Chemistry Reference
In-Depth Information
17.53
Trypsin cleaves peptides at the carboxyl side of Lys and Arg. The only fragment that
does not end in Lys is the dipeptide. Therefore, it must come at the C-terminus of
endorphin, and the C-terminal amino acid is Gln.
Cyanogen bromide cleaves peptides at the carboxyl side of Met. There is only one
Met in the structure (see the fifth trypsin fragment), and the cyanogen bromide
cleavage gives a hexapeptide consisting of the first six amino acids of this fragment.
The N-terminal amino acid must be Tyr. We now know the first ten and the last two
amino acids of
β
-endorphin.
Chymotrypsin cleaves peptides at the carboxyl side of Phe, Tyr, and Trp. Working
backward in the 15-unit fragment, we see that it starts at the ninth amino acid in the
last fragment from the trypsin digestion. We see that the first amino acid in this
fragment (Ser) must be connected to the Lys at the end of the fifth fragment from the
trypsin cleavage. Since this fragment begins with the N-terminal amino acid, we now
know the sequence of the first twenty and the last two amino acids. The partial
structure of
β
-endorphin must be
Tyr-Gly -Gly -Phe-Leu-Met -Thr-Ser-Glu-Lys-Ser-Gln-Thr-Pro-Leu-Val
Thr-Leu-Phe-Lys-(Lys)(Asn -Ala -His-Lys)(Asn -Ala -Ile-Val -Lys)-Gly -Gln
We cannot deduce, from the data given, the sequence of the amino acids in
parentheses. To put them in order, we must seek peptides from partial hydrolysis
that contain overlapping sequences of these amino acids.
17.54
Use Table 17.3 as a guide.
a.
With trypsin, we expect four fragments:
HSEGTFTSDYSK
YLDSR
R
AQDFVQWLMNT
b.
With chymotrypsin, we expect six fragments:
HSEGTF
TSDY
SKY
LDSRRAQDF
VQW
LMNT
17.55
The nucleophile is the carboxylate anion; the leaving group is chloride ion from the
benzyl chloride. Benzyl halides react rapidly in S
N
2 displacements.
H
O
S
N
2
-
NH
C
C
O
CH
2
Cl
R
1
protecting group
H
O
NH
C
C
O
CH
2
R
1
polystyrene backbone
