a. Sketch the titration curve you would expect for a -melanotropin (Problem 4). Assume the pKa s of the N- and C-termini are 7.9 and 3.8, respectively. For side chains, assume the pKa values given in Table 5.1.
In Table 5.1

b. Calculate to three decimal places the charge on a -melanotropin at pH values of 11, 5, and 1.
c. Calculate the pI (isoelectric point) of a -melanotropin.
For parts (b) and (c) of this problem, refer to the logic used to solve Problem 2.15. You may find a spreadsheet useful to solve part (c).
In Problem 2.15
Is the value of average charge you calculated in part b reasonable, given the pI you calculated in part a?

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TABLE 5.1 Name Properties of the common amino acids found in proteins pKa of lonizing Residue Mass Occurrence Side Chain Abbreviations 1- and 3-letter codes aCOOH Group A, Ala R, Arg N, Asn pKa of pKa of -NH3 Group (daltons) in Proteins (mol %) 9.7 Alanine Arginine Asparagine Aspartic acid D, Asp Cysteine Glutamine Glutamic acid E, Glu Glycine Histidine Isoleucine Leucine Lysine Methionine Phenylalanine F, Phe Proline Serine Threonine Tryptophan yrosine Valine 71.08 156.20 114.11 115.09 103.14 128.14 129.12 57.06 137.15 113.17 113.17 128.18 131.21 147.18 97.12 87.08 101.11 186.21 163.18 99.14 2.3 2.2 12.5 8.8 9.8 10.8 5.0 4.2 5.9 1.3 3.7 3.9 8.3 C, Cys O, Gln 2.2 2.2 2.3 9.7 9.6 9.2 9.7 9.6 9.0 9.2 4.2 G, Gly 7.9 2.4 5.5 IS 2.4 L, Leu K, Lys M, Met 2.2 10.0 2.0 4.0 P Pro S, Ser T, Thr W, Trp Y, Tyr V Val 10.6 9.2 10.4 9.4 5.6 2.4 1.5 10.1