The instability index calculates an estimate of the stability of the protein in a test tube. Statistical analysis of 12 unstable and 32 stable proteins revealed that there are certain dipeptides, whose occurence is significantly different in the unstable proteins compared with those in the stable ones. The authors of this method have assigned a weight value of instability to each of the 400 different dipeptides (DIWV). Using these weight values it is possible to compute an instability index (II) which is defined as:
where L is the sequence length and DIWV(x[i]x[i+1]) is the instability weight value for the dipeptide starting in position i.
A protein whose instability index is smaller than 40 is predicted as stable, a value above 40 predicts that the protein may be unstable.
Guruprasad K, Reddy BV, Pandit MW (1990) Correlation between stability of a protein and its dipeptide composition: a novel approach for predicting in vivo stability of a protein from its primary sequence. Protein Eng. 2 (2), 155-61.