Question

1. What is a peptide bond? 2. How does the primary structure of proteins differ from the secondary structure? A. Peptide Bonds A.1 Structure of glycylserine Structure of serylglycine A.2 Hydrolysis of serylglycine

Q.1 Why are heat and alcohol used to disinfect medical equipment?

Q.2 Why is milk given to someone who accidentally ingests a heavy metal ion such as silver or mercury?

Q.5 After working with HNO3, a student noticed that she had a yellow spot on her hand. What might be the reason?

Q.6 Which samples above give a negative biuret test? Why?

Q.7 What functional group gives a positive test in the xanthoproteic test?

Q.8 What tests could you use to determine whether an unlabeled test tube contained an amino acid or a protein?

Answer 1

1. A peptide bond is a chemical bond which is formed when two amino acid groups undergo dehydration reaction. Amino acids have carboxyl groups and amino groups. When the carboxyl group of one amino acid reacts with the amino acid group of another amino acid, one molecule of water is eliminated and a covalent bond is formed between the two amino acids. This is known as peptide bond. I have shown one such example in the figure attached.

2. Primary structure of a protein is the sequence of amino acids forming peptide bonds. The constituent amino acids of a protein forms peptide bonds with one another to form the backbone of the protein. This chain is the primary structure. After this, the amino acids in a peptide chain undergoes hydrogen bonding and other interactions to form several shapes mainly alpha helix and beta sheets which are characterised as the secondary structure of the protein.

A. peptide bonds are already explained in question 1.

A1. Glycylserine is the formed when amino acid glycine and serine forms a peptide bond. Serylglycine is formed when serine and glycine forms a peptide bond. The structures are shown in the image attached.

A2. Hydrolysis of serylglycine shall yield the two constituent amino acids i.e. serine and glycine. The reaction is addition of water to break the peptide bond. It is shown in attached figure.

Q1. Heat and alcohol are used to disinfect medical instruments because both of these factors denatures a protein and thus kills all the germs, fungi or bacteria etc. Denaturation of a protein means destruction of its tertiary and secondary structures and rendering it non-functional. Both heat and alcohol breaks hydrogen bonding structures of the protein. As said above hydrogen bonding between the amino acids are main reason of forming secondary structures as well as tertiary structures. So rupturing hydrogen bonding in a protein by heat and alcohol destroys its functional form and denatures it.

Q2. Milk contains various proteins. When a person consumes heavy metals such as silver or mercury, he/she is given milk so that the heavy atoms reacts to the proteins in milk rather than the proteins in that persons body. Heavy atoms have properties to react and disrupt protein structures. It can attack different proteins in a human body. So milk is given which has large amount of proteins and the milk proteins reacts with heavy atoms and the body is not harmed.

Q5. Nitric acid HNO3 reacts with amino acids containing aromatic groups to produce the yellow colour. Human skins have proteins such as keratines. These proteins contains many aromatic amino acid residues such as tyrosine. When HNO3 comes in contact with the skin, it reacts with those amino acids and makes the skin yellow. This is known as the Xanthoproteic reaction.

Q6. The question mentions "samples above", but none such samples are mentioned in the question.

Q7. As mentioned in the answer for Q5, amino acids containing aromatic rings such as tyrosine undergoes Xanthoproteic reaction. The reaction is nitration of the benzene ring of the amino acid. The reaction produces a yellow colored product. Thus it is know as the Xanthoproteic test.

Q8. Biuret test is a process of detecting the presence of peptide bonds. Since simple amino acids do not contain peptide bonds, only proteins does, the detection of proteins can be done using this test. Biuret reagent is sodium hydroxide, copper sulfate and potassium sodium tartrate. When a protein is present addition of this mixture shall result in a pink colouration.