CH3
It includes a positively charged amino group (NH3⁺) and a carboxylic acid group (COOH).
Step 3
Draw a structural formula for the other amino acid as it would exist in a solution of pH 12.
Answer
At pH 12, the amino acid will have a negatively charged carboxylate group and a neutral amino group. For instance, Cysteine would look like:
H2N
|
H2C-C—COO⁻
|
CH2SH
Here, the carboxylic acid group (COOH) has lost a proton to become a carboxylate (COO⁻).
Step 4
c. Nitrogen gas in the atmosphere cannot be used directly by plants to make amino acids. Give the formula of a nitrogen-containing molecule or ion which can be taken up by plants.
Answer
Plants can take up nitrogen in the form of nitrate ions (NO₃⁻) or ammonium ions (NH₄⁺).
One process by which nitrogen in the atmosphere is converted into these forms is called nitrogen fixation. In this process, bacteria such as Rhizobium convert atmospheric nitrogen (N₂) into ammonia (NH₃), which can then be further converted into ammonium or nitrate through nitrification.