Fruits,
Flowers, Striking Colors, and Medicinal Uses Too
In sucrose, starches, and other sugar polymers, the O-glycoside bonds attach sugars to sugars. Other major categories of glycosides are known in which the sugar binds to some other type of molecule. Probably the most common example is the structure of nucleotides, N-glycosides, in which the sugar binds to the nitrogenous, aromatic base, as found in ATP, many vitamins, DNA, and RNA. In glycolipids and glycoproteins, carbohydrates are attached to both lipids and proteins, respectively, by glycoside linkages.
The red and blue colors of some flowers are sugar derivatives, often called anthocyanins. These pigments involve various sugars bonded to the compound cyanidin and its derivatives. These compounds are water soluble because of the polar groups they possess. You may have done an acid–base titration of the pigment from red cabbage or from blueberry juice in a chemistry lab. In contrast, orange, yellow, and green plant pigments tend to be lipid in composition and insoluble in water.
Many flavors involve sugar glycosides. Two familiar ones are cin-namon and vanilla, in which the sugars bond to cinnamaldehyde (3-phenyl-2-propenal) and vanillin, respectively. Both of these com-pounds are aromatic aldehydes. The distinctive taste of the kernel in a peach or apricot pit (a bitter-almond flavor) is due to laetrile, a controversial substance suggested as a cancer treatment by some.
Many medically important substances have a glycosidic link-age as a
part of their structure. Digitalis, prescribed for irregular heartbeat, is a
mixture of several steroid complexes with sugars attached. Laetrile, a
benzaldehyde derivative with a glycosidic linkage to glucuronic acid, was once
thought to fight cancer, possibly because the cyanide moiety would poison the
fast-growing cancer cells. This treatment is not approved in the United States,
and it is likely that the cyanide causes more prob-lems than it solves. The
National Cancer Institute maintains a website at http :// www .cancer.gov; use
the search function there to find information about laetrile.
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