titration biochem lab

Are amino acids with positive side chains acidic or basic?

They are basic (so lysine, arginine, and histidine have basic side chains)

Which are the ionizable side groups?

The ones with charge (so negative charge and positively charged, which incluudes histidine

How many amino acids are known to exist? And how many of them are present in proteins?

200 amino acids known to exist, but only 21 are present in proteins

What is the 21st amino acid?

Selenocysteine (Sec) —> only amino acid containing an essential dietary micronutrient (selenium)

What bonds form the secondary structure? What bonds form the tertiary structure?

Hydrogen bonds (between the amino and carboxyl group) form the secondary structure

As for the tertiary structure, many types of noncovalent bonds between variable side chains form the tertiary structure

What is Ionization?

A molecule acquiring a negative or positive charge by gaining or losing protons

At some low rate, pure water dissociates into ___ ions, donating a proton to ________.

dissociates into two ions, donating a proton to another water molecule

H2O (two-way arrow) H⁺ + OH^-

Does water readily ionize?

No, as the K (equilibrium constant) is very small for pure water (1×1—14), meaning that the concentrations of H and OH are always equal. So, when H increases, OH decreases, and vice versa

What is the definition of an acid according to Bronsted?

A molecule that acts as a proton donor (acids give protons!)

How much do strong acids dissociate in solution?

They dissociate completely in solution, so their concentrations are equal to the concentration of hydrogen

How do you convert pKa to Ka?

pKa = -logKa or Ka = 10^(-pKa)

What is the equation for K_a (dissociation constant)

Ka = ([H+][A-])/[HA]

The smaller the value of pKa, the stronger or weaker the acid?

The stronger the acid

When the pKa is negative, is equilibrium steeply tilted toward the products or reactants?

Toward the products

ex. Strong acids, which completely dissociate in solution, steeply tilted towards ion products

In weak acids, which is when the pKa is positive (greater than zero), is equilibrium steeply tilted towards products or reactants?

Neither, both reactants and products are present in solution, as the weak acid only dissociates partially in solution

pH is a measure of the _____ ions in solution

measure of the hydrogen ions in solution

What is pKa?

The negative base 1- log of the acid dissociation constant

• the pH value at which a molecule will accept or donate a proton and therefore “buffer” drop in pH creating a plateau

When pH = pKa, the ratio is 1 for an ionizable group. What does this mean?

That the group will accept or donate a proton at this point, therefore creating a buffer zone

Knowing the pKa can help you predict what state of a group at a particular pH?

The ionization state

Amino acids have at least how many pKas?

They have at least two and an isoelectric point

What is the isoelectric point?

pH when the amino acid has no net charge (charges balance out)

At high pHs, are things protonated or deprotonated?

Deprotonated. They are protonated at low pH (when you have an excess of hydrogen ions)

When HCl, the pH drops rapidly initially, and then gradually drops in buffer zone. Why does this happen?

Because the ionizable group starts accepting protons as it is near it’s pKa