Section 3 Flashcards
What is a zwitterion?
A neutral molecule with both positive and negative charges
What groups are always present in an amino acid?
amino and carboxyl group
these are both bonded to the alpha carbon
If the pH is less than the pKa, then the _______ form of the compound predominates.
a) acid
b) conjugate base
If the pH is less than the pKa, then the ACID form of the compound predominates.
If the pH is greater than the pKa, then the conjugate BASE predominates
What are type I ionizable groups?
They are neutral in their acidic form (HA); they dissociate to form a proton (H+) and a conjugate base (A-) that is negatively charged.
Thus, type I groups carry a negative charge when the pH > pKa, and are neutral when pH < pKa.
Examples found in proteins include: carboxyl groups (the side chains of aspartate and glutamate, as well as the C-terminus), the sulfhydryl group in the side chain of cysteine, and the phenolic side chain of tyrosine.
What are type II ionizable groups?
They are positive in their acidic form (HA); they dissociate to form a proton (H+) and an uncharged conjugate base (A).
Thus, type II groups are uncharged when the pH > pKa, and carry a positive charge when pH < pKa.
Examples found in proteins include: amino groups (the side chain of lysine and the alpha-amino group of the N-terminus), the imidazole side chain of histidine, and the guanidinium group of arginine.
The correct operational relationship between pKa and pH is that:
a) Both are always < 7 for acids, and > 7 for bases.
b) Both are log functions
c) When pH = pKa, the compound in question will have a charge of +0.5
d) When pH = pKa, the ionizable compound in question (whether acid or base) will be half protonated and half deprotonated
d)
What is the isoelectric point (pI)?
the pH at which an amino acid or polypeptide is ionized and yet has no net charge.
the pH is calculated by averaging the pKa values for the ionizations states that straddle the electrically neutral species.
***important^ basically means when calculating pI you only take the pKa values from the two arrows beside the amino acid that has a net charge of 0 (the electrically neutral species)
How does a peptide bond form?
Through a dehydration reaction with the loss of a water molecule (the H from the amino group and the OH from the carboxyl group).
Therefore, when the N- and C-terminal groups on amino acids join together, they can no longer ionize by losing a hydrogen and thus do NOT contribute to the acid-base behaviour of peptides.
So basically this is C=O bonded to a NH
What does it mean when an amino acid is “ionizable”?
When we say that an amino acid is ionizable, it means that the amino acid has a functional group that can either accept or donate a proton (H+) depending on the pH of the environment. This ability to accept or donate a proton is a characteristic of acidic or basic functional groups.
