Amino Acids

Step 1: Understanding dipeptide formation.
A dipeptide forms when two different amino acids join through a peptide bond. The order of amino acids matters: A–B and B–A are different molecules.
Step 2: Considering chirality of amino acids.
Both phenylalanine and leucine are chiral (each has one stereocenter). Each chiral amino acid exists as L and D enantiomers when considered for maximum possibilities.
Step 3: Counting possible dipeptides.
Two amino acids (A and B) can produce four stereochemically distinct dipeptides:
1. L-Phe–L-Leu
2. L-Leu–L-Phe
3. D-Phe–D-Leu
4. D-Leu–D-Phe
Thus, the maximum number is 4.
Step 4: Conclusion.
Hence, phenylalanine and leucine together can form a maximum of four dipeptides.

Step 1: Definition of isoelectric point.
The isoelectric point (pI) is the pH at which an amino acid or peptide has no net charge. For amino acids with more than one dissociable group, pI is the average of the two pK values corresponding to the ionizable groups that contribute to the net charge.
Step 2: Formula for pI.
For lysine, the isoelectric point can be calculated as: Using the given pK values for lysine (pK = 2.2 and pK = 9.1), the average is:
Step 3: Conclusion.
Thus, the isoelectric point of lysine is 6.7.