Question

Tyr-Lys-Met Gly-Pro-Arg Asp-Trp-Tyr Asp-His-Glu Which one of the above tripeptides Leu-Val (a) is most negatively charged at pH 7? (b) will yield DNP-tyrosine when reacted with I-fluoro-2.4-dinitro acid? (e) contains the largest number of nonpolar R (d) contains sulfur? (e) will have the greatest light absorbance at 280 nm? A) (a) D, (b) A (c) E: (d) A, (e) C, B) (a) A: (b) D, (c) E, (d) A, (e) C, C)(a) D, (b) E. (e)A (a A

Answer 1

ans)

from above data that

first we have to note the pK_a and pI values of the amino acids

Amino Acid	pKa1 (α-COOH)	pKa2 (α-NH3+)	pKa3 (side chain)	pI
Tyr	2.20	9.11		5.66
Lys	2.18	8.95	10.53	9.74
Met	2.28	9.21		5.74
Gly	2.34	9.60		5.97
Pro	1.99	10.60		6.30
Arg	2.17	9.04	12.48	10.76
Asp	1.88	9.60	3.65	2.77
Trp	2.83	9.39		5.89
His	1.82	9.17	6.00	7.59
Glu	2.19	9.67	4.25	3.22
Leu	2.36	9.60		5.98
Val	2.32	9.62		5.96
Phe	1.83	9.13		5.48

Again consider the following points about the given tripeptides (all tripeptides are at pH 7.0):

Tripeptide	Amino Acid	Charge on α-COOH	Charge on α-NH3+	Charge on side chain	Overall charge of tripeptide at pH 7.0
A	Tyr	-	+1	-	+1
	Lys	-	-	+1
	Met	-1	-	-
B	Gly	-	+1	-	+1
	Pro	-	-	-
	Arg	-1	-	+1
C	Asp	-	+1	-1	-1
	Trp	-	-	-
	Tyr	-1	-	-
D	Asp	-	+1	-1	-3
	His	-	-	-1
	Glu	-1	-	-1
E	Leu	-	+1	-	0
	Val	-	-	-
	Phe	-1	-	-

a)

Tripeptide D is the most negatively charged at pH 7.0 (ans).

b)

The nucleophilic amine group of an amino acid attacks 1-fluoro-2,4-dinitrobenzene to give a complex. In case of the tripeptide, the DNP-tripeptide complex is obtained.

Upon acid hydrolysis, the tripeptide breaks down, producing the DNP-labelled N-terminal amino acid and unlabeled amino acids.

Since the free N-terminus of a tripeptide can react with 1-fluoro-2,4-dnitrobenzene to give the DNP-labelled amino acid, hence, tripeptide A will give DNP-tyrosine (since only tripeptide A has free N-terminus of the Tyrosine residue).

c)

Look up the structures of the amino acids in a text book or over the internet. Find out the amino acids which have non-polar hydrocarbon groups. Make a list.

The following amino acids have non-polar R groups: Valine, Leucine, Phenylalanine and Proline. Tripeptide E has Leu, Val and Phe and is the tripeptide with the maximum non-polar R groups.

d)

Methionine is a S-containing amino acid and hence, tripeptide A is the correct answer.

e)

Aromatic groups absorb strongly at 280 nm. Look for amino acids containing aromatic groups in the structure. These are: Tyr, Phe, Try and His.

Among these amino acids, Typ absorbs the strongest at 280 nm. Tripeptide C, containing both Tyr and Trp will absorb the strongest at 280 nm.