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Hydropathy index

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The hydropathy index of an amino acid is a number representing the hydrophobic or hydrophilic properties of its sidechain. It was proposed in 1982 by Jack Kyte and Russell F. Doolittle.[1]

The larger the number is, the more hydrophobic the amino acid. The most hydrophobic amino acids are isoleucine (4.5) and valine (4.2). The most hydrophilic ones are arginine (-4.5) and lysine (-3.9). This is very important in protein structure; hydrophobic amino acids tend to be internal (with regard to the protein's 3 dimensional shape) while hydrophilic amino acids are more commonly found towards the protein surface.


  1. Kyte J, Doolittle RF (May 1983). "A simple method for displaying the hydropathic character of a protein". J. Mol. Biol. 157 (1): 105–32. PMID 7108955

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