Chapter 26: Biomolecules: Amino Acids Peptides and Proteins

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Chapter 26: Biomolecules: Amino Acids Peptides and Proteins. L-glyceraldehyde. D-glyceraldehyde. Chirality of Amino Acids. Glycine, 2-amino-acetic acid, is achiral In all the others, the carbons of the amino acids are centers of chirality - PowerPoint PPT Presentation

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  • Chapter 26: Biomolecules: Amino Acids Peptides and Proteins

  • Chirality of Amino AcidsGlycine, 2-amino-acetic acid, is achiralIn all the others, the carbons of the amino acids are centers of chiralityThe stereochemical reference for amino acids is the Fischer projection of L-serineProteins are derived exclusively from L-amino acids

  • Draw Fisher diagrams of L-Alanine (R = CH3) and L-cysteine (R = CH2-SH) and assign stereochemistry as R or S

  • Amino Acids: pKa and Isoelectric points

  • 26.2 Isoelectric PointsIn acidic solution, the carboxylate and amine are in their conjugate acid forms, an overall cation

    In basic solution, the groups are in their base forms, an overall anion

    In neutral solution cation and anion forms are present

    This pH where the overall charge is 0 is the isoelectric point, pI

  • Titration Curves of Amino AcidspKas determined from titration curve

    If pKa values for an amino acid are known the fractions of each protonation state can be calculated (Henderson-Hasselbach Equation)

    pH = pKa log [A-]/[HA]

  • pI of any amino acid is the average of the two pKas that involve the neutral zwitterion

  • pI of any amino acid is the average of the two pKas that involve the neutral zwitterionpI of Histidine ?(pKas = 1.82, 9.17, 6.00)

    Charge of Histidine at pH = 7.0?Contains an imidazole ring that is partially protonated in neutral solutionOnly the pyridine-like, doubly bonded nitrogen in histidine is basic. The pyrrole-like singly bonded nitrogen is nonbasic because its lone pair of electrons is part of the 6 electron aromatic imidazole ring (see Section 24.4).

  • Peptides (listed from N to C terminus)

    Draw structure of Ala-Ser-Asp. What is the net charge of the peptide at pH 7.5?Ala, R = CH3; Ser, R = CH2OH; Asp (R = CH2CO2H

  • 26.10 Peptide SynthesisPeptide synthesis requires that different amide bonds must be formed in a desired sequenceThe growing chain is protected at the carboxyl terminal and added amino acids are N-protectedAfter peptide bond formation, N-protection is removed

  • Carboxyl Protecting GroupsUsually converted into methyl or benzyl estersRemoved by mild hydrolysis with aqueous NaOHBenzyl esters are cleaved by catalytic hydrogenolysis of the weak benzylic CO bond

  • Amino Group ProtectionAn amide that is less stable than the protein amide is formed and then cleaved after amide bond formedThe tert-butoxycarbonyl amide (BOC) protecting group is introduced with di-tert-butyl dicarbonateRemoved by brief treatment with trifluoroacetic acid

  • Peptide CouplingAmides are formed by treating a mixture of an acid and amine with dicyclohexylcarbodiimide (DCC)

  • 26.11 Automated Peptide Synthesis: The Merrifield Solid-Phase Technique

  • 26.13 Protein StructureThe primary structure of a protein is simply the amino acid sequence.The secondary structure of a protein describes how segments of the peptide backbone orient into a regular pattern.The tertiary structure describes how the entire protein molecule coils into an overall three-dimensional shape.The quaternary structure describes how different protein molecules come together to yield large aggregate structures

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