Understanding Micelles, pK values, pI, and the titration of Amino Acids
Understanding Micelles, pK values, pI, and the titration of Amino Acids CH 344
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This 4 page Class Notes was uploaded by Cody Brazel on Saturday September 5, 2015. The Class Notes belongs to CH 344 at Southeast Missouri State University taught by dr. Jacoby in Spring 2015. Since its upload, it has received 35 views. For similar materials see Biochemistry in Chemistry at Southeast Missouri State University.
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Date Created: 09/05/15
Notes for Biochemistry UI431 August 31September 4 2015 This week s lecture notes will focus primarily around the formation of micelles and the intricacies that the amino acid s exhibit with an emphasis on their Rgroups The formation of the icee F30 3 Note that the top blue bubbles of the molecme Phopholipid bilayer are the i phosphate groups and the strands lagging behind the phosphoryl groups are the fatty acid chains Since the phosphate phosphate groups are polar water group i molecules are attracted to them meaning that portion of the phospholipid is hydrophobic 7 glycerol backbone The fatty acids of a phospholipid are nonpolar Since they are nonpolar they are considered to be repelled by V hYdTOPhObiC t3 water or hydrophobic V Phospholipid Micelles are composed of a large 3dimensional grouping of amphipathic molecules usually the goto example molecule for this are phospholipids an example structure of a phospholipid is shown above courtesy of Wikipedia Amphipathic molecules have BOTH hydrophilic and hydrophobic portions Because of this water molecules are attracted to the hydrophilic portion of the molecule In this case a phosphoryl group When individual phospholipids are surrounded by water molecules they create a sort of quotbubblequot that I like to call a hydration bubble as shown in the picture below Hydrophilic I quothead group Hydrophobic alkyl group quotFlickering clusters of H20 0 molecules in bulk phase A Highly ordered H20 molecules form quotcagesquot around the hydrophobic alkyl chains Fig ure 27a Lehninger Principles of Biochemistry Fifth Edition 2008 W H Freeman and Company Now these bubbles are orderly Think about a bubble that you may blow from a soapy solution is very circular and orderly In another aspect you also need to factor that the fatty acid chains are not happy with being surrounded by water molecules and would love to group up with other fatty acids instead Taking this into account the quotmagicalquot formation of micelles can be explained Remember in chemistry class it was taught that ordered structures tend to go from orderly to disorderly due to thermodynamics Most of the time we are taught this concept through chemical reactions For example a reaction like this C quot9 AB would increase the entropy of the surroundings assuming the gibbs free energy is negative But natural processes may increase entropy without the need of a textbook example chemical reaction Think about it if the fatty acid portions are wanting to coalesce together they would have to displace the water molecules around them in order to make room By displacing their respective quotbubblesquot and then combining the orderly hydration bubbles are broken apart and multiple water molecules are dispersed Since we are going from a more orderly structure the bubbles to a less orderly structure displacement and breaking of the bubbles the entropy of the surroundings has been increased Since the entropy of the surroundings has increased the phospholipid system has decreased in energy and is therefore at a lower energy state something that natural systems usually strive to achieve It is this effort to achieve a lower energy state that accounts for the migration of single phospholipids to come together and form a micelle Amino Acids and Pk Values Something that many of you have noticed is that most of the sapling homework of CH3 Revolves around interpreting Pka values A Pka is the negative log of a chemical species Ka dissociationionization constant The higher the Ka value of an acid the greater in strength it is conversely the lower an acid s Pka the stronger in strength This relationship can be explained through mathematic examples Say we have acid quotxquot and acid quotyquot Acid quotxquot has a Pka value of 8 while acid y has a Pka value of 6 By taking the inverse each acid s Pka we can find the Ka of the acids Remember the inverse of a negative log is 1039l39L gl Acid quotxquot has a Ka value equal to 1039s Acid y has a Ka equal to 106 From simple observation it can be seen that acid quotyquot which has the lower Pka has the highest the Ka This shows that the lower to the Pka the better the acid Now the core behind much of Dr Jacoby s teachings has been based around the interaction of pH and the Pka of each ionizable group of the 20 amino acids Only 7 of the amino acids have an R group that is ionizable These 7 amino acids Y C K H R D and E along with all other amino acids can be found pages 77 and 79 in the class textbook Memorizing the 7 ka values on page 77 will require rote memorization and there is really no quoteasy route to memorizing them The structure and polarity of each amino acid can be found page 79 and these must also be memorized The most confusing part about each amino acid and their ionizable groups comes from questions concerning quotwhat is their charge at so and so pH or quotwhat is the pi of the polypeptide First off if the pH of an environment exceeds the pk value of any ionizable group on an amino acid that group is deprotonated Conversely if the pH falls below the pH of an ionizable group that group is protonated The charges of an ionizable group vary and it should be quite simple identifying those charges 0 A carboxyl group with a hydrogen is neutral and a carboxyl group without the hydrogen is nega ve o A Nitrogen with four bonds is positive and a nitrogen with 3 bonds is neutral There comes a time when a pH of an environment may be just right in which the entire amino acid or peptide has a total ionic charge of zero This point is called the isoelectronic point p or the zwitterion The equation for solving for the pi of a peptide is w The Two Pk values used in the equation MUST SURROUND the zwitterion curve Understanding a Titration Curve n gen chem a titration curve was quite simple to use While titrating sometimes you hit a plateau where the pH of the solution hardly changes These plateaus are where half of an ionizable group is transitioning from protonated to deprotonated In terms of amino acid titration the first plateau is usually the carboxyl group being deprotonated The vertical lines proceeding a plateau mark where a full transition has happened and the ionizable group has been fully deprotonated Below is a picture of histidine explaining this concept Please note that with each passing of a pK value the overall charge of the molecule changes Using deductive reasoning one can easily predict where the zwitterion of a peptide will be Howl like to determine the relative position of a zwitterion is through deprotonating the carboxyl group first which is after the first pK value Then I ask myself if the combined ionic charges equal to zero If not I then deprotonate the next most acidic group and ask myself the same question If the answer to quothave I reached a charge of O is quotyesquot I know that I reached the zwitterion I can then calculate the pl of the peptide by finding the two pK values that surround the zwitterion In this case the pKr value and the pK2 value surround the zwitterion Therefore the pl of the solution is equal 91760 to 785 pH With this information I can deduce that the highest amount of zwitterions appear at a pH of 785 Note that this titration curve has three pK points This is because Histidine has 3 ionizable groups If you are looking at an amino acid with two pK points than it must be one of the 13 amino acids that do not have a ka value HI I I Hi rill PETE T1111 l l Ha if39Ell Ellil inn HE fin infra t fil it H Wu H all i JL H t HE P it ESE t HR HfiliEl PHI Jpn 393me r Emil FE l grit It if 1 iquot Z s39 t H H H H J Ell m Hi 39 i i 3 PH 5 12 l ll 2J3 ill DH ieqtivaliagittel Illa As always if you ever need to contact me please email me at cibrazdazgmailcom