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Peptide Bonds, Amino Acids and Chemical Structures

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I need some help with biochemical questions:
1. How do you calculate the pH of a solution of lysine if the side chain amino group is 3/4 dissociated?
2. What is the isoelectric point of His?
3. For L-Ile, describe with drawings why it is 2S,3S?
4. What are the 6 atoms that are coplanar due to the resticted rotation about the peptide bond?
5. How many different tetrapeptide sequences are possible of the number of different amino acid types is 20?
6. Draw the chemical structures for tripeptides G-F-F and for F-F-G..are they the same?

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Solution Summary

This solution contains answers to the aforementioned biochemistry questions with clear explanations and attached diagrams for better understanding. All formulas and workings are shown.

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Solution:
1. The Henderson-Hasselbalch equation states that:

pH = pKa + log [A-]/[HA]

The pKa of the side chain amino group is 10.53. You can look this number up in a table of pKa values for the amino acids. Since 3/4 is dissociated, i.e. the form --NH2, then 1/4 must be protonated, i.e. the form --NH3+.

pH = 10.53 + log (3/4)/(1/4)
pH = 10.53 + log 3
pH = 10.53 + 0.48
pH = 11.01

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2. You can look this up in a table attached below. The pI of histidine is 7.59.

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3. See attached graphic. I've included a graphic that shows all of the four possible stereoisomers of isoleucine with each one labelled as to what it is. The first one is 2S,3S. Remember the rule. If you place the lowest priority atom in the back, ...

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