Question

One method for separating polypeptides utilizes their different solubilities. The solubility of large polypeptides in water depends upon the relative polarity of their R groups, particularly the number of ionized groups. The more ionized groups there are, the more soluble the polypeptide. Which polypeptide, (Gly)20 or (Lys-Ala)3, is more soluble at pH 7.0? O (Lys-Ala); O (Gly)20 Which polypeptide, (Glu)20 or (Phe-Met)z , is more soluble at pH 7.0? O (Glu)20 O (Phe-Met), Which polypeptide, (Ala-Ser-Gly), or (Asn-Ser–His),, is more soluble at pH 6.0? O (Ala-Ser-Gly), O (Asn-Ser–His), Which polypeptide, (Ala-Asp-Gly); or (Asn-Ser-His)s, is more soluble at pH 3.0? O (Ala-Asp-Gly), O (Asn-Ser-His),

Answer 1

Here pK_a value means the ability of an amino acid to donate the H⁺ ions when pH is increased in the medium to counteract the increasing OH^- ions in the solution. Lesser the pK_a value, stronger the acid. At low pH, all the groups capable of taking up H⁺ ions will become protonated and these protons will release on increasing the pH.

Following is a list of all the pK_a values of all the amino acids' side chains and alpha groups:-

1. alpha- NH₂ group = 9.60
2. alpha- COOH group = 2.34
3. R- NH₂ group (Lysine) = 10.60
4. R- COOH group (Glutamic acid) = 4.25
5. R- NH₂ group (Histidine) = 6.01
6. R- COOH group (Aspartic acid) = 3.86

Solutions are attached in the following pictures:

Part 1: (rude or (Lys- Ala), (Gly)₂0 Amino Acid Ipka of d-cool = 2.34] Ipka of d-NH₂ = 9.60] 20th Glycine 1st Glycine NH, “CH - NH- H-COAH ...... - Co NM –ch – OD Structure in Polypeptide : Peptide bond At Low PH Ph NH .... c bNH - CH - (Very High Htion concentration). AN - H “NH - H- -CONH-CH-coo Groups capable of accepting Htion will takeitup. HpH 7.0 HAN - CH-CONH-CH-CO S (this group will donate its it only when the pH will he equal or greathan greater than its pka ... Net charge on this peptide molecule - Zero (this group will donate its lition to counteract the increasing PH effect since it's Pka value is quite less (Lys Majz [pka veue of R-Wty of Lys = 10.6] , h, 9 CH - CNH - H - NH “H - . 51 Amino Acid str. in Polypeptide ORFC 1 - Co NH - H - CÓ NH -CH- top 4 Chiny ( 4 CH₂ CH3 (CH2) ON Hz ALLOW PHO @NH₂ NH3 At PH:7 603 -CONH-CH-CONH-CH-CONH - CH-CONH -CH-CONH-CH-coo 1- (CH). CH₂ (CH) Chiz NH₂ 4 Nhz Net charge: (+5. [More charged groups more solubility] Thuss (Lys- Alle is more soluble than (Gly) 2o at PH to

som now on, I will be writing Awino Auds name in the backbone Structure Chee the backbone polypeptide is same for au cases. Only the side chains ( groups way size chains and a groups will be mentioned) Part 2: To-oon group pka salur: 2.34 comme prevare: 4.25] -Gw-cool : Polypetide structure: H₂N-Gm Gm - Gle-Gw ... (Low PH) (cm) (on (1) con coor con coon Ph:10 : M - Ghe-Gow-com..... G-code aut al land co. Those groups lost their meion when ph crossed their pka rauses to counternet increasing on ion cone (High pH). Catalana com = -20 (10. godine tha Recor) + 1 (2002 1920 ) - 1 (one ( Mt-low - ), : NH, - Phí - Lu - - Mu ( - - - LH و - - - OOS-ty Au Non-polar groups, nothing to donate Ar pH:7.0 IN -Phe-bet-The-Met-the-her- (nr de aut in an Ney charge : Id Soluble This who is more than (the-kat)

Part 3 d-NH₂ group: 9.60 1 ERN &-coon group: 2.34 group of Histidine: 6.01] (Ala-ser-Gly)s . .. Ser-Gly-cool Structure: (LowpH) 1 H₂N - Ala-Ser-Gly CH2OH n. V CH, OH PH 6.0 - Ala-Sex-Gly . - ding dom ha chionn Sex-Gly - 10 он на concept is same as previous LR groups are nonpolar, sono donation Lego (Asn-ser-tis)s Structure Low pr) : My - Asn-Sen-His.... Ser-His - COOH -N саун H LON A pH: 6.0 : H₂N - Asn-Sex-His. – 1 Сн.. Сп,ой Sen- tis -cool Only Histidine will donate Сңoh lits tion since it's pka valueis same as the pH). CONT, Net charge: Hence both are insoluble at pH 6.0 Had the pre beena little below dort (Asn-fer- His) as would have been rece soluble. ka WA

а 4 м. РУолші: I (Ala-Asp-Gly)s x-NH2:9.60 e-coon a-cool: 2.34 | (Asp) 3.86 B (he's Structure at low pH: ...Ala-Asp-Gly-cool -Asp-Gly- I 10 CH, H. CH, CH₂ LOOM LOOH I REAL PH:30 h Aa-Asp-Gly Con EN - Ala- Asp-6 - - . . I % Сн, н · COOH Соо Nercharge : Lero (Asn-ser-His)s structure at low pH nzn - Asn-sex-tis. . Ser-this-coot | Doh - agok ど W3 のくれっ BU Struct. At PH:3.0 HN - Asn-ser- this Ser- tis - cool cron INH Croh -NM CONH NH coon plan d- arge Calculation : 5-this Amino -NH₂ group of Asa Amino Aid) -1 Acids this ar last post = (+5) : (Asn-der-tis)s is soluble name than (Ha-Asp-Gly's appraded
Hope this will help. Thank you!