Final answer:
The approximate pH at which the pentapeptide ala-asp-his-cys-lys would have an approximate net charge of 1.5 is 6.04. This pH is chosen because it is close to the pKa of histidine, allowing it to be partially protonated, while the other amino acids contribute to the desired net charge.
Step-by-step explanation:
To determine the pH at which the pentapeptide ala-asp-his-cys-lys would have an approximate net charge of 1.5, we must consider the pKa values of the ionizable side chains in the peptide and the pH environment. Amino acids like aspartic acid (Asp) and glutamic acid have low pKa values and are negatively charged at neutral pH, whereas basic amino acids like lysine (Lys), arginine (Arg), and histidine (His) have higher pKa values and are positively charged at neutral pH. The specific side chains of the amino acids in the pentapeptide will have different levels of protonation depending on the pH, thereby affecting the overall net charge of the peptide.
At a pH that is lower than the pKa of an acidic side chain, the group will be protonated and not carry a negative charge. Conversely, at a pH higher than the pKa of a basic side chain, the group will lose a proton and not carry a positive charge. Peptides and proteins often have an overall neutral charge at their isoelectric point (pl). However, as the question asks for a net charge of 1.5, which is not neutral, we are not looking for the pl but a pH where the charges from the acidic and basic side chains balance out to the desired net charge.
Considering the provided pKa values and the behavior of the amino acids, a pH around 6.04, which is close to the pKa of histidine, would result in histidine being partially protonated. Since the rest of the peptide's amino acids would also be in states that contribute to the net charge, pH 6.04 seems a reasonable estimate for achieving the desired net charge of 1.5 for the pentapeptide.