You are given poly-L-Serine and poly-L-Threonine. (repeating sequences of Ser-Se

Question

You are given poly-L-Serine and poly-L-Threonine. (repeating sequences of Ser-Ser-Ser and Thr-Thr-Thr). Poly-L-Serine has a large negative signal at 222 nm, but Poly-L-Threonine does not. How might you explain this result?

An undergraduate researcher you know is confused. They were told to measure the amount of helix in poly-L-Glutamate in 0.1 M phosphate. When they made the solution using H3PO4 and KH2PO4 they saw a nice CD signal indicative of alpha helix. The next week they tried again using identical reagents except that they were out of the potassium salt and had to substitute Na2HPO4. When they made a CD measurement they saw a large negative signal at 195 nm indicating random coil conformation. How would you explain the behavior of poly-Glu to the confused student so that it made sense? What simple step did the student likely forget while making up the solution?

Explanation / Answer

Q1. Answer

Peptides are studied using circular dichroism (CD), which shows the secondary structure of the protein. Different components (alpha helix, beta sheets and random coils) are shown differently.

In the case of alpha helices, the CD results show negative bands at 222 and 208 nm.

Now, when we study the two peptides Poly-L-Serine and Poly-L-Threonine,

Poly-L-Serine has an alpha helical conformation and hence it shows a strong negative band at 222nm. In the case of Poly-L-Threonine, since threonine is a branched amino acid, it most likely forms the Beta pleated secondary structure and hence no negative band at 222nm.

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