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Amino Acids (18.1-18.3)

Amino Acids (18.1-18.3).  Amino acids have the same basic structure. 20 common amino acids combine in different sequences to form thousands of different proteins. Amino acids differ here. All amino acids have. C g. General Structure. gamma carbon. Beta and gamma carbons. beta carbon.

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Amino Acids (18.1-18.3)

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  1. Amino Acids (18.1-18.3)  Amino acids have the same basic structure 20 common amino acids combine in different sequences to form thousands of different proteins Amino acids differ here All amino acids have Cg General Structure gamma carbon Beta and gamma carbons beta carbon alpha carbon alpha carbon Cb Ca Ca amino acid Carboxylic acid

  2. Which of the following are alpha amino acids and which are not?

  3. Classification of Amino Acids Nonpolar amino acids alanine glycine valine isoleucine leucine proline methionine tryptophan phenylalanine Polar uncharged amino acids serine threonine cysteine asparagine glutamine tyrosine Polar charged basic amino acids Polar charged acidic amino acids aspartic acid (aspartate) glutamic acid (glutamate) arginine histidine lysine

  4. Amino Acid Abbreviations

  5.  Amino acids are weak polyprotic acids (18.4) both the amino and carboxy groups are protonated at low pH. Carboxy loses proton first and then amino loses at high pH. “many protons” lower pH higher pH neutral pH  Amino acids are zwitterions (dipolar) zwitterionshave charged residues but are electrically neutral the amino and carboxy charges cancel each other out at neutral pH so aa wo/ charged side chains are uncharged at neutral pH Isoelectric point, pI pH where amino acid is electrically neutral depends on the R group also

  6. Amino Acid pKa’s

  7. pK2 net charge X +1 0 -1 X pI X pK1 Titration of Amino Acids Amino acids have titration curves like diprotic or triprotic acids Net charge +1 Net charge -1 Net charge 0

  8. Alanine similar to diprotic acid

  9. Histidine similar to triprotic acid

  10. Amino Acid Questions. Explain in detail with structures and equations where applicable. 1. Given a peptide with the following amino acid sequence, identify the polar amino acids, the aromatic amino acids, and the sulfur-containing amino acids. Val-Met-Ser-Ile-Phe-Arg-Cys-Tyr-Leu 2. Write equations to show the ionic dissociation reactions of the following amino acids: aspartic acid valine 3. Draw the predominant ionized forms of the following amino acids at pH 7: glutamic acid histidine arginine 4. Based on the pKa information for amino acids, is there any amino acid that could serve as a buffer at pH 6? If so, which one(s)?

  11. Amino acid questions continued. 5. Sketch a titration curve for the amino acid cysteine; indicate the pKa values for all titratable groups.

  12.  Amino acids have handedness (18.5-18.6) Chirality - an object or molecule that cannot be superimposed on its mirror image image is chiral your left and right hands are mirror images of each other. They are non-superimposable mirror images. You cannot exactly superimpose your left and right hands. Molecular handedness Molecules with carbon are frequently chiral carbon must have 4 different groups bound to it - called a chiral carbon or chiral center. The presence of one chiral carbon always produces a chiral molecule A molecule with more than one chiral carbon may or may not be chiral overall non-superimposable mirror images - chiral superimposable mirror images - achiral

  13. In the type of chiral center we are intersted in, carbon must have 4 different groups attached: Examples: PropaneChloroethane Amino acids Groups attached toCa of amino acids 1. COO- 2. H 3. NH3+ 4. R group The chiral atom that makes the overall molecule chiral Has 2 identical atoms attached(the hydrogens) achiral Has 2 sets of identical groups attached (the hydrogens and the methyl groups) achiral NH3+ NH3+ Ca CH3 CH3 Ca H COO- H COO- 4 different groups attached in all amino acids except glycine (the R group is hydrogen) Alanine enantiomers

  14. Label the chiral center in the following molecules if one is present. Two of the 20 common amino acids have two chiral carbon atoms in their structures. Identify these amino acids, draw their structures below, and label their chiral centers.

  15. Enantiomers - molecules that are non superimposable mirror images of each other Enantiomers have the same name but are distinguished by a prefix R vs. S system most commonly used D vs. L is older nomenclature only L amino acids are in making proteins (L for “Life”) Enantiomers have many identical physical properties: melting and boiling points same pI same solubility in water same density Enantiomers have some physical and chemical properities that differ rotate plane polarized light in opposite directions reactivity with other chiral molecules **biological activity **odors and tastes **activity as drugs

  16. Chiral center Chiral center L-Carvone (in spearmint) D-Carvone (in caraway) mirror Chiral center Chiral center (R)-Limonene (smell of oranges) (S)-Limonene (smell of lemons) mirror Examples of the differing biological activity of enantiomers:

  17. Chiral center Chiral center (R)-Thalidomide (sedative) (S)-Thalidomide (fetal abnormalities) mirror Must be sold as 97% S enantiomer mirror (R)-Naproxen (liver toxin) (S)-Naproxen (anti-inflammatory)

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