Transcript Chapter 2: Biomolecules in Water 水做為溶劑之特殊性 非共價作用力 pH

Chapter 2: Biomolecules in Water
水做為溶劑之特殊性
非共價作用力
解離常數與pH
緩衝溶液
水為生物的首選溶劑

Biological solvent: a medium
for metabolic reactions
– aquaporins: membrane channels
for water transport


Essential buffer to regulate
temperature and pH: high
specific heat capacity
and pH-regulating substances
dissolved in it
Participant in many
biochemical reactions:
photosynthesis and hydrolysis
of ATP
水是威力強大的小分子
Weak Chemical Bonds
Non-covalent interactions:
–
–
–
–
Hydrogen bonds氫鍵
Ionic interactions離子交互作用(靜電吸引力)
Hydrophobic interactions (厭水性交互作用)
van der Waals interactions (凡得瓦爾力)
這些弱的維繫力重要性何在?
*
“The more complex the system, the
weaker are the forces that govern its
behavior”
- J. R. Platt
較強
較弱
Forces: 核間力 原子力 共價鍵
非共價鍵
重力
_________________________________________
Structure: 原子
簡單
核原子 簡單化合物 生物巨分子 宇宙
複雜
The Four Weak Interactions
*
共通特質：
1. 非共價
2. 可逆
3. 專一
The Polar Nature of Water

Given the difference in
electronegativity between
oxygen and hydrogen
(3.5 - 2.1 = 1.4) and its
shape, water is a polar
molecule with a dipole
moment of 1.85D
– the net charge on oxygen
is -0.66 and that on each
hydrogen is +0.33
1. Hydrogen Bonds





< 5% of the bond strength of a O-H (20 vs. 460
kJ/mole)
Much longer bond distance: 0.18 vs. 0.096 nm
Highly directional
Based on the dipole moment of water
Account for the unusual properties of water as both
substance and solvent
Common Hydrogen Bonds
X-H:::A
X= Hydrogen bond donor N, O, S
A= Hydrogen bond acceptor O, N
Peptides
*
Hydrogen bonding in ice
Ice crystal: static
Water: dynamic
breaking and
forming
冰為何浮
在水上？
重要生物分子間氫鍵
Hydrogen Bond Strength is Highly
Directional
Fig. 2-5
2. Ionic Interactions


They are about 5-10% the strength of a
carbon-carbon bond (20-40 vs. 350
kJ/mole)
Ionically stabilized compounds (like NaCl)
are readily dissolved in solvents with a
high dielectric constant (like water)
Why?
F= Q1Q2 / er2
e: dielectric constant
r: distance between the charge groups
Solvent Properties of H2O
Ionic compounds (e.g.,KCl) and lowmolecular- weight polar covalent compounds
(e.g., C2H5OH and CH3COCH3) tend to
dissolve in water
 The underlying principle is electrostatic
attraction of unlike charges; the positive
dipole of water for the negative dipole of
another molecule, etc.

– ion-dipole interaction: e.g., KCl dissolved in H2O
– dipole-dipole interactions: e.g., ethanol or acetone
dissolved in H2O
– dipole induced-dipole interactions: weak and
generally do not lead to solubility in water
Dipole-Dipole
Ion-Dipole
Salts Dissolve in Aqueous
“Cages of Hydration”
Fig. 2-6
還有什麼因素讓鹽很容易溶於水？
氧既是非極性，又如何支持生命？
3. Hydrophobic Interactions



They are about 2% the strength of a carboncarbon bond (8 vs. 350 kJ/mole)
Water becomes highly ordered around
hydrophobic groups
Greasy chains stay together to minimize
entropy loss by water
水的退出有利酵素與受質作用
4. van der Waals Interactions



They are about 1% the strength of a
carbon-carbon bond (4 vs. 350 kJ/mole)
Attraction between atoms by transientlyinduced dipoles
Repulsion when the electron clouds bump
together