Solution to PS 3 #1
a) a helices containing amino acid
residues with ionizable side chains are sensitive to changes in pH because
pH determines whether or not the side chain is charged. Identical
residues on side chains of adjacent amino acids in a polypeptide create
electrostatic repulsion that precludes packing of a polypeptide into an
a-helix.
The side chain of Asp has a pKa of 3.9, therefore at pH's less than 3.9
nearly all of the side chains in poly-Asp are uncharged (fully protonated)
and the polypeptide chain can form an
a-helix.
At pH 5.0 or above nearly all of the Asp side chains are charged
(-1); elecrostatic repulsion between neighborong charges thus prevents
formation of helices and causes the polypeptide to assume a fully extended
b-strand
conformation (where the side chains are as far part as possible).
b) The pKa of the side chain of Lysine is 10.5. When the pH is
sufficiently above 10.5 most of the sidechains of poly-Lys are uncharged
and the polypeptide would likely form an a-helix.
At lower pH values the polycationic molecule (lots of + charges) would
assume an extended configuration.
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