. Biophysical science. Biophysics. 15:2/ X-ray Analyses of Proteins and Nucleic Acids 273 rotated into one another (except when R is a proton). A carbon atom, which is sterically asymmetric in this fashion, is called optically active because in solution it rotates polarized light. The two stereoisomers are labeled D and L for dextro-rotary and levulo-rotary, respectively. Most test tube syntheses give equal amounts of the D and L isomers, but most living cells produce only one variety or the other. With very few exceptions, the amino acids polymerized into proteins in the living cell are all Z
. Biophysical science. Biophysics. 15:2/ X-ray Analyses of Proteins and Nucleic Acids 273 rotated into one another (except when R is a proton). A carbon atom, which is sterically asymmetric in this fashion, is called optically active because in solution it rotates polarized light. The two stereoisomers are labeled D and L for dextro-rotary and levulo-rotary, respectively. Most test tube syntheses give equal amounts of the D and L isomers, but most living cells produce only one variety or the other. With very few exceptions, the amino acids polymerized into proteins in the living cell are all Z,-a-amino acids. Two amino acid molecules may react to eliminate a molecule of water, thereby forming a peptide bond. Schematically, this can be represented as OH. NH2 O Peptide Bond The peptide bond so formed is very stable and the molecule is called a dimer or dipeptide. It is then possible to attach this molecule to other amino acid molecules, forming chains, or polypeptides. When these chains include 50 or more amino acids, they are called proteins. In some cases, the chains may be branched or cross linked; many proteins contain, in addition, a few small molecules other than amino acids. Molecular weights of proteins vary from several thousand into the millions. The number of different amino acids conceivable has no known limit. A very large number have been synthesized in test tubes. Of these, approximately 20 Z,-a-amino acids make up almost all of the proteins of all living cells. Other amino acids occur in nature, especially in bacteria, but they are the exceptions rather than the rule. The 20 amino acids tabulated in the table on pages 275-277 are the building units which are polymerized to form complex polypeptide chains called proteins. The physical form and arrangement of these polypeptide chains is discussed further in Section 5 of this chapter. The various proteins differ from one another in the number and order of the various amino acids in the polypeptide chains
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