The representation of the molecular structure by a system (sequence) of amino acids has been used to establish quantitative structure-property/activity relationships (QSPR/QSAR) which can be used for (i) bioactivities of epitope-peptides, (ii) antibacterial potencies of polypeptides, and (iii) the binding affinity of peptides that bind to the class I major histocompatibility complex molecule HLA-A*0201. The representation of the peptide structure has been done via 1-letter abbreviations of amino acids, i.e., A (alanine), C (cysteine), D (aspartic), etc.
This approach allows classifying amino acids according to their function in a biochemical process (promoters of increase or decrease of an endpoint).