Lecture 2

Question 1

Human growth hormone and receptor:

Answer 1

1. binding of hGH to growth hormone receptor transmits a signal across the cell membrane, which stimualtes cell growth
2. complementary binding surfaces between hGH to growth hormone receptor
3. highly specific noncovalent bonding interactions

Question 2

Properties of noncovalent bonds:

Answer 2

• 10-100x weaker than covalent
• allow non-permanent interactons
• many bonds together are quite stable allowing 3-D structure of a molecule to be maintained yet flexible
• ~150-400 kj/mol

Question 3

Noncovalent interactions nature:

Answer 3

all electrostatic in nature, charge-charge

Question 4

δ- or δ+ indicates:

Answer 4

a partial negative or positive charge in molecules that share electrons unequally

Question 5

Difference between charge-charge interactions and van der Waals:

Answer 5

charge-charge interactions are stronger over large distances than van der Waals (strong over closer distances)

Question 6

Charge-charge (dependence of energy on distance):

Answer 6

1/r

Question 7

Charge-dipole (dependence of energy on distance):

Answer 7

1/r^2

Question 8

Dipole-dipole (dependence of energy on distance):

Answer 8

1/r^3

Question 9

Charge-induced dipole (dependence of energy on distance):

Answer 9

1/r^4

Question 10

Dispersion (van der Waals) (dependence of energy on distance):

Answer 10

1/r^5

Question 11

Hydrogen bond (dependence of energy on distance):

Answer 11

bond length is fixed

Question 12

Relative bond energies in noncovalent interactions:

Answer 12

charge-charge > hydrogen bond > van der Waals

Question 13

Charge-charge interactions:

Answer 13

(ionic bonds, salt bridges) is formed by the interavtion between two opposite charges

Question 14

Coulomb’s Law:

Answer 14

force of interaction is the product of the charges over their distance of separation squared (q = charge, r = distance, k = constant)

Question 15

Negative force:

Answer 15

attraction

Question 16

Positive force:

Answer 16

repulsion

Question 17

Energy required to join/separate two charged particles:

Answer 17

Question 18

Ionic bonds in water:

Answer 18

tend to come apart in water because water has a large dielectric constant, ε.

Question 19

Medium/ dielectric constant:

Answer 19

surrounding molecules that screen charges from each other, water has a high dielectric constant (80) while organic liquids have a lower dielectric constant

Question 20

Electrolytes:

Answer 20

free ions (i.e. Na+ and Cl-)

Question 21

Properties of dipole interactions:

Answer 21

carry no net charge, but have asymmetrical distribution of charge

Question 22

Polar:

Answer 22

assymetric distribution of charge

Question 23

Dipole moment:

Answer 23

μ, which expresses the magnitude of the polarity

Question 24

Polarity of water:

Answer 24

• polar molecule with a strong dipole moment
• oxygen has a high electronegativity and draws electrons away from two hydrogens in water molecules
• partial negative charge on O
• partial positive charge on each H
• two dipole moments, one vector sum

Question 25

Polar molecules in aqueous environment:

Answer 25

can be attracted by nearby ions or other polar molecules

Question 26

Dipole interactions:

Answer 26

interaction between nearby ions or other polar molecules with polar molecules, these are shorter range

Question 27

Properties of oxygen:

Answer 27

has a high electronegativity, so electrons are pulled towards itself

Question 28

Charge-dipole interactions:

Answer 28

ion + polar molecule

Question 29

Dipole-dipole interactions:

Answer 29

polar molecular + polar molecule

Question 30

Induced dipoles:

Answer 30

a molecule without net charge or dipole moment can become polar in presence of electrical charge (charged molecule or dipole). ie. Benzen rings are polarizeable: electrons can be displaced within the ring

Question 31

Induced-dipole interactions:

Answer 31

temporary charge fluctuations, producing forces that can attract molecules to each other, effective over very short distances

Question 32

Benzen rings:

Answer 32

1. neither a net charge nor permanent dipole moment, but a nearby charge can induce a redistribution of electrons within the benzene ring, producing an induced dipole moment 2. planar molecules like benzene have a strong tendency to stak because fluctuations in the electron clouds of the stacked rings give rise to mutually attractive induced dipoles (van der Waals)

Question 33

Polarizeable:

Answer 33

a molecule in which a dipole can be induced

Question 34

Induced dipole interactions:

Answer 34

polarizeable molecule + dipole molecule

Question 35

Charge-induced dipole interactions:

Answer 35

anion/cation + dipole molecule

Question 36

Dipole-induced dipole interactions

Answer 36

polar molecule + dipole molecule, shorter range than permanent dipole interactions

Question 37

van der Waals:

Answer 37

nonpolar + nonpolar

Question 38

Properties of van der Waals:

Answer 38

* electron charge distribution is not static * attraction increases as two atoms come closer to each other * **optimum**: van der Waals contact distance * Closer than contact distance leads to repulsion * 2-4 kJ/mol

Question 39

Energy of attraction and repulsion (graph):

Answer 39

Question 40

van der Waals strength:

Answer 40

stronger if they act as a team

Question 41

Hydrogen bonding:

Answer 41

interaction of a hydrogen (covalently bound to electronegative atone) with a pair of nonbonded electrons on another atom (typically O or N)

Question 42

Hydrogen bond donor:

Answer 42

* (negative charge) hydrogen bond between electronegative atone and hydrogen, induces slightly positive charge * cannot exist with carbon

Question 43

Hydrogen bond acceptor:

Answer 43

(negative) interaction between hydrogen and electronegative atone electrons

Question 44

Properties of hydrogen bonds:

Answer 44

* have both covalent and noncovalent features * electrons are shared between acceptors and donors * charge-charge interactions between the H and acceptor

Question 45

Structure of water:

Answer 45

each water molecule can make up to 4 hydrogen bonds. Each molecule is a donor and an acceptor

Question 46

Melting/boiling point relative to hydrogen bonds:

Answer 46

more hydrogen bonds = higher melting/boiling point

Question 47

Polar solvents can dissolve ionic molecules:

Answer 47

the negative end of dipole can interact with cations and the positive end can interact with anions

Question 48

The tendency of ionic compounds to dissolve in water:

Answer 48

1. formation of hydration shells replaces the ionic interaction 2. the dielectric constant of water decreases the force between oppositely charged ions that would pull them back together

Question 49

Example of ionic molecules dissolved in polar solvent:

Answer 49

Na+ and Cl- are free ions in water surrounded by hydration shells: * Cl- with H+ * Na+ with O-

Question 50

H-bonding allows:

Answer 50

uncharged, yet polar molecules to dissolve in water

Question 51

Hydrophillic:

Answer 51

polar molecules that can form hydrogen bonds with water molecules, water-loving

Question 52

Hydrophobic:

Answer 52

non-polar, non-ionic substances don't form H-bonds and they don't form hydration shells

Question 53

Enthalpy in hydrophobic molecules:

Answer 53

* water forms ordered "cages" around non-polar molecules, which is energetically unfavorable * hydrophobic nonpolar surfaces tend to aggregate, thus releasing some ordered H2O molecules, enabling the release of H2O to form more H-bonds with bulk H2O

Question 54

Amphipathic:

Answer 54

have both hydrophobic and hydrophillic properties: * polar head can interact with water * nonpolar components hide from water

Question 55

Amphipathic molecules in aqueous solution:

Answer 55

* form micelles from single tail groups * form spherical structures and bilayer vesicles from double tail groups

Question 56

Acids:

Answer 56

are H+, proton, donors

Question 57

Bases:

Answer 57

are H+, proton, acceptors

Question 58

Strong acids:

Answer 58

tend to lose their hydrogen protons, dissociate, more completely than weak acids

Question 59

Equilibrium acid dissociation constant (Ka):

Answer 59

tendency to donate or dissociate more readily

Question 60

Higher Ka:

Answer 60

stronger acid

Question 61

Lower pKa:

Answer 61

stronger acid

Question 62

pKa = pH

Answer 62

50% protonated and 50% deprotonated

Question 63

Nucleic acid acidity:

Answer 63

very acidic because of phosphoric backbone

Question 64

pH of living cell:

Answer 64

7.2 - 7.4 (except in stomach lysosomes) controlled by buffers in the body

Question 65

As the pH increases:

Answer 65

more -OH present, an acid will become more dissociated

Question 66

As the pH decreases:

Answer 66

more H+ present, acids and bases become protonated

Question 67

Affects of pH on proteins:

Answer 67

as pH changes, the degree of protonation or deprotonation of acidic or basic groups on amino acids changes, depending on their pKa