Chem 142              Hmwk Set 2 Key

 

1-    The characteristic pH at which the net electric charge is zero is the isoelectric pH, designated as pI. Draw the structure of glycine at pH

i)              Well below the pI

ii)            Well above the pI

iii)            What is the net charge of the amino acid at any pH below the pI.

The structure of glycine at low and high values of pH is shown below. At a pH equal to the pI equimolar concentrations of positively and negatively charged species are present.  Thus at its pI glycine, like all molecules, is neutral.  At pH below the pI, the ÐCOO- group begins to pick up a proton and the glycine is in its fully protonated, +1 charge state.

 

 

 

  1. What is the difference between a protein and a polypeptide chain?

 

The word polypeptide refers to any polymer of peptide bond-linked (ie amide-linked) alpha-amino carboxylic acids.  The word protein denotes a subset of polypeptides.  In general the word is used to imply the subset of polypeptide that folds into a unique 3-dimensional structure that is biologically functional (or was derived via mutation or modification of a functional molecule).

 

  1. A measure of the hydrophobicity of an amino acid is the free energy of transferring it from water to, for example, olive oil.  You have a test tube filled with an aqueous solution of tryptophan.  You pour oil over the top of it and shake like crazy (to equilibrate it).  How would you then determine the free energy of transfer?

 

Because tryptophan has a relatively hydrophobic, aromatic sidechain, the amino acid will migrate into the oil until, if you shake long enough, there will be equilibrium between aqueous and oil phases.

Trp (aq) ó Trp (in oil)

Equilibrium constant can be obtained from the relative concentrations.

Thus, to determine DG you simply measure this equilibrium constant.  How do you do this?  You simply measure the concentration of tryptophan in each of the two solutions and take the ratio.  The concentration of tryptophan can be measured via a variety of approaches.  The first two that come to my mind are to do it spectroscopically (tryptophan absorbs at 280nm) or by knowning how much tryptophan youÕd put into the water originally, and measuring the final amount by drying the water and weighing it.