Protein Engineering Questions and Answers for Campus interviews on “Chemical Synthesis of Peptides – 2”.
1. What is the total number of dipeptides and tripeptides that can be formed when two solutions of different amino acids are mixed?
A. Twenty-four
B. Sixteen
C. Ten
D. Twelve
Answer: D
Clarification: The total number of dipeptides and tripeptides that can be formed when two solutions of different amino acids are mixed is twelve. E.g. if those two different amino acids are trp and cys then the possible combinations of dipeptides and tripeptides are trp-cys, trp-trp, cys-cys, cys-trp, cys-cys-cys, trp-trp-trp, trp-cys-cys, cys-trp-cys, cys-cys-trp, trp-trp-cys, trp-cys-trp, and cys-trp-trp. This can also be calculated by using the formula given below,
Formula: n=xy
Where x=number of different amino acids used,
y=number amino acid positions in the peptides.
Hence, n=23+22
n=12.
2. The side group of serine is protected by which of the following protecting groups?
A. A tertiarybutyloxycarbonyl (tBOC.
B. 2,2,5,7,8-pentamethylchroman-6-sulphonyl group (PMC.
C. A triphenylmethyl group (Trt)
D. A tertiary butyl group (tBu)
Answer: D
Clarification: The side group of serine is protected by the tertiary butyl group (tBu). Serine has a hydroxyl group in its side chain. Hence, it cannot be protected by tertiarybutyloxycarbonyl (tBOC., 2,2,5,7,8-pentamethylchroman-6-sulphonyl group (PMC., or triphenylmethyl group (Trt).
3. The coupling reaction in the SPPS is catalyzed by which of the following?
A. 1,4-dicyclohexylcarbodiimide
B. 2,3-dicyclohexylcarbodiimide
C. 1,2-dicyclohexylcarbodiimide
D. 1,3-dicyclohexylcarbodiimide
Answer: D
Clarification: In the coupling reaction the FMOC protected amino acid is reacted with the last amino acid attached to the polyamide. The coupling reaction in the SPPS is catalyzed by 1,3-dicyclohexylcarbodiimide. In the reaction process, 1,3-dicyclohexylcarbodiimide (DCC. is itself reduced to 1,3-dicyclohexylurea (DCU).
4. In the deprotection step of SPPS, the FMOC group is removed with the help of which of the following reagent?
A. Fluorobenzene
B. Trifluoroacetic acid
C. 1,3-dicyclohexylcarbodiimide
D. Piperidine
Answer: D
Clarification: In the deprotection step of SPPS, the FMOC group is removed with the help of piperidine. Piperidine is a cyclic secondary amine. This is a trans amidification reaction.
5. How many combinations of tripeptides can be formed if two solutions of different amino-acids are mixed?
A. Twelve
B. Sixteen
C. Thirty-two
D. Eight
Answer: D
Clarification: Eight different combinations of tripeptides can be formed if two solutions of different amino-acids are mixed. E.g. if those two different amino acids are trp and cys then the possible combinations of tripeptides are cys-cys-cys, trp-trp-trp, trp-cys-cys, cys-trp-cys, cys-cys-trp, trp-trp-cys, trp-cys-trp, and cys-trp-trp. This can also be calculated by using the formula given below,
Formula: n=xy
Where x=number of different amino acids used,
y=number amino acid positions in the peptides.
Hence, n=23
n=8.
6. The side group of cysteine is protected by which of the following protecting groups?
A. 2,2,5,7,8-pentamethylchroman-6-sulphonyl group (PMC.
B. A tertiary butyl group (tBu)
C. A tertiarybutyloxycarbonyl (tBOC.
D. A triphenylmethyl group (Trt)
Answer: D
Clarification: The side group of cysteine is protected by a triphenylmethyl group (Trt). The side chain of cysteine contains a sulfhydryl group. Hence, it cannot be protected by tertiarybutyloxycarbonyl (tBOC., 2,2,5,7,8-pentamethylchroman-6-sulphonyl group (PMC., or triphenylmethyl group (Trt).
7. If the yield for each amino acid addition in SPPS is 99 % then, what will be the overall yield of a 40 amino acid peptide?
A. 64.1 %
B. 76.8 %
C. 86.9 %
D. 66.9 %
Answer: D
Clarification: If the yield for each amino acid addition in SPPS is 99 % then, the overall yield of a 40 amino acid peptide would be around 66.9 %. This can be calculated by using the formula given below.
Formula: Y=yx((frac{y}{100}))n-1
Where Y=the overall yield
y=yield for each amino acid addition
n=number of amino acids in the peptide
Hence, Y=99x((frac{99}{100}))40-1
Y=66.9%.
8. How many combinations of tetrapeptides can be formed if two solutions of different amino-acids are mixed?
A. Twelve
B. Nine
C. Twenty-four
D. Sixteen