2. 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

3. 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

4. 水是威力強大的小分子

5. 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 ? *

6. “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: 原子核原子 簡單化合物 生物巨分子 宇宙 簡單 複雜

7. The Four Weak Interactions * 共通特質： 1. 非共價 2. 可逆 3. 專一

8. 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

9. 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

10. Common Hydrogen Bonds * Peptides X-H:::A X= Hydrogen bond donor N, O, S A= Hydrogen bond acceptor O, N

11. Hydrogen bonding in ice 冰為何浮 在水上？ Ice crystal: static Water: dynamic breaking and forming

12. 重要生物分子間氫鍵

13. Hydrogen Bond Strength is Highly Directional Fig. 2-5

14. 2. Ionic Interactions 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= Q 1 Q 2 / r 2 : dielectric constant r: distance between the charge groups

15. Solvent Properties of H 2 O  Ionic compounds (e.g.,KCl) and low- molecular- weight polar covalent compounds (e.g., C 2 H 5 OH and CH 3 COCH 3 ) 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 H 2 O –dipole-dipole interactions: e.g., ethanol or acetone dissolved in H 2 O –dipole induced-dipole interactions: weak and generally do not lead to solubility in water

16. Dipole-Dipole Ion-Dipole

17. Salts Dissolve in Aqueous “Cages of Hydration” Fig. 2-6 還有什麼因素讓鹽很容易溶於水？

18. 氧既是非極性，又如何支持生命？

19. 3. Hydrophobic Interactions  They are about 2% the strength of a carbon- carbon bond (8 vs. 350 kJ/mole)  Water becomes highly ordered around hydrophobic groups  Greasy chains stay together to minimize entropy loss by water

23. 水的退出有利酵素與受質作用

24. 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 transiently- induced dipoles  Repulsion when the electron clouds bump together