Question 1. All of the amino acids in MIL would be enantiomers of our amino acids.
Because only one of
the 20 genetically encoded amino acids is achiral, and all of the others have the L-configuration, chiral
amino acids in MIL would have the mirror image D-configuration. The only exception to this generalization
on either side of the mirror is the amino acid glycine. Because glycine, unlike the other amino acids, does
not have a stereogenic carbon atom, it does not exist in D/L enantiomeric forms. It would then be the only
amino acid that Alice’s enzymes could digest on both sides of the mirror. The structure of glycine as well
as a general structure representative of the 19 other genetically encoded, chiral amino acids are shown in
Figure 5.
Question 2. On our side of the mirror, glucose exists in its D-enantiomeric form. Starch is a polymer
formed from D-glucose monomer units joined together by á-1,4-glycosidic linkages. The structures of á-D-
glucose, and that of its mirror image, the áa-anomer of L-glucose are shown in Figure 6.
Glucose would exist in its mirror image, L-enantiomeric, form in MIL, so starch there would be a polymer
formed from L-glucose molecules joined together by á-1,4-glycosidic linkages. Because Alice’s enzymes
could only fit the D-enantiomer of glucose, she would be unable to digest the MIL starch formed from the
L-enantiomer.
Question 3. The glycerides that Alice could digest in MIL would be limited to those that are achiral
because the chiral glycerides would exist as the mirror image form of those on the other side of the mirror.
Inspection of the structures shown in question 3 indicates that the 1-monoglycerides have a stereogenic
center and would, therefore, be indigestible for Alice. The 2-monogylcerides are achiral and could be
digested by Alice.
1,2-diglycerides contain a stereo-genic center regardless of the nature of the two R-groups and would be
indigestible. Whether the 1,3-diglycerides are chiral or achiral depends on the nature of the two R-groups
(meso); if these are identical, the molecule contains a plane of symmetry and is achiral (meso); if the R-
groups are not identical, the plane of symmetry is lost and the diglyceride is chiral and indigestible for
Alice.
The chirality of the triglycerides is totally dependent on the nature of the three R-groups. If all three R-
groups are different, the triglyceride is chiral (inedible). If the fatty acids on carbons 1- and 3- are the
same, the molecule would be achiral and could be safely eaten by Alice.
It should be noted, however, that if a glyceride were to undergo hydrolysis, glycerol and one or more fatty
acids would form. Because these compounds are individually achiral, they would be edible substances for
Alice.
Question 4. The stereochemistry of R–(l)–carvone and S–(d)–carvone is shown in the structures in Figure
7. The Cahn-Ingold-Prelog priority ratings (Solomons 2004) for the groups attached to the stereogenic
carbon atoms of these enantiomers are specified by the numbers 1 through 4 adjacent to these groups.
Question 5. Because the sweetener used in the sugar-free gum, xylitol, tastes
sweet to Alice on both sides of the mirror, we can conclude that it must be an
achiral molecule. The structure of xylitol is shown in Figure 8.
Question 6. Alice’s headache could be safely relieved by any of the analgesics
that are achiral. Because acetyl salicylic acid (aspirin) and acetaminophen both
contain a plane of symmetry, they are achiral and would be safe for Alice to use
in MIL. Both the sodium salt of naproxen and ibuprofen contain a stereogenic
carbon atom making them questionable for Alice’s use. In fact, however,
ibuprofen is sold on our side of the mirror as a racemic mixture and so should be
safe for Alice to use in MIL (Sen 1996). The situation is more dangerous for Alice
with the naproxen sodium salt. This compound is sold as the S-enantiomer; the
R-enantiomer, which would be used in MIL, is reported to be harmful to the liver
(Caron 1994).
Question 7. When the Professor says that Alice can eat anything that is “like our
air” he is telling Alice that she can eat anything that is achiral. Both oxygen and
water are achiral molecules and would be the same on either side of the mirror.
Question 8. Thiamine (vitamin B-1), whose structure is shown in question 8, is achiral and could be safely
consumed by Alice on either side of the mirror. Â-carotene, although not itself a vitamin, is a source of
vitamin A. It is an achiral molecule and could function effectively, forming vitamin A for Alice in MIL.
Of the molecules shown in question 8, ascorbic acid (vitamin C) is chiral and would be unavailable for
Alice in MIL. The d-á-tocopherol molecule (vitamin E) is unusual. Although it is a chiral molecule, both the
isomer shown and its enantiomer have been found to be efficacious (Brigelius-Flohe 1999). This is
probably because the effectiveness of vitamin E is associated with the ring system, not with the attached
alkyl chain. Students, not being aware of this information, are likely to assume that Alice could not use
vitamin E in MIL.
Frank J. Dinan (e-mail: dinan@canisius.edu) is a professor in the Department of Chemistry and
Biochemistry at Canisius College, 2001 Main Street, Buffalo, NY 14208; and Gordon T. Yee (e-mail:
gyee@vt.edu) is an associate professor in the Department of Chemistry at Virginia Polytechnic Institute
and State University, MC0212 Blacksburg, VA 24061.
References
Brigelius-Flohe, R., and M.G. Traber. 1999. Vitamin E: Function and metabolism. FASEB Journal
13(10):1145–1155.
Budavari, S., ed. 1996. The Merck Index. Whitehouse Station, N.J.: Merck and Company.
Caron, G., et al. 1994. Kinetic resolutions concentrate the minor enantiomer and aid measurement of high
enantiomeric purity. Tetrahedron: Asymmetry 5(1):83–92.
Sen, S.E., and K.S. Anliker. 1996. 1H NMR Analysis of R/S ibuprofen by the formation of diastereomeric
pairs: Microscale stereochemistry for the undergraduate laboratory. Journal of Chemical Education
73(6):569–572.
Solomons, T.W.G., and C.B. Fryhle. Organic Chemistry. New York: John Wiley and Sons.
Yee, G.T. 2002. Through the looking glass and what Alice ate there. Journal of Chemical Education
79(5):569–571.
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