4. The Cy1 methyl protons of Val94 in folded T4 lysozyme have a chemical shift o
ID: 1041867 • Letter: 4
Question
4. The Cy1 methyl protons of Val94 in folded T4 lysozyme have a chemical shift of-0.8 ppm. In the unfolded protein, they have a chemical shift of +0.9 ppm, close to the chemical shift for the Cyl methyl protons on an isolated Val residue in solution. The structure of lysozyme reveals that Val94 is adjacent to Phe158. The structure of Val is shown below with the Cyl labeled. The structure of phenylalanine is also shown. CH-CH-CO NH2 H3Cy2 L-valine phenylalanine (a) Provide an explanation for the pronounced upfield shift of the resonances for the methyl protons in the folded protein. Use diagrams to explain.Explanation / Answer
4)
a) The chemical shift is related to the resonance frequency of a particular nucleus.
Upfield (shielded) a nucleus whose chemical shift is decreased, feels weaker magnetic field due to the addition of electron density, magnetic induction, etc in neighboring atoms or groups.
Shielding is when the nucleus experiences a weaker magnetic field around it. This can be caused by other atoms “getting in the way” of the nucleus and the magnetic field, or the nucleus itself having a low spin?flip energy. Due to the weaker magnetic field it experiences, a nucleus with more shielding will have a lower ppm and therefore lie on the right side of the chemical shift scale.
High electron density of neighboring atoms may shield the nucleus from an external magnetic field.
A Cy1 methyl protons of Val194 in folded T4 lysozyme have a chemical shift of -0.8 ppm. In folded lysozyme methyl protons are adjacent to the phenylalanine (phenylalanine have a high electron density) therefore these protons shields strongly compared to the unfolded lysozyme.The methyl protons in the folded protein pronouns the upfield chemical shift.
b)
Folded T4 lysozyme (-0.8 ppm) = upfield chemical shift
Unfolded T4 lysozyme (+0.9 ppm) = downfield chemical shift
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