4. Effect of pH on the Conformation of -Helical Secondary Structures The unfoldi

Question

4. Effect of pH on the Conformation of -Helical Secondary Structures The unfolding of the helix of a poly- peptide to a randomly coiled conformation is accompanied by a large decrease in a property called specific rotation, a meas- ure of a solution's capacity to rotate circularly polarized light. Polvglutamate, a polypeptide made up of only L-Glu residues, has the -helix conformation at pH 3 . When the pH is raised to 7, there is a large decrease in the specific rotation of the solu- tion. Similarly, polylysine (L-Lys residues) Is an helix at pH 10, but when the pH is lowered to 7 the specific rotation also decreases, as shown bv the following graph. 2 5 Helix Poly(Glu) Helix Random conformation Poly(Lys) Random conformation 0 2 46 8 10 12 14

Explanation / Answer

Amino acids are the structural component of proteins. In each amino acid, carbon is bonded to an amino group, a carboxylic group, a hydrogen atom and an R group which is different for different amino acids. Acidic amino acids have an extra - carboxylic group in the side chain or extra amino group in the side chain. Glutamate is an amino acid with carboxylic group in the side chain and lysine has amino group in the side chain.

The presence of carboxylic group causes a transition between - COOH and - COO- group. This transition of state of carboxylate ions occur according to the presence of hydrogen ions in the solution. The concentration of hydrogen ions in the solution is decided by the pH of the solution. Similarly, amino groups transit between NH2 and - NH4+ ions. This is also affected by the pH of the solution.

The ionic forms of the carboxylate and ammonium groups decide the conformation of a polypeptide. The alpha helical structure is stabilized by these ionic interactions along with hydrogen bonds. Change in the ionic state of these groups affect the alpha helical structure. In turn, change in the ionic groups is caused by the pH of the solution. So, pH affects the conformation of these polypeptides.

The stability of a particular group of ion is largely affected even in a narrow range. As transition in the group of ion occurs over a narrow range, so there is change in structure of polypeptides over this range.

Related Questions

Navigate

• Browse (All)
• Browse 4
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.