Now let's look more closely at two groups of biologically important compounds - amino acids and nucleotides - whose structures include the nitrogen-containing functional groups. Amino acids contain an amine functional group and a carboxylic acid group. Under most biological conditions, amino acids exist as ZWITTERIONS, where the amino group is positively charged and the carboxylic acid group is negatively charged. In these structures, the R group represents one of 20 side chains found in amino acids of living systems.

It's helpful to classify amino according to their side chain structure. Some amino acids have a NONPOLAR side chain. Alanine is one example. The hydrocarbon side chain, C-H-3, is what makes this structure nonpolar. Eight other biological amino acids also have nonpolar side chains. If one of the hydrogen atoms on alanine's side chain is replaced by an alcohol group, we get the amino acid SERINE. The alcohol group makes the side chain POLAR because the electrons in the carbon-oxygen bond are closer to the oxygen. Five other biological amino acids also have polar side chains. Finally, some amino acids have ionic side chains. If one of the hydrogen atoms on alanine's side chain is replaced by carboxylic acid group, we get ASPARTIC ACID. The names and structures of all 20 amino acids are presented on the Reference Sheet that accompanies this activity. The structure of an amino acid's side chain influences its physical properties, but it also determines its chemical reactivity. We'll take a closer look at serine's role in the biological activity of aspirin shortly.

Nucleotides are the other nitrogen-containing biomolecules we'll consider. Nucleotides have three components: a nitrogeneous base, a pentose sugar, and a phosphate group. When the structure consists of only the pentose sugar and the nitrogenous base, it's called a NUCLEOSIDE.

We can classify nucleotides according to the pentose sugar they contain. Nucleotides containing the pentose sugar RIBOSE are called RIBONUCLEOTIDES, while those containing deoxyribose are called DEOXYRIBONUCLEOTIDES.

Nucleotides use one of five different nitrogenous bases. Two of the bases, adenine and guanine, are called PURINES. They're found in ribonucleotides and deoxyribonucleotides. The other three bases are PYRIMIDINES. Cytosine is found in both ribonucleotides and deoxyribonucleotides. Uracil is found only in ribonucleotides, and thymine is found only in deoxyribonucleotides. Later in the course, when we study the gene, we'll see how ribonucleotides combine to form RNA, or ribonucleic acid, and how deoxyribonucleotides combine to form DNA, or deoxyribonucleic acid.

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