Aliphatic index (AI) refers to the amount of aliphatic side chains (valine, alanine, isoleucine and leucine) present in a protein and signifies the thermostability of the protein. AI can be calculated using formula AI ¼ XðAÞ þ a XðVÞ þ ½bxðXðIÞ þ XðLÞwhere, X is mole percent of alanine (A), valine (V), isoleucine (I) and leucine (L). The numerical value of a, and b are 2.9 and 3.9, which is the relative volumes for V and L/I side chain, respectively