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Amino acids in polypeptides

Charged polar side chains

Amino acids with charged (basic or acidic) side chains are very polar and they are nearly always found in the outside of protein molecules.

  1. Acidic side chains

    Carboxyl groups in water loose an H+ ion to become negatively charged.

    Acidic side chains

    They are therefore acid.

    Amino acids with acidic side chains:

    • Aspartic, Asp, D
    • Glutamic, Glu, E
  2. Basic side chains

    Amines in water combine with an H+ ion to become positively charged

    Basic side chains

    They are therefore basic.

    Amino acids with basic side chains:

    • Lysine, Lys, K
    • Arginine, Arg, R
    • Histidine, His, H

    Arg: The NH3+ group of Arg is very basic because its positive charge is stabilized by resonance.
    His: The nitrogens of the cycle have a relatively weak affinity for an H+ and are only partly positive at neutral pH.

Uncharged polar side chains

Amino acids with uncharged polar side chains are relatively hydrophilic and are usually on the outside of proteins.

Nonpolar side chains

Amino acids with non polar side chains tend to cluster their side chains together on the inside of proteins.
They are: G, P, A, V, L, I, F, M, W, C.
Paired cysteins allow disulphide bonds.

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