Properties of water

1. Properties of water • Polarity • Hydrogen bonding potential • Specific heat, heat of vaporization • Nucleophilic • Ionization It is the unique combination of properties of water that make it the perfect solvent for biological systems. We will discuss each of these properties in more detail.

2. Water and pH • Water - one of the most important molecules in life. • 70% of the bodies mass is water • 2/3 of total body water is intracellular (55-66% body weight of men and 10% less for women) • The rest is interstitial fluid of which 25% is in the blood plasma. pH - The body tightly controls both the volume and pH of water. • The bicarbonate system is crucial for blood maintenance • changes of pH greater than 0.1 are dangerous and can lead to coma -diabetics

3. Life on Earth would not be possible without water Its chemical and physical properties actually defy some fundamental laws of physicsAlmost all biochemical reactions require water!

5. Water is an ideal biological solvent • Water is close to a tetrahedral shape with the unshared electrons on the two sp3-hybridized orbital are in two corners and the hydrogen in others Compared to a tetrahedron, CH4 (109o) or NH3 the bond angle is smaller (109.5o and 107o vs.104.5o) Due to the bent shape of the molecule, there is an uneven distribution of electrons. It is a polar molecule, -a dipole

6. Water is a polar molecule • Water has a dipole moment • “like dissolves like” Oxygen is more electronegative than hydrogen so there is an uneven distribution of charge in this H-O bond Uneven distribution is called a dipole and the bond is said to be polar

7. Water has hydrogen bonding potential • H-bonds are non-covalent, weak interactions • H2O is both a Hydrogen donor and acceptor • One H2O can form up to four H-bonds

8. Kq q 1 2 F = 2 D r What makes this molecule important? • solvent ability - easily disrupts ionic compounds • dielectric constant (D) is high (measure of the ability to keep ions apart) • Coulombs Law - The force of attraction • between two charged pairs

9. Kq q 1 2 F = 2 D r What makes this molecule important? • solvent ability - easily disrupts ionic compounds • dielectric constant (D) is high (measure of the ability to keep ions apart) • Coulombs Law - The force of attraction • between two charged pairs • K = proportionality constant • 8.99 x 109 J.M.C2 • q = charge and sign of ion • D = 75.5 (water), 24.3 (EtOH)1.9 (Hexane), 2.4 (interior of protein).

10. Kq q 1 2 F = 2 D r What makes this molecule important? • solvent ability - easily disrupts ionic compounds • dielectric constant (D) is high (measure of the ability to keep ions apart) • Coulombs Law - The force of attraction • between two charged pairs • large electronegativity creates a strong ionic type bond (dipole). • Liquid water has a higher density than solid water (ice). Is this normal? Think of why this is important? • orderliness - solvating shells • ability to take place in many hydrogen bonds (up to 4 at a time)

11. Kq q 1 2 F = 2 D r What makes this molecule important? • solvent ability - easily disrupts ionic compounds • dielectric constant (D) is high (measure of the ability to keep ions apart) • Coulombs Law - The force of attraction • between two charged pairs - Strong nucleophile - • to prevent unwanted attack by water, proteins often “hide” reactions from water by creating a hydrophobic core • Specific heat and heat of vaporization is high for water due to extensive H bonding

12. Water and H-bonds • High specific heat Lots of heat is needed to break H-bonds and raise H2O temperature. Therefore, H2O is a good insulator. High heat of vaporization Lots of heat is needed to vaporize H2O. Therefore, sweat cools.

13. Other bonds can also be Polar Fig. 2.2

14. Water is nucleophilic • Water participates in many chemical reactions • it is electron rich • it is a weak nucleophile • it is present in high concentration Water weakly ionizes

15. CO2 buffers our blood • Water reacts with CO2 to form an important blood buffer • We breath in and out gaseous CO2 • In the blood, CO2 reacts with water to form the buffering compound H2CO3 –carbonic acid • Disturbances in blood buffering system leads to acidosis (pH below 7.1) or alkalosis (pH above 7.6) • Bleeding in lungs – death! • “a few good men”

16. CO2 buffers our blood • Primary buffering compounds at physiological pH is H2CO3 (carbonic acid) and HCO3- (bicarbonate)

17. CO2 buffers our blood • We will come back to this after we talk about pH and buffers

18. Noncovalent Bonds • The London dispersion force is the weakest intermolecular force. • temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles. • sometimes called an induced dipole-induced dipole attraction. • cause nonpolar substances to condense to liquids and to freeze into solids when the temperature is lowered sufficiently.

19. Noncovalent Bonds - London forces • Because of the constant motion of the electrons, an atom or molecule can develop a temporary (instantaneous) dipole when its electrons are distributed unsymmetrically about the nucleus. • A second atom or molecule, in turn, can be distorted by the appearance of the dipole in the first atom or molecule (because electrons repel one another) which leads to an electrostatic attraction between the two atoms or molecules

20. Noncovalent Bonds - van der Waals • the van der Waals force is the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules • The term includes: • dipole–dipole forces • London (instantaneous dipole–induced dipole) forces • induced-dipole induced-dipole (dispersion forces)

21. Noncovalent interactions in biomolecules • Ionic • H-bond • van der Waals • Hydrophobic • Ionic>H-bond, hydrophobic>van der Waals

22. Noncovalent Bonds – van der Waals • The major noncovalent interactions in cells are electrostatic (ionic), hydrophobic, hydrogen bonds, and van der Waals. • Remember: • London forces are minimal unless tight association occurs • Most covalent bonds are 8 to 10 times stronger yet the overall shape and effectiveness of large molecules are due to the much weaker non-covalent bonds.

23. Ionic Bonds or salt bridges... into the 21st century? • Simple magnetic attraction between • Carboxy and amino groups, metals… • The force of attraction (F) depends on distance and relative shielding • Water and salts weaken bond • Strongest single noncovalent bond

24. Hydrogen bonds result from the interactions of strong covalent bonds between hydrogen and a highly electronegative atom (N and O) strongest bonds are when the arraignment is linear. The hydrogen is “shared” by a the covalently bonded atom and another electronegative atom You must be able to identify the donor and acceptor

25. Dipolar water dissociates in solution • association of dipolar water with typical biological side groups due to Hydrogen bonding • A-hydroxyl groups • B-keto groups • C-carboxylate ions • D-ammonium ions

26. Van der Waals (dipole-induced interactions) • Next to London dispersion forces, these are the weakest of the nonionic bonds but are important due to the large number of van der Waal interactions in a protein These bonds originate from very small dipole moments generated in atoms as electrons move around the nucleus.

27. Van der Waals These are small ionic, dipolar interactions The energy of the attraction is related to the distance between nuclei The average separation between atoms or molecules is the sum of the van der Waals radii

28. Van der Waals Space filling models use the van der Waal Radii to depict sizes

29. van der Waals in nature • The ability of geckos to climb on sheer surfaces has been attributed to van der Waals force • A gecko can hang on a glass surface using only one toe. Efforts continue to create a synthetic "gecko tape" that exploits this knowledge • A recent study suggests that water molecules of roughly monolayer thickness (present on all surfaces) also play a role

30. Hydrophobic interactions • The association of relatively nonpolar molecular groups in an aqueous environment. • Driven by the order of water entropy • The lack of interactions with a polar molecules with decreases the randomness of the order of water. ( an increase in entropy)

31. Hydrophobic interactions • Water forms cage-like structure around hydrocarbons forming shells of highly ordered water • Shell formation is due to water forming hydrogen bonds with each other • Aggregation of hydrophobic moleculules reduces total surface area and results in less order (increase in entropy) • Minimization of the • hydrophobic portions of the molecule permits the water max degrees of freedom (a minimization of entropy increase)

32. Why are non-polar molecules insoluble in water? Highly ordered water forms “cages: around the hydrophobic lipid

33. Why are non-polar molecules insoluble in water? A: It is energetically unfavorable to solvate a non-polar molecule. Non-polar (hydrophobic) compounds interfere with the structure of water - HOW? Water molecules surrounding a hydrophobic molecule are ordered but cannot make hydrogen bonds or ionic interactions to the non-polar molecule. This reduces the entropy (disorder) of water and is thermodynamically unfavorable and is called the hydrophobic effect. Highly ordered water forms “cages: around the hydrophobic lipid

34. Molecular associations are often accompanied by the release of water molecules that are ordered at the molecular surface. Release of ordered molecules is entropically favorable. Highly ordered water interacting with enz & Substrate Disorded water displaced by interaction By solvating themselves through self association, hydrophobic molecules, decrease the level of order of the system (shells of hydration) entropy is increased! Enzyme - Substrate interaction stabilized by hydrophobic interactions - less shells of hydration for new complex

35. Thus it is entropically favorable to have several hydrophobic pieces come together so only on shell is formed. This is the hydrophobic effect. Thus nonpolar molecules aggregation is not due to attraction to other hydrophobic compounds but due to inability to interact with water Very important in maintaining protein structure • hydrophobic portions of proteins are solvated by “hiding” inside the molecule away from the water. • This is the driving force for the formation of ampipathic molecules forming lipid bilayers membranes and vesicles

36. The End Any Questions?