1D: NutraSweet: Bioorganic Molecules

Many, many, many different organic chemical compounds exist naturally in humans, animals, plants, soils, ocean water, etc. Of special interest are the biochemical processes involving these compounds that occur regularly to keep us, as well as the natural world that we depend on alive and well. The more we understand the characteristics of organic biomolecules, the wiser we will be. Whether we then act more wisely raises questions of considerable debate, questions that legitimately could be addressed in science courses, but are usually left to social and political scientists and philosophers.

When attention is turned to human biomolecules, the ones studied most frequently are grouped according to their functional group characteristics and/or their biofunction. Familiar names are carbohydrates (sugars), lipids (fats), proteins, DNA (nucleic acids), and vitamins. (Chapter 11 in Atkins and Jones has sections on proteins, carbohydrates, and DNA.)

Proteins, complex carbohydrates, and DNA are polymers, where small units are hooked together by bonds into much larger units that contain scores or hundreds of the monomer units. The figures below show the bonding linkages present in proteins, where individual amino acids such as phenylalanine and aspartic acid are bound in sequences through amide (peptide) linkages and one type of starch where individual sugar units are connected together with a spacial type of ether linkage called an acetal or ketal.

Figure 9

R | ( -C-C=O | NH-)n

Amino acids (AA) are organic molecules that contain both an amine and a carboxylic acid functional group. Proteins are polymers constructed by linking together, in various combinations and ratios, the 20 or so naturally occurring amino acids. The -NH2 amine group can react with the -COOH acid group to give an amide group -(C=O)NH- and water (H2O). This provides a bond between two amino acids, leaving one amino acid with a free -NH2 group that could bond to a -COOH from a new

R
|
( -C-C=O
|
NH-)

and the other amino acid with a free -COOH group that could bond to a new amino acid’s -NH₂ group. This process can repeat itself over and over.

Figure 10

There is considerable variation among the structures and properties of the individual naturally occurring amino acids, but they all have in common a so-called "a-amino acid" relationship,

NutraSweet is called aspartame. Aspartame’s structure is a combination of two amino acids, aspartic acid and phenylalanine, and can be called a dipeptide. The "natural" state of phenylalanine in aspartame has been modified by converting the acid grouping (-COOH) to an ester grouping (-COOCH₃). Similar to the reaction of an amine and acid to give an amide and water, alcohols and acids react to give esters and water; therefore, to obtain the methyl ester of phenylalanine, the phenylalanine can be reacted with methyl alcohol (methanol, CH₃OH).

< face="Arial">Interim Summary

Before proceeding to a discussion of sweeteners, a consolidation of some of the structural aspects of organic molecules seems prudent. Aspartame is a relatively simple organic molecule, but has layers of structural features that need to be appreciated before a full understanding of its overall structure can be grasped. The hybridizations of the carbon, oxygen, and nitrogen determine the three-dimensionality of those several atoms. The ability to rotate rather freely about single bonds allows the molecule to seek an overall more stable conformation, a conformation where structure may easily differ depending upon whether aspartame is in a solid form or in solution with water or other solvents. There are several polar regions that may attract or repel each other intramolecularly or intermolecularly either in the solid form or in solution. The several functional groups (amine, acid, amide, ester, and phenyl group) are responsible for various polar and nonpolar regions.

Aspartame has two chiral carbon centers that require a specific arrangement of the four groups attached to that carbon. In contrast, a mirror image arrangement would provide a different three-dimensional structure and its biological characteristics might differ dramatically.

These various structural features, in their entirety, define the structure of aspartame. X-ray crystallographic analysis of the solid form of the compound reveals the entire three-dimensional structure of aspartame, including bond lengths and bond angles. But, as suggested earlier, aspartame may assume different conformations in solution and in the presence of other biological units, such as "taste buds", so the solid structure can only provide hints to how aspartame might taste sweet when you and I consume it.