Amino Acids

More documents

Recommendations

Info

Chapter 4 . Amino Acids If we recognize that [Cl - ] =[NH 3 + ] (because Cl - and the amino acid are in equimolar amounts), the charge neutrality equation may be simplified to: [COO - ] =[H + ] and combined with (1) and (2) we have : [H pH = 2.4 + log + ] 0.3−[H + ] If we make the assum ption that [H + ]
Chapter 4 . Amino Acids At the pI, [ COO − ] = [NH 3 + ] Using the Henderson - Hasselbalch equations pH = pKCOOH + log [COO− ] [COOH] and pH = pK + + log NH 3 [NH2] + [NH 3 ] and recognizing that [COOH] = [Amino acid] total -[COO − ] and that [NH 2] = [Amino acid] total -[NH 3 + ] upon substituting, solving for [ COO − + ] and [NH 3], and setting these terms equal at pH = pI we find that pI = pKCOOH + pK + NH 3 2 2.4 + 9.6 pI = = 6.0 2 or 10. Quantitative measurements of optical activity are usually expressed in terms of the specific rotation, [a]D 25, defined as [ α ] D 25 = Measured rotation in degrees × 100 (Optical path in dm) ×(conc. in g/ml) For any measurement of optical rotation, the wavelength of the light used and the temperature must both be specified. In this case, D refers to the "D line" of sodium at 589 nm and 25 refers to a measurement temperature of 25°C. Calculate the concentration of a solution of L-arginine that rotates the incident light by 0.35° in an optical path length of 1 dm (decimeter). Answer: [α ] D 25 = Measured rotation in degrees × 100 (Optical path in dm) ×(conc. in g/ml) [Arg] = 0.35°×100 [α ] D 25 × 1dm [α ] D 25 for L-arginine = 12.5° (from Table 4.2) Thus, [Arg] = 0.35°×100 = 2.8 g/ml 12.5°× 1dm 11. Absolute configurations of the amino acids are referenced to D- and Lglyceraldehyde on the basis of chemical transformations that can convert the molecule of interest to either of these reference isomeric structures. In such reactions, the stereochemical consequences for the asymmetric centers must be understood for each reaction step. Propose a sequence of reactions that would demonstrate that L(-)-serine is stereochemically related to L(-)-glyceraldehyde. Answer: The aldehyde group of L(-)-glyceraldehyde is converted to L-glyceric acid by oxidation and L-glyceric acid is converted to 2-hydroxypropanoic acid. 2-Hydroxypropanoic acid is converted to 2-bromopropanoic acid by nucleophilic substitution using concentrated HBr. The reaction results in alteration in stereochemistry. 2-Bromopropanoic acid is subsequently converted to L-(-)-alanine by ammonolysis and alteration in stereochemistry. Finally, L-(-)alanine is convert serine. The reaction sequence is shown below. 55
Page 1 and 2: Chapter 4 Amino Acids . . . . . . .
Page 3 and 4: Chapter 4 . Amino Acids Methionine
Page 5 and 6: Chapter 4 . Amino Acids 47
Page 7 and 8: Chapter 4 . Amino Acids Problems an
Page 9 and 10: Chapter 4 . Amino Acids The equival
Page 11: Chapter 4 . Amino Acids Using the H
Page 15 and 16: Chapter 4 . Amino Acids Answer: Dow
Page 17 and 18: Chapter 4 . Amino Acids 7. Of the f
Page 19: Chapter 4 . Amino Acids Summary Ami

Amino Acids

Create successful ePaper yourself

Delete template?

Save as template?