The instability index (II) predicts whether a protein will be stable in a test tube based on its amino acid sequence. Developed by Guruprasad and colleagues in 1990, it remains one of the most widely used sequence-based stability predictors in protein science.
The method works by analyzing dipeptide patterns—pairs of consecutive amino acids—that occur more frequently in unstable proteins. A low instability index suggests the protein will remain intact during expression, purification, and storage. A high value warns that the protein may degrade quickly.
For a comprehensive analysis that includes instability index along with molecular weight, pI, and other properties, use our Protein Parameters calculator.
How does the instability index work?
The calculation assigns a weight value to each of the 400 possible dipeptide combinations based on how often they appear in unstable versus stable proteins. These weights are called Dipeptide Instability Weight Values (DIWV).
The formula
The instability index is computed as:
II=L10i=1∑L−1DIWV(xixi+1)
Where L is the sequence length and DIWV(xixi+1) is the instability weight for the dipeptide at position i.
Dipeptide weights
The DIWV values were derived from statistical analysis of 12 unstable and 32 stable proteins. Dipeptides that appeared significantly more often in unstable proteins received higher weight values.
Some dipeptides strongly destabilize proteins (high DIWV), while others are neutral or stabilizing (low or negative DIWV). The sum of all dipeptide contributions, normalized by sequence length, produces the final index.
Understanding the results
The output table shows one row per input sequence with columns for protein ID, sequence length, and instability index.
The threshold of 40 separates stable from unstable predictions:
II < 40: Protein is predicted to be stable in vitro
II ≥ 40: Protein may be unstable with a short half-life
Most well-folded globular proteins have instability indices between 20 and 40. Values below 20 suggest very stable proteins, while values above 50 indicate likely degradation issues.
Input requirements
Protein sequences: Enter one or more sequences in FASTA format
Supports batch processing of multiple sequences
Accepts .fasta, .fa, .fas, .pdb, .csv, and .txt file uploads
Can fetch sequences directly from RCSB PDB
Use cases
The instability index helps guide protein production and engineering decisions:
Expression screening: Prioritize protein variants predicted to be stable for recombinant production
Mutation analysis: Compare wild-type and mutant proteins to identify destabilizing substitutions
Protein engineering: Guide design of stabilizing mutations by tracking II changes
Shelf-life prediction: Estimate whether purified proteins will remain stable during storage
Limitations
The instability index was developed from a small dataset (44 proteins) and is based on in vivo stability data. Keep these limitations in mind:
In vitro conditions differ from the cellular environment where the training data originated
Buffer composition, pH, temperature, and additives affect actual stability but are not captured by the index
The method cannot account for post-translational modifications or cofactor binding
Very short sequences may give unreliable predictions due to limited dipeptide sampling
We recommend using the instability index as one factor among several when evaluating protein stability. Experimental validation remains essential for critical applications.
