I don't understand this zwitterion form . Here is the question. write the zwitterion form of the amino acid cysteins shown in the follwing figure. If the pI for cysteine is 5.1 at what pH will the zwitterions be the predominant form? Can you please show me step by step on how to solve this?
To find the zwitterion form of the amino acid cysteine and determine the pH at which it is predominantly present, you can follow these steps:
Step 1: Understand the concept of a zwitterion
A zwitterion is a molecule or ion that has both a positive and a negative charge within the same molecule or ion. In the case of amino acids, the zwitterion form occurs when the carboxyl group (-COOH) loses a hydrogen ion (H+) and becomes negatively charged (-COO-), while the amino group (-NH2) gains a hydrogen ion (H+) and becomes positively charged (-NH3+).
Step 2: Examine the structure of cysteine
Cysteine is an amino acid that contains both an amino group (-NH2) and a carboxyl group (-COOH), along with a side chain (-CH2SH). The specific structure of cysteine as shown in the figure is necessary for determining the zwitterion form.
Step 3: Identify the protonatable and deprotonatable functional groups
In cysteine, the carboxyl group (-COOH) can lose a hydrogen ion (H+) to become negatively charged (-COO-), while the amino group (-NH2) can gain a hydrogen ion (H+) to become positively charged (-NH3+). The side chain does not participate in this ionization.
Step 4: Write the zwitterion form
Combine the negatively charged carboxyl group (-COO-) and the positively charged amino group (-NH3+) to form the zwitterion form of cysteine, which is:
NH3+ - CH2 - COO-
This form has a positive charge on the amino group and a negative charge on the carboxyl group.
Step 5: Determine the pH at which the zwitterions are predominant
The isoelectric point (pI) of an amino acid is the pH at which it exists as a zwitterion. In this case, the pI of cysteine is given as 5.1.
Since the zwitterion form occurs when the amino group is positively charged and the carboxyl group is negatively charged, it implies that at pH values below the pI, the amino acid is predominantly in the positively charged form, and at pH values above the pI, it is predominantly in the negatively charged form.
Therefore, the pH at which the zwitterions are the predominant form is equal to the pI of cysteine, which is 5.1.
To summarize, the zwitterion form of cysteine is NH3+ - CH2 - COO-, and the pH at which the zwitterions are the predominant form is 5.1.