Histidine Standard Lab Report
IIn part A, the unknown 30 was identified as Histidine by using a thin layer chromatography. Among the four amino acid standards (Ala, Gly, His, and Ser), the average Rf value of the unknown (0.31) was closest to that of standard Histidine (0.29) and its Rf percent error was lowest (6.9%). Furthermore, the color, shape, and size of the unknown clearly corresponded to that of Histidine standard. Overall, the experiment was successful in identifying the unknown amino acid, for the obtained Rf and color were very distinct from each other. Because Histidine was a basic amino acid, it had three ionizable groups: a carboxyl group, an amino group, and a basic side chain. The titration curve (graph 2) clearly corresponded to the result obtained from
part A. In graph 2, there were three different plateaus and the midpoint of each plateau (Pka) was where the amino acid was in half-protonated form and half-deprotonated formed. At pKa 1 = 1.8, the carboxylic group in the amino acid was the first group that lost proton since it was most acidic. At pka 2 = 5.4, the proton of the basic side chain in turn dissociated. As enough amount of OH- added into the solution, the amino group was last group to lose proton at pKa 3 = 10.04. Based on this information, the ionization form of the amino acid molecule in each pKa was constructed. In figure 3, at pKa 1, the amino acid was in half-form of A and half-form B. At pKa 2, the amino acid was in half-form of B and half-form. At pka 3, half of the amino acid was in the C form and half was in the D form. In short, the titration curve was successful in confirming the identity of the unknown 30.
Lastly, the molar mass of the unknown was 188.5 g/ mole. By comparing the experimental molar mass of the unknown with the literature value, Histidine has a lowest percent error molar mass (21%) among all four standards. Alanine, Glycine, and Serine were eliminated, for their percent errors were very high. Therefore, by determining the molar mass of the unknown, the identity of the unknown 30 as Histidine was further supported. In order to obtain a better result (slower percent error),
part A. In graph 2, there were three different plateaus and the midpoint of each plateau (Pka) was where the amino acid was in half-protonated form and half-deprotonated formed. At pKa 1 = 1.8, the carboxylic group in the amino acid was the first group that lost proton since it was most acidic. At pka 2 = 5.4, the proton of the basic side chain in turn dissociated. As enough amount of OH- added into the solution, the amino group was last group to lose proton at pKa 3 = 10.04. Based on this information, the ionization form of the amino acid molecule in each pKa was constructed. In figure 3, at pKa 1, the amino acid was in half-form of A and half-form B. At pKa 2, the amino acid was in half-form of B and half-form. At pka 3, half of the amino acid was in the C form and half was in the D form. In short, the titration curve was successful in confirming the identity of the unknown 30.
Lastly, the molar mass of the unknown was 188.5 g/ mole. By comparing the experimental molar mass of the unknown with the literature value, Histidine has a lowest percent error molar mass (21%) among all four standards. Alanine, Glycine, and Serine were eliminated, for their percent errors were very high. Therefore, by determining the molar mass of the unknown, the identity of the unknown 30 as Histidine was further supported. In order to obtain a better result (slower percent error),