Hydropathy index

The hydropathy index of an amino acid is a number representing the hydrophobic or hydrophilic properties of its side-chain. It was proposed in 1982 by Jack Kyte and Russell Doolittle.

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.

Hydropathy Index for the twenty natural amino acids (Kyte and Doolittle)

A	R	N	D	C	Q	E	G	H	I	L	K	M	F	P	S	T	W	Y	V
1.8	-4.5	-3.5	-3.5	2.5	-3.5	-3.5	-0.4	-3.2	4.5	3.8	-3.9	1.9	2.8	-1.6	-0.8	-0.7	-0.9	-1.3	4.2

Amino acids sorted by increasing hydropathy index (based on previous table from Kyte and Doolittle)

R	K	N	D	Q	E	H	P	Y	W	S	T	G	A	M	C	F	L	V	I
-4.5	-3.9	-3.5	-3.5	-3.5	-3.5	-3.2	-1.6	-1.3	-0.9	-0.8	-0.7	-0.4	1.8	1.9	2.5	2.8	3.8	4.2	4.5

Hydropathy index from Eisenberg consensus scale (ECS)

R	K	D	Q	N	E	H	S	T	P	Y	C	G	A	M	W	L	V	F	I
-2.5	-1.5	-0.90	-0.85	-0.78	-0.74	-0.40	-0.18	-0.05	0.12	0.26	0.29	0.48	0.62	0.64	0.81	1.1	1.1	1.2	1.4