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Free MCAT flashcard deckAmino acid charge at physiological pH

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Card 1 of 27
Question
Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
In the four-group scheme, glycine's side chain at pH 7.4 is [class].
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Answer
In the four-group scheme, glycine's side chain at pH 7.4 is nonpolar.
Skeletal structure of glycine (Gly, G) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
Its –H side chain has no polar or ionizable group, so it is grouped as nonpolar (some texts set glycine apart because the side chain is so small).

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  1. 1
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, glycine's side chain at pH 7.4 is [class].
    In the four-group scheme, glycine's side chain at pH 7.4 is nonpolar.
    Skeletal structure of glycine (Gly, G) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its –H side chain has no polar or ionizable group, so it is grouped as nonpolar (some texts set glycine apart because the side chain is so small).
  2. 2
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, alanine's side chain at pH 7.4 is [class].
    In the four-group scheme, alanine's side chain at pH 7.4 is nonpolar.
    Skeletal structure of alanine (Ala, A) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its methyl side chain is pure hydrocarbon, with nothing that ionizes.
  3. 3
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, serine's side chain at pH 7.4 is [class].
    In the four-group scheme, serine's side chain at pH 7.4 is polar.
    Skeletal structure of serine (Ser, S) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its –OH makes it more polar than a hydrocarbon, but it carries no charge at pH 7.4.
  4. 4
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, proline's side chain at pH 7.4 is [class].
    In the four-group scheme, proline's side chain at pH 7.4 is nonpolar.
    Skeletal structure of proline (Pro, P) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its side chain is a hydrocarbon ring looping back to the backbone nitrogen, with no polar group.
  5. 5
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, valine's side chain at pH 7.4 is [class].
    In the four-group scheme, valine's side chain at pH 7.4 is nonpolar.
    Skeletal structure of valine (Val, V) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its isopropyl side chain is pure hydrocarbon, the kind that buries in the protein core.
  6. 6
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, threonine's side chain at pH 7.4 is [class].
    In the four-group scheme, threonine's side chain at pH 7.4 is polar.
    Skeletal structure of threonine (Thr, T) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its –OH makes it more polar than a hydrocarbon, but it carries no charge at pH 7.4.
  7. 7
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, cysteine's side chain at pH 7.4 is [class].
    In the four-group scheme, cysteine's side chain at pH 7.4 is polar.
    Skeletal structure of cysteine (Cys, C) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its –SH (pKa ≈ 8.3, free amino acid) is mostly protonated, and hydrophobicity scales rank it with the nonpolar residues.
  8. 8
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, leucine's side chain at pH 7.4 is [class].
    In the four-group scheme, leucine's side chain at pH 7.4 is nonpolar.
    Skeletal structure of leucine (Leu, L) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its isobutyl side chain is pure hydrocarbon, the kind that buries in the protein core.
  9. 9
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, isoleucine's side chain at pH 7.4 is [class].
    In the four-group scheme, isoleucine's side chain at pH 7.4 is nonpolar.
    Skeletal structure of isoleucine (Ile, I) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its sec-butyl side chain is pure hydrocarbon, the kind that buries in the protein core.
  10. 10
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, asparagine's side chain at pH 7.4 is [class].
    In the four-group scheme, asparagine's side chain at pH 7.4 is polar.
    Skeletal structure of asparagine (Asn, N) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its amide makes it more polar than a hydrocarbon, but it carries no charge at pH 7.4.
  11. 11
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, aspartate's side chain at pH 7.4 is [class].
    In the four-group scheme, aspartate's side chain at pH 7.4 is acidic.
    Skeletal structure of aspartate (Asp, D) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its carboxyl (pKa ≈ 3.9, free amino acid) is mostly deprotonated at pH 7.4, so it is negative.
  12. 12
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, glutamine's side chain at pH 7.4 is [class].
    In the four-group scheme, glutamine's side chain at pH 7.4 is polar.
    Skeletal structure of glutamine (Gln, Q) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its amide makes it more polar than a hydrocarbon, but it carries no charge at pH 7.4.
  13. 13
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, lysine's side chain at pH 7.4 is [class].
    In the four-group scheme, lysine's side chain at pH 7.4 is basic.
    Skeletal structure of lysine (Lys, K) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its amino group (pKa ≈ 10.5, free amino acid) is mostly protonated at pH 7.4, so it is positive.
  14. 14
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, glutamate's side chain at pH 7.4 is [class].
    In the four-group scheme, glutamate's side chain at pH 7.4 is acidic.
    Skeletal structure of glutamate (Glu, E) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its carboxyl (pKa ≈ 4.3, free amino acid) is mostly deprotonated at pH 7.4, so it is negative.
  15. 15
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, methionine's side chain at pH 7.4 is [class].
    In the four-group scheme, methionine's side chain at pH 7.4 is nonpolar.
    Skeletal structure of methionine (Met, M) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its thioether sulfur carries no group that ionizes, so it sorts with the hydrocarbons.
  16. 16
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, histidine's side chain at pH 7.4 is [class].
    In the four-group scheme, histidine's side chain at pH 7.4 is basic.
    Skeletal structure of histidine (His, H) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Classed basic, yet with pKa ≈ 6 (free amino acid) it is mostly deprotonated and neutral at pH 7.4.
  17. 17
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, phenylalanine's side chain at pH 7.4 is [class].
    In the four-group scheme, phenylalanine's side chain at pH 7.4 is nonpolar.
    Skeletal structure of phenylalanine (Phe, F) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its ring has no heteroatoms, though five-group schemes such as Lehninger's give aromatics their own group.
  18. 18
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, arginine's side chain at pH 7.4 is [class].
    In the four-group scheme, arginine's side chain at pH 7.4 is basic.
    Skeletal structure of arginine (Arg, R) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its guanidinium (pKa ≈ 12.5, free amino acid) stays protonated, positive across the physiological range.
  19. 19
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, tyrosine's side chain at pH 7.4 is [class].
    In the four-group scheme, tyrosine's side chain at pH 7.4 is polar.
    Skeletal structure of tyrosine (Tyr, Y) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its –OH (pKa ≈ 10.1, free amino acid) stays protonated, and Lehninger's five groups list it as aromatic.
  20. 20
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In the four-group scheme, tryptophan's side chain at pH 7.4 is [class].
    In the four-group scheme, tryptophan's side chain at pH 7.4 is nonpolar.
    Skeletal structure of tryptophan (Trp, W) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Its indole is mostly hydrocarbon, though five-group schemes such as Lehninger's give aromatics their own group.
  21. 21
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In free cysteine, the side-chain pKa is about [number].
    In free cysteine, the side-chain pKa is about 8.3.
    Skeletal structure of cysteine (Cys, C) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Above 7.4, so at physiological pH the side chain is mostly protonated and neutral.
  22. 22
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In free aspartate, the side-chain pKa is about [number].
    In free aspartate, the side-chain pKa is about 3.9.
    Skeletal structure of aspartate (Asp, D) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Below 7.4, so at physiological pH the side chain is mostly deprotonated and negative.
  23. 23
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In free lysine, the side-chain pKa is about [number].
    In free lysine, the side-chain pKa is about 10.5.
    Skeletal structure of lysine (Lys, K) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Above 7.4, so at physiological pH the side chain is mostly protonated and positive.
  24. 24
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In free glutamate, the side-chain pKa is about [number].
    In free glutamate, the side-chain pKa is about 4.3.
    Skeletal structure of glutamate (Glu, E) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Below 7.4, so at physiological pH the side chain is mostly deprotonated and negative.
  25. 25
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In free histidine, the side-chain pKa is about [number].
    In free histidine, the side-chain pKa is about 6.
    Skeletal structure of histidine (His, H) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    The side-chain pKa closest to neutral, so histidine is the side chain proteins buffer with.
  26. 26
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In free arginine, the side-chain pKa is about [number].
    In free arginine, the side-chain pKa is about 12.5.
    Skeletal structure of arginine (Arg, R) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    The most basic side chain, so it stays positive across the entire physiological range.
  27. 27
    Skeletal structure of an amino acid, drawn as the un-ionized free acid with the amino group at left and the carboxylic acid at right; the side chain (R group) is shaded and drawn with heavier bonds.
    In free tyrosine, the side-chain pKa is about [number].
    In free tyrosine, the side-chain pKa is about 10.1.
    Skeletal structure of tyrosine (Tyr, Y) in the form that predominates at pH 7.4, with a protonated amino group (NH3+) at left and a carboxylate (COO−) at right; the side chain (R group) is shaded and drawn with heavier bonds.
    Above 7.4, so at physiological pH the side chain is mostly protonated and neutral.

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