Multiple buffering

Ahern's Biochemistry #2 - Buffers

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• published: 02 Oct 2014
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1. Contact me at kgahern@davincipress.com / Friend me on Facebook (kevin.g.ahern) 2. Download my free biochemistry book at http://biochem.science.oregonstate.edu/biochemistry-free-and-easy 3. Take my free iTunes U course at https://itunes.apple.com/us/course/biochemistry/id556410409 4. Check out my free book for pre-meds at http://biochem.science.oregonstate.edu/biochemistry-free-and-easy 5. Lecturio videos for medical students - https://www.lecturio.com/medical-courses/biochemistry.course 6. Course video channel at http://www.youtube.com/user/oharow/videos?view=1 7. Check out all of my free workshops at http://oregonstate.edu/dept/biochem/ahern/123.html 8. Check out my Metabolic Melodies at http://www.davincipress.com/ 9. My courses can be taken for credit (wherever you live) via OSU's ecampus. For details, see http://ecampus.oregonstate.edu/soc/ecatalog/ecourselist.htm?termcode=all&subject;=BB 10. Course materials at http://oregonstate.edu/instruct/bb450 Highlights Buffers and Henderson Hasselbalch 1. The HAC/Ac- system acts as a buffer because it resists change in pH (that is the definition of a buffer). When protons are added to the system, Ac- gobbles them up. When protons are removed from the system by NaOH, HAc releases protons to make up the difference. A buffer system will be at maximum capacity when the concentration of HAc equals that of Ac- (Weak Acid = Salt). 2. Addition of protons (from HCl, for example) to the HAc GOES TO H+ + Ac- system will drive the reaction to the left due to Le Chatelier's principle. This will INCREASE the amount of HAc and DECREASE the amount of Ac-. The total amount of HAc and Ac- will remain the same in a system (unless, of course one adds additional HAC or Ac- ). Addition of NaOH (strong base) the same system will drive the reaction to the right, causing there to be LESS HAc and MORE Ac- 3. Adding a strong acid to a buffer system causes salt (A-) to be converted into acid (HA). For every molecule of HCl added, one A- is converted to one HA. If one had 500 molecules of HA and 500 molecules of A- in solution and one added 10 molecules of HCl, the resulting solution would have 510 molecules of HA and 490 molecules of A-. Adding a strong base (such as NaOH) to a buffer system causes HA to be converted to A-. For every molecule of NaOH added, one HA is converted to one A-. Thus, if one had 500 molecules of HA and 500 molecules of A- in solution and one added 10 molecules of NaOH, the resulting solution would have 490 molecules of HA and 510 molecules of A-. 4. For a system in its buffering range (+/- 1 pH unit of pKa), addition of each HCl molecule will cause one mole/molecule salt to convert to one mole/molecule weak acid. Conversely, addition of each NaOH molecule will cause one mole/molecule weak acid to convert to one mole/molecule salt. 5. Buffers are maximally effective when the pH = pKa. Within one pH unit on either side of the pKa, a buffer is reasonably effective. 6. Gain or loss of a proton by a molecule causes the molecule to gain or lose one full charge. Carboxylic acid systems (such as are present in amino acids) have two forms: COOH and COO-. The COOH is the acid (most protons) and the COO- is the salt (fewer protons). Amine systems (also in amino acids) have two forms: NH3+ and NH2. NH3+ is the acid and NH2 is the salt. 7. The loss/gain of a proton changes amino acids from a charge of zero to a charge of -1 (carboxyl groups) or a charge of zero to a charge of +1 (amine groups). If a molecule changes charge, it may drastically affect the interactions it has with neighboring amino acids in a protein - causing a great increase or decrease in attraction/repulsion depending on the charge. Thus, it is important to be able to predict charge on groups commonly found in biological molecules. 8. The Henderson Hasselbalch equation tells us we can predict the ratio of salt to acid as a function of pH if we know the pKa. Consequently, we can predict the charge on amino acids in a protein as the pH changes. Subtle changes in pH in the body can have drastic changes in protein structure and function. We'll see soon how hemoglobin undergoes drastic changes between the lungs and actively respiring tissue. The pH changes are not very large, so it is important to recognize that very small pH changes can make a big difference. 9. It is important for you to be able to use the Henderson-Hasselbalch equation to make predictions relating to pH, pKa, and/or salt/acid. Students should understand what happens in a titration and how buffering occurs near pKa values. Some molecule have multiple ions they can lose and thus have multiple pKa values and multiple buffering regions. Each buffering region in a titration curve will appear as a flattened out region around the relevant pKa. 10. You will not have to calculate the exact values of logarithms, but you should be able to tell by looking if a logarithm is positive, negative, or zero.