Highlights Lecture #3 Spring 2017

Highlights Buffers

1. A buffer is a solution containing a weak acd and its salt that resists changes in pH. Remember that pH is a measure of the concentration of hydrogen ions (pH = -log[H+]), so the addition or subtraction of hydrogen ions to a buffer solution has a smaller effect on the pH than the addition or subtraction of hydrogen ions to a solution of pure water.

2. Buffers act by providing protons to replace the lost protons when a strong base is added (by HA releasing H+) to replace the protons lost by forming water or by absorbing the protons added from a strong acid, like HCl. In the latter case, A absorbs the H+ to become HA.

3. This is important – for every proton added by a strong acid, one HA is created by the buffer and one A is lost. Conversely, for every proton removed by NaOH, one A is created and one HA is lost. This is one of the most important points students overlook about buffer solutions.

4. When the amount of salt equals the amount of acid in the Henderson-Hasselbalch equation, their ratio equals one and the log term therefore equals zero. Thus, when salt = acid for a buffer, pH = pKa. When the pH is less than the pKa, there will be more acid than salt. When the pH is greater than the pKa, there will be more salt than acid.

5. Note for a buffer that when [salt] = [acid], the maximum capacity of the buffer is reached. That is, at this point, the buffer will resist changes in pH more strongly than at any other point.

6. Buffers are effective when the pH of the solution in which they are found is within about 1 pH unit of the pKa of that buffer.

7. Proteins have optimal activity at fairly specific pH values. For example, pepsin, which is an enzyme that is active in stomach (where there is a low pH, due to a lot of acid) has a maximal ability to catalyze reactions at about the pH of stomach acid. Most enzymes have their maximal ability to catalyze reactions at around the pH of body fluids – about 7.4.

Highlights Amino Acids

1. Amino acids are the building blocks of proteins. 20 amino acids (with rare exceptions) are used to make every protein on earth.

2. All amino acids except glycine contain an asymmetric carbon and thus are stereoisomers. The forms are called D and L.

3. With rare exceptions, all amino acids found in cells are in the L configuration.

4. All amino acids have an alpha carboxyl, an alpha amino group, an alpha carbon group, and an R group. Amino acids differ from each other ONLY in the R groups they contain.

5. We group amino acids in this class into four groups – non-polar, polar, acidic, and basic (note that these are not strong bases, so I sometimes refer to them as amine-containing). You should know the names of the 20 amino acids and the groups they belong to. You do not need to memorize the structures of the amino acids, but there are some things about some of their R groups you should know.

6. Specifically, you should know which amino acids have R-groups that can ionize and what their charge is when they ionize.

7. Carboxyl R-groups, found on aspartic acid and glutamic acid, for example, can exist as COOH (no charge) or COO (negative charge).

8. Amine groups, such as found on lysine, arginine, and histidine, come in a variety of forms, but we will abbreviate them as NH3+ or NH2 (no charge).

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