tailieunhanh - Biochemistry, 4th Edition P13

Biochemistry, 4th Edition P13. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | What Are the Spectroscopic Properties of Amino Acids 83 FIGURE The ultraviolet absorption spectra of the aromatic amino acids at pH 6. From Wetlaufer D. B. 1962. Ultraviolet spectra of proteins and amino in Protein Chemistry 17 303-390. Amino Acids Can Be Characterized by Nuclear Magnetic Resonance The development in the 1950s of nuclear magnetic resonance NMR a spectroscopic technique that involves the absorption of radio frequency energy by certain nuclei in the presence of a magnetic field played an important part in the chemical characterization of amino acids and proteins. Several important principles emerged from these studies. First the chemical shift1 of amino acid protons depends on their A DEEPER LOOK The Murchison Meteorite Discovery of Extraterrestrial Handedness The predominance of L-amino acids in biological systems is one of life s intriguing features. Prebiotic syntheses of amino acids would be expected to produce equal amounts of L- and D-enantiomers. Some kind of enantiomeric selection process must have intervened to select L-amino acids over their D-counterparts as the constituents of proteins. Was it random chance that chose L- over D-isomers Analysis of carbon compounds even amino acids from extraterrestrial sources might provide deeper insights into this mystery. John Cronin and Sandra Pizzarello have examined the enantiomeric distribution of unusual amino acids obtained from the Murchison meteorite which struck the earth on September 28 1969 near Murchison Australia. By selecting unusual amino nh3 nh3 CH3 CH2 ch C COOH CH3 CH2 C COOH CH CH3 ch3 ch3 2-Amino-2 3-dimethylpentanoic acid Isovaline acids for their studies Cronin and Pizzarello ensured that they were examining materials that were native to the meteorite and not earth-derived contaminants. Four a-dialkyl amino acids a-methylisoleucine a-methylalloisoleucine a-methylnorvaline and isovaline were found to have an L-enantiomeric excess of 2 to 9 . This may be the

• Sinh học
• Nền tảng của hóa sinh học
• trao đổi chất protein
• RNA trao đổi chất
• nguyên tắc của hóa sinh
• Axit amin
• công nghệ DNA