Carbohydrates hydrogen-bonding functionalities

All of these functions are made possible by the characteristic chemical features of carbohydrates (1) the existence of at least one and often two or more asymmetric centers, (2) the ability to exist either in linear or ring structures, (3) the capacity to form polymeric structures via glyeosidie bonds, and (4) the potential to form multiple hydrogen bonds with water or other molecules in their environment. 

Two characteristics of carbohyhdrate structure are often given special attention when constructing potential functions. Although hydrogen bonding and "anomeric effects" are certainly in ortant in other compounds, they are thought to be especially important for carbohydrates. 

Our initial, small models of an isolated cellulose chain ranged from the dimer (cellobiose) to the octamer. The dynamics of these fragments have thus far been simulated only in vacuum, using different potential energy functions such as those of MM2(85) (9) and AMBER (10), with and without contributions from electrostatic terms and hydrogen bonds, etc. (The program DISCOVER, customized for carbohydrates and for operation on the Alliant FX/80 computer, has been used (12).) Generally, the time span for the simulations has been of the order of several hundred picoseconds to 1 nanosecond. 

In the nucleoside and nucleotide crystal structures, the OH O hydrogen bonds are the most common. In general, these molecules do not crystallize as readily as do the carbohydrates, and in consequence, all the data, with only one exception, are from X-ray analyses. In addition, only about a quarter of the crystal structures reported are of sufficient quality to permit either direct or indirect location of all the functional hydrogen atoms. As is often the case the larger the molecule, the less accurate are the crystal structure analyses, hence most of the data refer to compounds with less than 12 carbon atoms. 

In the carbohydrates, the water molecules most commonly have fourfold, nearly tetrahedral coordinations, which function as crossing-points for infinite or finite chains, thereby forming network structures. In this sense, the inclusion of water molecules in carbohydrate structures makes the hydrogen bonding more ice-like. 

We conclude the discussion of the organic molecules in biological systems by turning our attention to lipids, biomolecules that are soluble in organic solvents. Unlike the carbohydrates in Chapter 27 and the amino acids and proteins in Chapter 28, lipids contain many carbon-carbon and carbon-hydrogen bonds and few functional groups. 

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