The descriptors ‘α’ and ‘β’ are also used in carbohydrate nomenclature to specify configuration of cyclic monosaccharides [1, P-102.3.4.2.1]. You may remember that aldehydo-glucose, the open-chain form of glucose, has four chiral centres. Consider the structures (a) and (b):
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| (a) | (b) |
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Upon cyclisation of either enantiomer, an extra chiral centre is created at the position 1. This centre is referred to as anomeric centre [2, 2-Carb-6.1] and two resulting stereoisomers are anomers. For example, cyclisation of aldehydo-D-glucose (a) brings about two major forms of D-glucose, (c) and (d):
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| (c) | (d) |
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Since the six-membered ring can be regarded a derivative of the parent heterocycle pyran, these cyclic forms are called pyranoses [1, P-102.3.4.1]. Note that the hydroxy group at C-1 in diagram (c) is below (behind) the plane of the paper (computer screen); the complete carbohydrate name is α-D-glucopyranose. Cf. this name with systematic (2S,3R,4S,5S,6R)-6-(hydroxymethyl)tetrahydro-2H-pyran-2,3,4,5-tetrol (based on pyran) or slightly shorter (2S,3R,4S,5S,6R)-6-(hydroxymethyl)oxane-2,3,4,5-tetrol (based on Hantzsch-Widman parent oxane). In the structure (d) the hydroxy group at C-1 is above (in front of) the plane, and this isomer is called β-D-glucopyranose.
As you can see, instead of five stereodescriptors, the carbohydrate names for (c) and (d) appear to use only two: ‘D’ gives us the stereochemistry of C-5 (configurational atom) while ‘α’ or ‘β’ specify the stereochemistry of C-1 (anomeric centre). No magic here: this only works because the ‘gluco’ bit — referred to as configurational prefix [2, 2-Carb-4.3] — already has the relative configurations of the stereocentres C-2, C-3, C-4 and C-5 built in. Configurational prefixes really are just another type of stereodescriptors.
Recall that in steroids and some other natural products, the descriptors ‘α’ or ‘β’ refer to configurations at specific atoms, and therefore have to follow their corresponding locants. In theory, there could be as many of ‘α’ or ‘β’ as chiral centres in the ring system. In carbohydrate names, the meaning of ‘α’ and ‘β’ remains the same, viz. “below” and “above” the ring — in standard orientation. (I’ll come to it in a minute.) However, as there is only one ‘α’ or ‘β’ per monosaccharide name, no locants are needed.
By historical reasons, carbohydrate chemists still use Fischer and Haworth projections. I prefer the Mills depiction [1, P-102.3.6] which is computer-interpretable and consistent with the rest of diagrams used in organic chemistry. I allow myself to introduce what I think should be the standard orientation of a cyclic monosaccharide, exemplified by β-D-glucopyranose (d) as shown below.
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| (d) | (e) |
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The ring lies in the plane of the paper (or in the plane of the computer screen). The α-face of the ring it below (behind) this plane, the β-face is above (in front) of it [3]. The observer is at the β-face of the ring. The numbering of the ring carbon atoms goes clockwise (↻) and the hydroxymethyl group attached to the configurational atom C-5 points up, i.e. towards the observer. The hydroxy group at the anomeric centre C-1 also goes up, thus it’s the β-anomer*.
Now let’s look at the mirror image of (d), β-L-glucopyranose (e). Here the ring carbon atoms are numbered anticlockwise (↺) and the hydroxymethyl group at C-5 still goes up. Since the anomeric hydroxy group is pointing up, it’s the β-anomer†.
The five-membered ring monosaccharides are called furanoses because they can be considered derivatives of furan. Observe (what I call) the standard orientation of furanoses such as β-D-ribofuranose (f) and α-L-arabinofuranose (g):
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| (f) | (g) |
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In (f), the ring carbon atoms are numbered clockwise (↻) and the hydroxymethyl group attached to the configurational atom C-4 points up — this is a D-sugar. The hydroxy group at the anomeric centre C-1 also points up, therefore it’s the β-anomer. To the astronomically-minded, the arrangement of the atoms C-1 through C-5 in this orientation must remind that of the first five stars in the Big Dipper, from α (Dubhe) to ε (Alioth), respectively. How’s that for a mnemonic?
In (g), which is an L-sugar‡, the ring numbering goes anticlockwise (↺) and the hydroxymethyl group at the configurational atom C-4 points up. The hydroxy group at the anomeric centre C-1 goes down, hence it’s the α-anomer.
There are other uses of the symbols ‘α’ and ‘β’ in chemical nomenclature: as locants [1, P-14.3.1], as allotrope descriptors [4, IR-3.4.2] and, in case of ‘β’, as a type of end-group in carotenoids [5, 3.1—3.3]. I’ve been thinking that chemists may want to expand their arsenal of glyphs. Why don’t we use, say, the Chinese letters 下 (Pinyin: xià) “down” and 上 (shàng) “up” ? I find them more intuitive than “stereochemical” ‘α’ and ‘β’. Then we would name the structures (d) 上↻-glucopyranose and (g) 下↺-arabinofuranose.
| * | The IUPAC assigns ‘α’ and ‘β’ configurations based on ‘cis’ and ‘trans’ relationship of oxygens at configurational atom and anomeric centre in the Fischer projection [1, P-102.3.4.2.2; 2, 2-Carb-6.2]. I think using the Fischer projection only confuses the matter. Note that in Mills projections, ‘α’ anomers are ‘trans’ and ‘β’ anomers are ‘cis’ in a sense of ring system cis-trans isomerism. I find the method that I first encountered in Binkley [3] — ‘α’ below and ‘β’ above the plane of the ring — much easier to understand. |
| † | Alternatively, β-L-glucopyranose could be drawn in such a way that the ring carbon atoms are numbered clockwise, but in this case the β-face will be below the plane of the diagram:
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| ‡ | Most naturally occurring monosaccharides are the D-isomers. Nevertheless, there are exceptions: for instance, arabinose is more commonly found in L-form. |
References
- Favre, H.A. and Powell, W.H. Nomenclature of Organic Chemistry: IUPAC Recommendations 2013 and Preferred IUPAC Names. Royal Society of Chemistry, Cambridge, 2014.
- McNaught, A.D. (1996) Nomenclature of carbohydrates (IUPAC recommendations 1996). Pure and Applied Chemistry 68, 1919—2008.
- Binkley, R.W. Modern Carbohydrate Chemistry. Marcel Dekker, New York, 1988, pp. 20—23.
- Connelly, N.G., Hartshorn R.M., Damhus, T. and Hutton, A.T. Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005. Royal Society of Chemistry, Cambridge, 2005.
- IUPAC and IUPAC-IUB (1975) Nomenclature of carotenoids (rules approved 1974). Pure and Applied Chemistry 41, 405—431.
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