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Chemistry 14C
Summer 2016 Final Part A Solutions
Page 1
Statistics: High score, average, and low score will be posted on the course web site after exam grading is
complete.
Some questions have more than one answer, even though only one answer may be listed here.
To see the final course grade cutoffs, consult the grading scale on the Chemistry 14C course web page.
1.
2.
The phenomenon or reason that allows MRI to differentiate or distinguish between tissues in an
organism is differences in 1H-NMR relaxation times that vary between tissue types.
OH
3.
OH
CH3
SH
(a)
182oC
(b)
CH3O
161oC
OCH3
102oC
101oC
158oC
OCH3
HO
166oC
HO
OH
239oC
HO
OH
210oC
(c)
80oC
218oC
49oC
177oC
4.
Aromatic stacking (pi stacking) and London forces.
5.
Several answers are possible, so long as the hydrogen bonding has been removed. Two possible
answers:
6.
7.
Nitrogen lone pair density (and hence ability to accept a hydrogen
bond) is less at the amide nitrogen (due to resonance) and the Cl2CNH2
nitrogen (due to the chlorines’ electron withdrawing inductive effect).
Note that the question asks about hydrogen bonding, and not proton
transfer, so curved arrows are inappropriate.
This is not a valid hydrogen bond because carbon and selenium are not sufficiently electronegative to
give the hydrogen atom the large δ+ necessary for a hydrogen bond donor.
Chemistry 14C
Summer 2016 Final Part A Solutions
Page 2
13
8.
In the C-NMR spectrum of the synthetic DNA structure, one signal is a quartet and two signals have
a chemical shift of at least 160 ppm. Quartets are due to methyl groups. According to the Typical
Chemical Shifts table, only carbonyl carbons have a chemical shift of at least 160 ppm.
9.
Molecule D. Among the given answer choices, this molecule's conjugate base has the greatest number
of resonance contributors, plus the nitrogen’s electron-withdrawing inductive effect.
10. HOF. Fluorine has the strongest electron-withdrawing inductive effect. The atomic radius of oxygen is
the same for each conjugate base.
11. (CH3)3N. Nitrogen is less electronegative than oxygen, and has a smaller atomic radius than sulfur or
phosphorus.
12. (a) Protonation occurs at nitrogen atom F instead of oxygen atom G because oxygen is more
electronegative than nitrogen.
(b) Protonation occurs at nitrogen atom F instead of nitrogen atom H because the electron density of
nitrogen atom H is delocalized by resonance (or because protonation at nitrogen atom H decreases
conjugation).
(c) Protonation occurs at nitrogen atom F instead of sulfur atom I because nitrogen has a smaller
atomic radius than sulfur.
13. I know that fumaric acid is the strongest acid because the given pKa of furmaric acid is less than the
pKa of Seroquel XR.
14. Keq greater than 1. A proton transfer equilibrium favors the weakest acid, which is Seroquel XR.
15. Based on these pKa values, the inductive effect of an alkyl group is electron donating. The acid without
the alkyl group (HCOOH) has a more stable conjugate base than the acids with the alkyl group, so the
alkyl group is enhancing the electron density at the carboxylate group (–CO2-) by an electron-donating
inductive effect.
16. Lipid: A hydrophobic molecule of biological origin.
17. (a) None (or phospholipid)
(b) Phospholipid
(c) Wax, triacylglyceride, and phospholipid
(d) Prostaglandin, steroid
18.
19. (a) Lactose, maltose, and sucrose
(b) Galactose and ribofuranose
20. (a) Threonine
(b) Lysine
Chemistry 14C
Summer 2016 Final Part A Solutions
H2N
O
N
21.
Page 3
OH
H2N
N
O
H
O
22. When a group of amino acids are part of a β-sheet or α-helix, they use an N-H bond as the hydrogen
bond donor, and a C=O bond as a hydrogen bond acceptor.
23. (a) Adenine
(b) Purine
(c) Aromatic All DNA and RNA nucleobases are aromatic.
24. (a) In a Watson-Crick base pair the heterocyclic base in question 23 forms two hydrogen bonds.
O
N
deoxyribose
deoxyribose
N
N
N
N
H
N
N
H
O
CH3
H
(b) Assuming an even distribution of all the DNA bases in the human genome, the human genome has
7.5 x 109 hydrogen bonds between all the base pairs. A-T base pair = 2 hydrogen bonds, and G-C
base pair has three hydrogen bonds. Therefore the average base pair has 2.5 hydrogen bonds,
times 3.0 x 109 base pairs = 7.5 x 109 hydrogen bonds.