Download Organic Biochem Midterm: Buffers, Amino Acids, Peptides, Carbs - Prof. Marcy Hernick and more Exams Biochemistry in PDF only on Docsity! Name:________________________ Student Number:__________________ Biochemistry 2144 – Organic Biochemistry Midterm Examination – March 4, 2009 Note: Be sure to show all work (including equations) and watch significant figures. All answers must be in the space provided below (if needed, use the backsides of the pages for scratch paper). 1. Buffers. The amino acid glycine is often used as a buffer (amino group pKa = 9.6). (a) Write out the dissociation reaction for glycine (focus on the amino group only). (4 points) (b) In what pH range can glycine be used as an effective buffer due to its amino group? (4 points) pH 8.6 – 10.6 (c) In a 0.1 M solution of glycine at pH = 8.8, what fraction (decimal) of glycine has its amino group in the –NH3 + form? (5 points) log 8.8 = 9.6 + log [B]/[BH+] 10‐0.8 = [B]/[BH+] = 0.16 Fr [BH+] = 1/1.16 = 0.86 (d) How much 10 M KOH must be added to 1.0 L of 0.2 M glycine at pH 9.0 to bring its pH to exactly 10.0? (5 points) log 9 = 9.6 + log [B]/[HB+] 10‐0.6 = [B]/[HB+] = 0.25 Fr [B] = 0.25/1.25 = 0.2 10 = 9.6 + log [B]/[HB+] 100.4 = [B]/[HB+] = 2.5 Fr [B] = 2.5/3.5 = 0.71 So, Fr [B] we need is 0.71 – 0.2 = 0.51 and 0.51 x 0.2 M = 0.10 M NaOH needed For dilution: M1V1=M2V2 (10 M)(x mL) = (0.1 M)(1000 mL) ; so x = 10 mL 2. Buffers. Phosphoric acid (H3PO4), a triprotic acid, has 3 pKa values: 2.14, 6.86 and 12.4. (a) What molar ratio of HPO42‐ to H2PO4‐ in solution will produce a pH of 7.00? (5 points) log 7.00 = 6.86 + log [A‐]/[HA] 0.14 = log [A‐]/[HA] 1.38 = [A‐]/[HA] OR [HA]/[A‐] = 0.72 (b) What is the pH of a solution that contains 0.25 M Na2HPO4 and 0.40 M NaH2PO4? (5 points) log pH = 6.86 + log 0.25/0.40 pH = 6.86 + log 0.625 pH = 6.86 – 0.20 = 6.66, or ~6.7 3. Amino acids. Answer the following questions regarding the properties of amino acid side chains as they exist at pH 7.4 (Note: No pKa shifts due to environment for this question). (a) Name and provide the 3 letter abbreviation for 2 non‐polar amino acids. (4 points) Glycine (Gly), Alanine (Ala), Proline (Pro), Valine (Val), Leucine (Leu), Isoleucine (Ile), Methionine (Met), Phenylalanine (Phe), Tryptophan (Trp) (b) Name and provide the 3 letter abbreviation 2 polar (uncharged) amino acids. (4 points) Serine (Ser), Threonine (Thr), Cysteine (Cys), Asparagine (Asn), Glutamine (Gln), Tyrosine (Tyr) (c) Name and provide the 3 letter abbreviation 2 negatively charged amino acids. (4 points) Aspartate (Asp), Glutamate (Glu) (d) Name and provide the 3 letter abbreviation 2 positively charged amino acids. (4 points) Lysine (Lys), Arginine (Arg), Histidine (His)
