hw13solcorrect - H omework #13 with Solution Sketches...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
3.091 Fall Term 2004 Homework #13 with Solution Sketches November 30, 2004 (no weekly test on this material ± to be covered on the final) Biochemistry BIO-1. Amino acids exist in aqueous solution as zwitterions because the carboxyl group ionizes to CO 2 - and the amino group at the other end is protonated and becomes positively charged to –NH 3 + . The result is a zwitterion, an ion that has both a positive and a negative charge. BIO-4. The forms of serine at the pH indicated are: H O H O + NH 3 C C OH H 2 N C C O - CH 2 CH 2 OH OH at pH = 1 at pH = 13 BIO-5. Amino acids with nonpolar R groups will be able to form hydrophobic areas in the protein structure. Serine with a polar R group and lysine with a charged R group cannot contribute to the hydrophobic pocket. Responses (d) and (e). BIO-6. (a) Alanine hydrophobic (b) Glutamic acid hydrophilic (c) Arginine hydrophilic (d) Methionine hydrophobic (e) Threonine hydrophilic BIO-9. Since the pH lies below the isoelectric point of tyrosine, we would expect that on average, a tyrosine molecule would be positively charged. BIO-11. The structure of cysteine at pH = 7 shows that the side group is protonated. So we must conclude that even though the pK a is 8.33, the sulfhydryl (-SH) is acting as an acid. The isoelectric point , pI , is the pH at which the zwitterion is the dominant species. Let’s start with extreme acid conditions, pH < 1.96. Under these circumstances according to the LeChatelier Principle the amino acid will be fully protonated in an effort to try to consumer the proton excess in solution. The resulting structure of the amino acid is
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
3.091 Fall Term 2003 Homework #12 with Solution Sketches page 2 H O + NH 3 C C OH CH 2 SH , which is net positive ( cationic ). In slightly more alkaline solutions, 1.96 < pH < 8.33, the carboxylic acid sheds its proton in order to neutralize OH - . The resulting structure of the amino acid is H O + NH 3 C C O - CH 2 SH , which is the zwitterion . Hence we calculate the value of pI to be the average of 1.06 and 8.33 = 5.14. Just to complete the analysis, consider still greater basicity, 8.33 < pH < 10.78. Over this range of composition the resulting structure of the amino acid is H O + NH 3 C C O - CH 2 S - , which is net negative ( anionic ). Finally, at extreme alkaline conditions,
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 11/02/2009 for the course CHEMISTRY 3.091 taught by Professor Donsadoway during the Fall '04 term at MIT.

Page1 / 11

hw13solcorrect - H omework #13 with Solution Sketches...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online