Acetic acid

Ahern's Biochemistry #1 - Aqueous Solutions / pH

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• published: 30 Sep 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 1. Covalent bonds are VERY strong bonds that hold atoms/molecules together. Covalent bonds are the 'glue' that holds together biomolecules. 2. Hydrogen bonds are much weaker bonds that are also important in biological molecules. Hydrogen bonds arise from uneven sharing of electrons betweeen, for example a nitrogen and a hydrogen or an oxygen and a hydrogen. In each case, the hydrogen ends up with a partial positive charge and the other atom has a partial negative charge. The partial positive charge of the hydrogen may be attracted to a partial negative charge on another oxygen or hydrogen. These bonds are weaker than covalent bonds, but are VERY important in stabilizing protein and DNA structures. 3. Water has its relatively high boiling point due to its numerous hydrogen bonds. The double helix of DNA is held together by hydrogen bonds between the individual bases. 4. Hydrogen bonds are additive, so they provide great stability in numbers (as in across a chromosome), but less stability locally (one can easily pull them apart when DNA needs to replicatie). 5. Hydrogen bonds stabilize proteins. Since protein function depends on protein structure, forces that disrupt hydrogen bonds (such as heat) tend to disrupt protein structure and function. It is because of this that cooking food kills bacteria, because it denatures their proteins. Since proteins are the "workhorses" of cells, loss of protein function means loss of cell function, which means death. 6. pH is a measure of the proton concentration in a solution. The pH is the negative log of the hydrogen ion concentration. The lower the pH, the higher the hydrogen ion concentration and the stronger the acid. pH + pOH = 14. The pOH is the negative log of the hydroxide ion concentration. The pKa is, therefore, the negative log of the Ka. The lower the pKa for an acid system (such as the acetic acid system discussed in class), the stronger the acid is. 7. Acetic acid (HAc above) is a weak acid, meaning that it doesn't completely dissolve in water. HCl, by contrast is called a strong acid because it completely dissociates in water. 8. Weak acids (written as HA) dissociate in water to a limited extent. This ionization can be written as HA = H+ + A- (where = refers to the equiilibrium symbol) 9. For dissociation of such weak acids, the Henderson-Hasselbalch equation applies. It states that pH = pKa + log {[A-]/[HA]} where the bracketed items (such as [A-]) refers to the concentration of that item. 10. Remember that pH is defined as the negative log of the hydrogen ion concentration -log[H+] and the pKa is defined as the negative log of the acid dissociation constant, Ka -log(Ka) 11. pKa is a measure of the strength of an acid. The lower the pKa, the stronger the acid. 12. Very strong acids like HCl that completely ionize in water do not have pKa values. 13. I shall use the term "acid" in a weak acid system to refer to the molecule containing the most protons and the term "salt" to refer to the acid which has lost a proton. For the acetic acid system, HAc = H+ + Ac- HAc is the acid, and Ac- is the salt. We could write this equation more generally for ANY weak acid as HA = H+ + A- 14. Acetic acid (HAc above) is a weak acid, meaning that it doesn't completely dissolve in water. HCl, by contrast is called a strong acid because it completely dissociates in water. 15. The Henderson Hassselbalch equation is pH = pKa + log ([Salt]/[Acid]). Please see the extra problems on the schedule page for practice. 16. Gain or loss of a proton by a molecule causes the molecule to gain or lose one full charge. From organic chemistry, 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. Note that the NH3+ is the acid and NH2 is the salt.