biochemistry question. please help The peptide sequence shown below, containing

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biochemistry question. please help The peptide sequence shown below, containing a methionine followed by a long stretch of 26 Leu amino acids, does not readily dissolve in water. Based on the amino acid make-up of this peptide. provide an entropy-based explanation as to why its solubility in water is un favorable Met-Leu-Leu-Leu-Leu-(Leu)22 The picture below depicts a membrane protein. Of the peptide sequences shown below. which would you expect to be a part of this a-helix ? Explain your answer. i.) Asp-Lys-Gln-Glu-Val-Ala-Val-Leu Asp-Ile-Ala-Arg-Glu-Phe-Leu-Asp Asp-Leu-Arg-Val-Gln-Ala-Glu-Ile

Explanation / Answer

2.

The natural tendency toward dispersal does lead some peptide molecules to move into the water and some water molecules to move into the hexane. When a peptide molecule moves into the water, London forces between peptide molecules and hydrogen bonds between water molecules are broken. New attractions between peptide and water molecules do form, but because the new attractions are very different from the attractions that are broken, they introduce significant changes in the structure of the water. It is believed that the water molecules adjust to compensate for the loss of some hydrogen bonds and the formation of the weaker hexane-water attractions by forming new hydrogen bonds and acquiring a new arrangement.

Overall, the attractions in the peptide molecules move into the water are approximately equivalent in strength to the attractions in the separate substances. For this reason, little energy is absorbed or evolved when a small amount of a peptide is dissolved in water. To explain why only very small amounts of peptide dissolve in water, therefore, we must look at the change in the entropy of the system. It is not obvious, but when peptide molecules move into the water layer, the particles in the new arrangement created are actually less dispersed (lower entropy) than the separate liquids. The natural tendency toward greater dispersal favors the separate peptide and water and keeps them from mixing. That's why its solubility in water is unfavorable.

3.

II.

Because in first and third peptide sequence ( alpha helix) contains amino acid unit which are more water soluble

In case of second peptide sequence, more amino acids are less water soluble and hence it will like to stay in lipid layer.

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