The aliphatic index of a protein is defined as the relative volume occupied by aliphatic side chains (alanine, valine, isoleucine, and leucine). It may be regarded as a positive factor for the increase of thermostability of globular proteins. The aliphatic index of a protein is calculated according to the following formula:
Ikai A (1980) Thermostability and aliphatic index of globular proteins. J Biochem. 88 (6), 1895-8.