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Representation of the Stereochemistry of Amino Acids in Textbooks E. J. ~ehrman,'G. E. Means, and H. Zhang Department of Biochemistry, The Ohio State University, Columbus, OH 43210
The Fischer convention for the representation of carbohydrate stereochemistry in two dimensions has been used widely for amino acids as well. While carbohydrates almost always are represented correctly, this is not the case with the amino acids. The Fischer convention, as originally used ( I ) , specifies that the most oxidized carbon atom should appear a t the top of the structure with the chain extending vertically downward. Horizontal substituents are regarded as projeding above the plane and the vertical ones below it. If the rule governing the position of the most oxidized carbon and the longest chain is not followed, the stereochemistry is not immediately apparent to most readers because it is perhaps not generally realized that there are 12 valid projections for a single chiral center (Za, 3) (Fig. 1).Although 'Author to whom correspondence should be addressed.
+NH~+
+NH3+m-
*NH+ W-
R
4 1
COW -0oC
H 3
TR RT 2
H
R
H
4
5
any of these 12 is correct, it is very difficult to see, except by use of a tetrahedral model, whether the D- rather than the L-isomer is shown. Therefore, it is certainly best to choose a further restriction (such as the most oxidized carbon up and the chain down) to make the stereochemistry obvious. Not all textbooks adhere to the most oxidized carbon orientation rule for the amino acids. As a result, a variety of projections have appeared which evidently use only that part of the convention that specifies in which direction vertical and horizontal substituents project. (We have found only one case in which L-amino acids are represented as the D-isomers). We think that the situation is confusing to the reader and especially so when it is necessary to represent the stereochemistry of the second chiral center in Lisoleucine and Lthreonine. The table shows the projections used by recent texts and whether isoleucine and threonine are correctly shown. We note that the structures for these amino acids have been known for some time and that the proofs are admirably discussed by Greenstein and Winitz (Zb). . . We recommend either a return to the standard Fischer convention or the amino-carboxyl linear (AC linear) projection. The AC linear model has the significant advantage that polypeptides can be written in linear fashion with correct stereochemistry. Figure 2 compares the standard Fischer and the AC linear projections for L-isoleucine and L-threonine.
C m
Coo-
Coo-
6
H3%z M
Et
aandard Fischer
~~6$:~ H
Me
+. CO(t
C
H
R+COCT
NH;
NH3+
NH3+
10
11
12
F gure 1.The 12 Fischer projenions of an L-amlnoac d. Form 1 is the standard F scher project on. Form 3 is the AC linear project on that we recommend. There are 12 nondentcal m rror mages of lnese projections for the D-isomer,
282
Journal of Chemical Education
Figure 2. Projections of L-isoleucine and of L-threonine in the standard Fischer and the AC iinearforms.,
Textbook Representations of L-Amino Acids 1st Author
Ref.
Projeciion (Fig.1)
Ilea
th?
Abeles
4
3(wedge)
+
x
Armstrong
5
11
+
Bohinski
6
5
x
+ +
Campbell
7
1
x
Conn
8
11
+ not specified
9
1
+
Lehninger
10
1
x
+ +
Maiihews
11
3
x
x
1
x
+
Devlin
Rawn
12
Smith
13
Stryer
14
1
x
+
Voet
15
12
+
+
Zubay
16
11
+
x
D-forms (mirror image of 5)
= correct; x = incorrect
Aronson already h a s discussed some o f these problems (17);t h e table demonstrates t h a t o u r r e i t e r a t i o n i s warranted.'
Literature Cited 1. Fbeher,E. Ber 1891,24,2685-2687. 2. (a) Greenstein, J. P ; WmitE, M. Chemishy offhe h i m Acids, Vol. 1 ,Wilq: NY, 1961, p 50; (b)Chemistry offhe AmimAelda: pp 176,183. 3. Sheifwieser, A,; Heathcock, C. H. htmduetion to Olganie Chomiaw; Maemillan: NY,1981, p 134. 4. Abe1ea.R. H.;Frey,PA.; Jeneka,W. P.Bioehrmisw;Jones 8Bartlett: Boston, 1992,
frantispiffe. E B.Bioehrmishy, 3rd ed., Oxhrd: NewYork, 1989, p 61. 8. Bohinski, R. C. Modern C o w p t s in Bioekmishy, 5th ed., Ally" & Bsmn: Boston, 1987, p 66. 7. Campbell, M. K. Biochemistry; Saunders; New Ywk, 1991, p 83. 8. Conn, E. E.; Stumpf,P. K;Bruening, G.; Doi, R E.OutlinesofBt(loehmiatry,5thpd.. W h y : New Yo*, 1987, p65. 9. Devlin, T. M. Tbrtbwk afBioehemistry: Wiley: NwYark. 1992, p28. 10. Lehnulger, A. L.; Nelaon, D. I.; Cm, M. M. F7ineiplea ofBioehrmishy: Worth New Ywk, 1993, p 115. 11. Mstthews, C. K; vsn Holde, K E. BioehemisUy; Benjaminlcummings: Redwood 5. Amtrong,
14. 15. 16. 17. 18.
s&,
L. ~ i o e h e r n i s t j3rd , ed., ~rieman:New Yark, 1988, p 18. Voet, D.;Voet, J. G. Biochemistry; Wiley: New York, 1990, p Ml. Zubay, G. Biochemistry, 3rd ed.; W. C. B m m : Dubuque. IA, 1963;Vol. 1, p 4 9
Amneon, J.N.Bimkem.Educ 1974.2. 76. Edison, A. S. TIES ISW, 25,216217.
Volume 70 Number 4 April 1993
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