Biochemistry, 4th Edition P34. 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 Structure and Chemistry of Nitrogenous Bases 293 Hypoxanthine FIGURE The common purine bases adenine and guanine in the tautomeric forms predominant at pH 7. The Properties of Pyrimidines and Purines Can Be Traced to Their Electron-Rich Nature The aromaticity of the pyrimidine and purine ring systems and the electron-rich nature of their carbonyl and ring nitrogen substituents endow them with the capacity to undergo keto-enol tautomeric shifts. That is pyrimidines and purines exist as tautomeric pairs as shown in Figure for uracil and Figure for guanine. The keto tautomers of uracil thymine and guanine vastly predominate at neutral pH. In other words pKa values for ring nitrogen atoms 1 and 3 in uracil Figure are greater than 8 the pKa value for N-3 is . In contrast the enol form of cytosine predominates at pH 7 and the pKa value for N-3 in this pyrimidine is . Similarly for guanine Figure the pKa value is for N-1 and less than 5 for N-3. These pKa values specify whether protons are associated with the various ring nitrogens at neutral pH. As such they are important in determining whether these nitrogens serve as H-bond donors or acceptors. Hydrogen bonding between purine and pyrimidine bases is fundamental to the biological functions of nucleic acids as in the formation of the double-helix structure of DNA see Section . The important functional groups participating in H-bond formation are the amino groups of cytosine adenine and guanine the ring nitrogens at position 3 of pyrimidines and position 1 of purines and the strongly electronegative oxygen atoms attached at position 4 of uracil and thymine position 2 of cytosine and position 6 of guanine see Figure . Another property of pyrimidines and purines is their strong absorbance of ultraviolet UV light which is also a consequence of the aromaticity of their heterocyclic ring structures. Figure shows characteristic absorption spectra of FIGURE .
