CHAPTER 2 PROTEIN
Section1 Amino Acids
Outline
4.1,Amino Acids,Building Blocks of Proteins
4.2,Acid-Base Chemistry of Amino Acids
4.3,Reactions of Amino Acids
4.4,Optical Activity and Stereochemistry of
Amino Acids
4.5,Spectroscopic Properties of Amino Acids
4.6,Separation and Analysis of a.a,Mixtures
Amino Acids
Building Blocks of Proteins
----C-C-C-C-COOH
γ β α
----C-C-C-C(NH2)-COOH α- Amino Acid
----C-C-C (NH2)- C - COOH β- Amino Acid
----C-C (NH2)- C - C - COOH γ- Amino Acid
In protein chemistry,we use Greek letter nomenclature.
Amino Acids Can Join Via Peptide Bonds
20 Common Amino Acids
You should know names,structures,pKa
values,3-letter and 1-letter codes
Non-polar amino acids
Polar,uncharged amino acids
Acidic amino acids
Basic amino acids
Non-polar amino acids
Polar,uncharged amino acids
Acidic amino acids
Basic amino acids
Uncommon Amino Acids
We'll see some of these in later chapters
Hydroxylysine,hydroxyproline - collagen
Carboxyglutamate - blood-clotting proteins
Pyroglutamate - bacteriorhodopsin
Phosphorylated amino acids - signaling
device
Amino Acids Not Found in Proteins
4.2 Acid-Base Chemistry
Amino Acids are Weak Polyprotic Acids
H2A+ + H2O? HA0 + H3O+
Ka1 = [ HA0 ] [ H3O+ ]
__________________________
[H2A+ ]
4.2 Acid-Base Chemistry
The second dissociation (the amino group in
the case of glycine):
HA0 + H2O? Aˉ+ H3O+
Ka2 = [ Aˉ ] [ H3O+ ]
_______________________
[ HA0 ]
pI (isoelectric point)
The pI is the pH at which net charge is zero
( net charge = 0 )
AA的 pI公式推导
H +
H2N C
COO﹣
H
R
+ H +
COOˉ
C
R
H+H3N
COOH
K′1
K′2
C+ H3N H
R
+
K2= [H+] * [AA-] / [AA± ]
K1 = [H+] * [AA± ] / [AA+]
K1 = [H+] * [AA± ] / [AA+]
K2= [H+] * [AA-] / [AA± ]
[AA+] = [H+] *[AA± ] / K1
[AA-] = K2 * [AA± ] / [H+]
反应平衡时,[AA+] = [AA-]
[H+] * [AA± ] / K1 = K2 * [AA± ] / [H+]
[H+]2 = K2* K1 取负对数,-2lg [H+] = -lgK1 -lg K2
∵ PH=-lg [H+],PK1 = -lg K1,P K2 = - lg K2
∴ 2PH = P K1 +P K2
PI = PH = ( P K1 + P K2 ) / 2
An acidic amino acid pI=(pK1+pKR)/2
A basic amino acid pI=(pKR+pK2)/2
Reactions of Amino Acids
Carboxyl groups form amides & esters
Amino groups form Schiff bases and
amides
Side chains show unique reactivities
– Cys residues can form disulfides and can be
easily alkylated
A.
B,Biuret reaction(双缩脲反应）
Protein( or peptides) + CuSO4 碱性 紫红 色
C,Folin reaction
Tyr + Folin reagent 碱性 蓝色 化合物
D,Sakoguchi reaction(坂口反应）
Arg + α-萘酚 NaBrO 红色 产物
E,Pauly reaction
His (or Tyr) + Pauly reagent 橘红色
F,Glyoxalate reaction(乙醛酸反应）
Trp + 乙醛酸 +浓硫酸 紫红色 物质（环）
G,Cys reaction
Cys + HNO2-铁氰化钠（甲醇） 红色
G,Salkowski reaction(黄色反应）
Tyr ( or Trp) +浓 HNO3 黄色（酚类）
H,Millon reaction
Tyr + Millon reagen 粉红色（暗红色
）
Trp + Millon reagent 黄色至红色橙黄色（醌类）
I,醋酸铅反应
Cys
R-SH + 2NaOH R-OH + Na2S + H2O
Na2S + Pb2+ 2PbS + 2Na+
PbS + 2HCl PbCl2 + H2S
J,Ehrlich reaction (偶氮反应）
Tyr Ehrlich reagent 橙红
His 鲜红反应名称
Reaction
试剂
Reagent
颜色 反应有关基团 有关反应的
protein or aa
双缩脲反应
Biuret
加 NaOH及少量稀 CuSO4
紫色或粉红色肽键 All protein or
poly peptides
Millon HgNO3＆
Hg(NO3)2,HNO3
混合物共热红色 苯环 Tyr
Trp
乙醛酸反应
glyoxalate
乙醛酸试剂及浓硫酸紫色 吲哚基 Trp
茚三酮反应
ninhydrin
茚三酮 兰紫色 自由氨基及羧基
aa
Folin 碱性硫酸铜及磷钨酸 -钼酸兰色 酚基、吲哚基 Tyr
Sakoguchi
坂口
a-萘酚，次溴（
氯）酸钠红色 胍基 Arg
反应名称
Reaction
试剂
Reagent
颜色 反应有关基团 有关反应的
protein or aa
黄色反应
Salkowshi
浓 HNO3和碱 黄色 -?棕色苯环 Tyr
Trp
Pauly 对氨基苯磺酸盐酸 ＆ HNO2,
HNO3 混合物
0~4℃
橘红色 咪唑、酚基 Tyr
His
乙醛酸反应
glyoxalate
乙醛酸试剂及浓硫酸紫色 吲哚基 Trp
偶氮反应
Ehrlich
橙红鲜红酚基咪唑
Tyr
His
Cys HNO2-铁氰化钠
，甲醇溶液红色 巯基 Cys
醋酸铅 NaOH,Pb2+、
HCl
黑色沉淀臭鸡蛋味巯基 Cys
Stereochemistry of Amino Acids
All but glycine are chiral
L-amino acids predominate in nature
D,L-nomenclature is based on D- and L-
glyceraldehyde
D-型 L-型
Levorotatory
Levo
dextrorotatory
dextro
Spectroscopic Properties
All amino acids absorb in infrared region
Only Phe,Tyr,and Trp absorb UV
Absorbance at 280 nm is a good
diagnostic device for amino acids
Separation of Amino Acids
Mikhail Tswett,a Russian botanist,first
separated colorful plant pigments by
‘chromatography’
Many chromatographic methods exist for
separation of amino acid mixtures
– Ion exchange chromatography
– High-performance liquid chromatography