Chemistry Reference
In-Depth Information
combination of these methods can "sequence" a protein, and the methods have been
automated.
Polypeptides are usually synthesized by the
Merrifield solid-phase technique
.
An
N-protected amino acid is linked by an ester bond to a benzyl chloride-type unit in a
polystyrene. The protecting group is then removed, and the next N-protected amino acid is
linked to the polymer-bound one. The cycle is repeated until the peptide is assembled, after
which it is detached from the polymer. The most common N-protecting group is the
t
-butoxycarbonyl (Boc) group:
(CH
3
)
3
COC(=O)-, which is connected to the amino group
using di-
t
-butyldicarbonate and can be removed from the amino group using mild acid. The
reagent used to attach each N-protected amino acid to the growing peptide chain is
dicyclohexylcarbodiimide
.
Detachment of the peptide chain from the polymer is
accomplished using HF.
Two features that affect
secondary protein structure
(molecular shape) include the
rigid, planar geometry and restricted rotation of the peptide bond, and interchain or
intrachain hydrogen bonding of the type C=O
⋅⋅⋅
H-N. The
α
helix
and the
pleated sheet
are
common protein shapes.
Proteins may be
fibrous
or
globular.
The structure and polarity of the particular
amino acid R groups and their sequence affect the solubility properties and
tertiary
structure
of proteins.
Quaternary structure
refers to the aggregation of similar protein
subunits.
Reaction Summary
Dissociation of Amino Acids
H
H
H
HO
-
HO
-
-
-
R
CCO
2
H
NH
3
R
CCO
2
NH
3
R
CCO
2
NH
2
H
+
H
+
+
+
dipolar ion
Esterification
H
H
-
+ H
+
R
CCO
2
NH
3
+
R'OH
R
CCO
2
R'
NH
3
+O
H
2
+
+
Acylation
H
-
H
R
CCO
2
HO
-
2
-
+Cl
-
R
CCO
2
NH
3
+
R'
C
Cl
R'
C
NH
+2
H
2
O
+
O
O
