Question

it's a bio-chem problem.

"I have attached a titration of a mock amino acid with OH-. I have labeled the different charge states of the amino acid as I, II, III, IV. I have labeled different points on the titration curve as A, B,...G. I want you to tell me which form(s) of the species I,II,III,IV predominates (is present at greatest concentration) at each of the lettered points on the curve. I would also like you to calculate the PI for the molecule.

Relevant pKa's are below:

pKa of alpha NH3 group=9.4

pKa of alpha COOH= 1.95

pKa side chain NH3=approx 10

FOO COOH H₂Ntch CH2 CH2 HANTCH Çoo" H2N-CH CHO CH2 CH2 CH2 CH2 CH2 NH3 NH3 2 o pH 6 4 - B 1/ 0 А 0.5 1 1 1.5 2 2.5 Equivalents of OH

Answer 1

Ans:

Which form predominates out of I, II, III, and IV

Ans:

Points on the curve                        Species present

A                                                             I

B                                                            II

C                                                            II

D                                                           III

E                                                           IV

F                                                           IV

G                                                          IV

Species IV will be present abundantly as compared to other species.

pI calculation;

Ans:

Since the amino acid give is basic, we will have to add pKa of the alpha-amino group and pKa of side chain;

pI = pKa1 + pKa2/2

pI = 9.4 + 10/2

    = 9.7