It is important to understand the overall net charge on a molecule at a specific pH, especially when carrying out native gel electrophoresis. At pH 9, what will be the predominant form of the amino acid lysine?

Select the structure that best represents your answer. Note: pka1 = 2.18, pka2 = 8.95, pkaR = 10.53

Question

It is important to understand the overall net charge on a molecule at a specific pH, especially when carrying out native gel electrophoresis.  At pH 9, what will be the predominant form of the amino acid lysine?

Select the structure that best represents your answer. Note: pka1 = 2.18, pka2 = 8.95, pkaR = 10.53

BlackTom AI · Accepted Answer

Question restatement: The problem asks, at pH 9, which structure best represents lysine’s predominant form, given pKa values: pKa1 = 2.18 (carboxyl group), pKa2 = 8.95 (alpha amino group), pKaR = 10.53 (side chain amino group).

Step-by-step analysis of lysine’s ionizable groups at pH 9:
- Carboxyl group (pKa 2.18): Since pH 9 is well above 2.18, this group will be deprotonated (COO–).
- Alpha amino group (pKa 8.95): pH 9 is just above the pKa, so this group is partially deprotonated. Using Henderson-Hasselbalch for the NH3+/NH2 couple: [NH2]/[NH3+] ≈ 10^(9 − 8.95) ≈ 1.12. This means roughly half of the alpha amino groups are protonated (NH3+) and roughly half are neutral (NH2). Therefore, the alpha amino group contributes about +0.47 to +0.53 charge on average, depending on the exact fraction protonated.
- Side chain amino group (pKaR 10.53): pH 9 is below this pKa, so this group remains mostly protonated (NH3+). The ratio [NH2]/[NH3+] ≈ 10^(9 − 10.53) ≈ 0.029, so ≈ 97% protonated (NH3+), contributing nearly +1 charge.

Net charge expectation:
- Carboxyl: −1 charge
- Alpha amino: approximately +0.47 to +0.53 charge on average (due to partial protonation)
- Side chain amino: approximately +0.97 to +1 charge (mostly protonated)
Total average charge ≈ −1 + (0.5) + (1.0) ≈ +0.5 to +0.6. In practical terms, lysine at pH 9 remains positively charged overall, with the carboxylate deprotonated, one amino group partially protonated (alpha), and the side-chain amino mostly protonated, yielding a net positive charge.

What this implies for selecting the correct structure:
- The correct structure should depict a deprotonated carboxyl group (–COO–) and both amino-containing groups in their protonated states as much as possible, with the side-chain amino group clearly protonated (NH3+), and the alpha amino group largely protonated (NH3+) but with a small fraction as NH2 due to pH just above its pKa. The overall molecule should carry a positive net charge.

Evaluation of the options:
- Option

BCMB20005_2025_SM1 Week-5 Pre-Lab Quiz

View Explanation

Log in for full answers

Similar Questions

In a consumer society, many adults channel creativity into buying things

Economic stress and unpredictable times have resulted in a booming industry for self-help products

People born without creativity never can develop it

A product has a selling price of $20, a contribution margin ratio of 40% and fixed cost of $120,000. To make a profit of $30,000. The number of units that must be sold is: Type the number without $ and a comma. Eg: 20000

Which of the following statement regarding cost is correct:

Under the assumptions used in cost-volume-profit analysis, as the activity increases:

The CVP and break-even analysis are supported by a range of important assumptions. Which of the following is not an assumption of the CVP:

The break-even point is that at which:

More Practical Tools for Students Powered by AI Study Helper

Homework AI Solver

Stylized AI Paper Writer

Plagiarism Checker Assistant

Citation AI Academic Writing Tool

In-Class Translation Assistant

AI Note Generator

AI Quiz Answers

Past Exam Questions from University Test Bank

Smart Practice Assistant

Adaptive Practice

Making Your Study Simpler