Aliphatic index
The aliphatic index of a protein is a
measure of the relative volume occupied by aliphatic side chain of
the following amino acids: alanine, valine, leucine and isoleucine.
An increase in the aliphatic index increases the thermostability of
globular proteins. The index is calculated by the following formula.
X(Ala), X(Val), X(Ile) and X(Leu) are the amino acid compositional fractions. The constants a and b are the relative volume of valine (a=2.9) and leucine/isoleucine (b=3.9) side chains compared to the side chain of alanine [Ikai, 1980].