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Amino acids
Amino acids
•Tetrahedral alpha () carbon (C) with
H, amino group, carboxyl group, and
side chain (so-called R group).
•Neutral solution (pH 7): The Carboxyl
group –COO- and the amino group –
NH3+. A neutral molecule called a
zwitterion.
Peptide bond
•Amino acids in proteins joined by peptide bonds.
•Head-to-tail fashion
•Eliminating a water molecule
•Forming a covalent amide linkage
•Polypeptides and proteins
20 amino acids
•Nonpolar or hydrophobic amino acids
•Neutral (uncharged) but polar amino acids
•Acidic amino acids (which have a net
negative charge at pH 7.0)
•Basic amino acids (which have a net positive
charge at neutral pH)
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Nonpolar amino acids
(a) Nonpolar (hydrophobic)
• With alkyl (烷基) chain R groups (alanine, valine,
leucine, and isoleucine).
• With its unusual cyclic structure (proline).
• One of the two sulfur-containing amino acids
(methionine).
• Two aromatic (芳香基) amino acids (phenylalanine
and tryptophan).
(a) Nonpolar (hydrophobic)
Polar amino acids
•These amino acids are usually more soluble in
water than the nonpolar amino acids. Glycine
contain R group that can form hydrogen (H) bonds
with water.
•Tyrosine displays the lowest solubility in water of
the 20 common amino acids.
•Proline is very soluble in water.
•Alanine and valine are about as soluble as
arginine and serine.
Polar amino acids
(b) Polar, uncharged
•The amide groups of asparagine and glutamine;
the hydroxyl groups of tyrosine, threonine, and
serine; and the sulfhydryl group of cysteine are all
good hydrogen bond-forming moieties.
•Glycine, the simplest amino acid, has only a single
hydrogen for an R group, and this hydrogen is not
a good hydrogen bond former. The solubility
properties are mainly influenced by its polar amino
and carboxyl groups, and it is best considered a
member of the polar, uncharged group.
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(b) Polar, uncharged
Acidic amino acids
•Aspartic acid and glutanic acid.
•R group contain a carboxyl group.
•Have a net negative charge at pH 7.
•Play several important roles in proteins.
Basic amino acids
•Histidine, arginine, and lysine.
(c) Acidic
•Side chains with net positive charges at neutral pH.
•Histidine side chains play important roles as proton
donors and acceptors in many enzyme reaction.
•Histidine-containing peptides are important
biological buffers.
•Arginine and lysine side chains, which are
protonated under physiological conditions,
participate in electrostatic interactions in proteins.
(d) Basic
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Uncommon amino acids
•Several amino acids occur only rarely in
proteins.
Some amino acids are not found
in proteins
collagen and gelatin protein
Iodinated amino acids;thyroglobuline
a protein produced by thyroid gland
In protein isolated
from corn
In several proteins
involved in blood
clotting
In a unique lightdriven protonpumping protein
called
bacteriorhodopsin
Certain muscle proteins contain methylated amino acids
•Certain amino acids and their derivatives,
although nor found in proteins, nonetheless
are biochemically important.
Certain protein involved in cell growth
and regulation
In histone
proteins
associated with
chromosomes
Acid-base properties of amino acids
A part of coenzyme A
neurotransmitter
•The common amino acids are all weak
polyportic acids.
•All the amino acids contain at least two
dissociable hydrogens.
Neurotransmitters
and regulators
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Glycine
Glycine
Amino acids undergo typical reactions
•Carboxyl group reaction (COOH)
•Amino group reaction (NH3+)
Glutamic acid
Lysine
5
Amino acids side chains undergo
specific reaction
•A number of reactions of amino acids are
noteworthy because they are essential to the
degradation, sequencing, and chemical synthesis of
peptides and proteins.
Green fluorescent Protein (GFP) & Aequorin
•
Serine
•
Tyrosine
•
Glycine
Amino acids are chiral molecules
•Asymmetric (不對稱的 ) or chiral (對掌
性) (from Greek cheir, meaning “
hand”
)
•Mirror-image isomer or enantiomers (鏡
像異構物)
•Optical activity: dextrorotatory (+) (右旋)
and levorotatory (-) (左旋)
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Chiral molecules are described
by two systems
•D,L system
-- Mirror-image isomer or enantiomers
•R,S system
-- Non-image isomer or diastereomers (非
鏡像異構物)
R,S system
•The configuration of molecules with
more than one chiral center can be
more easily, completely, and
unambiguously.
•Several amino acids have two chiral
centers: isoleucine, threonine,
hydroxyproline, and hydroxylsine.
•R-right; S-left
•Higher atomic numbers having higher priorities.
-- SH > OH > NH2 > COOH > CHO > CH2OH > CH3
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Spectroscopic properties of
amino acids 分光光譜
•Spectroscopic methods
-- measure the absorption and emission
of energy of different frequencies by
molecules and atoms.
•Phenylalanine, thyrosine and tryptophan
exhibit significant ultraviolet (UV)
absorption above 250 nm.
Nuclear Magnetic Resonance
(NMR) 核磁共振
•A spectroscopic technique that involves the
absorption of radio frequency energy by
certain nuclei in the presence of a magnetic
field, played an important part in the
chemical characterization of amino acids
and proteins.
Chromatography 層析
•Partition properties
•Electrical charge
•Ion exchange chromatography, gas
chromatography (GC), and high-performance
liquid chromatography (HPLC).
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