circular dichorism question.. 1. A common feature of many DNA binding proteins i

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circular dichorism question..

1. A common feature of many DNA binding proteins is the ‘leucine zipper’. It is two similar sequences of 30-35 amino acids containing multiple leucines. Each sequence forms a right-handed -helix, the two helices wrap around each other to form a left-handed super helix. A number of studies have been carried out of this ‘coil-coil’ structure with model peptides. In one such study, the circular dichroism spectrum at 0°C had a large positive peak at 195 nm and negative minima at 208 nm and 222nm. This was interpreted as being the spectrum of a completely helical structure.

At 0°C, the molar ellipticity at 222nm is -33,000 deg cm2 dmol-1. At 80°C, the molar ellipticity at 222 nm is essentially zero. Interpret the observation at 80°C.

A.The two peptides forming the zipper are completely denatured

B.The two peptides forming the zipper have formed a stable -helix

C.The two peptides forming the zipper have aggregated and formed coil-coil-coil-coil structure

D.The two peptides forming the zipper formed -sheets

Continued from question 1. At 55°C, the molar ellipticity at 222nm is -16,000 deg cm2 dmol-1. What percentage of the peptide is -helical at this temperature?

A.19.6%

B.28.2%

C.48.5%

D.63.8%

Explanation / Answer

A.The two peptides forming the zipper are completely denatured

C.48.5%

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