TABLE 31 Properties and Conventions Associated with the Comm

TABLE 3-1 Properties and Conventions Associated with the Common Amino Acids Found in Proteins pKa values Hydropathy occurrence in Abbreviation pK2 pKR symbol Mr CooH) (-NHG) (R group) pl index proteins (96) Amino acid Nonpolar, aliphatic R groups Glycine 75 2.34 5.97 0.4 Alanine 89 2.34 6.01. Proline Pro P 115 1.99 10.96 Valine Val V 117 2.32 Leucine Leu L 131 2.36 9.60 Isoleucine Methionine Met M 149 2.28 9.21 Aromatic R groups henylalanine Tyrosine Tyr Y 181 2.20 9.11 10.07 Tryptophan Trp W 204 2.383 9.39 Polar, uncharged R groups Serine Ser 105 2.21 5.87 -0.7 Threonine Thr T 119 2.11 Cysteine Cys C 121 1.96 10.28 8.18 Asparagine Asn N 132 2.02 Glutamine Gin Q 146 2.17 9.13 5.65 3.5 Positively charged R groups Lysine Lys K 146 2.18 8.95 10.53 9.17 Histidine Arginine Arg R 174 2.17 Negatively charged R groups Aspartate Asp D 133 1.88 9.60 3.65 2.77 3.5 5.3 Glutamate Glu E 147 2.19 9.67 4.25 3.22 3.5 6.3

Solution

The given tripeptide, GlyArgPhe, at pH 7 would have the following major species:

-NH₂ of glycine (charge +1)

-R group of arginine (charge +1)

-COOH of phenylalanine (charge zero)

Therefore, the major contributing species at pH =7 would be NH₃⁺ of glycine having pKa= 9.60, and R group of arginine having pKa = 12.48

To find the concentration of each of these species, the Henderson-Hasselbach equation will be used:

pH = pKa + log [A^-]/HA

Concentration of NH₃⁺ of glycine

7 = 9.60 + log [NH₃⁺]/ 0.1

or, [NH₃⁺] =2.51 x 10^-4 M

Concentration of  R group of arginine

7 = 12.48 + log [R group]/0.1

or, [R group] = 3.31 x 10^-7 M