Amino Acid & Peptide Explorer

Analyze an unmodified linear peptide of 1–500 standard residues. Choose the encoding explicitly: one-letter input accepts letters and whitespace; three-letter input accepts contiguous triplets or runs separated by whitespace or hyphens. Each run must contain complete triplets. Unknown residues and other characters are rejected, with positions counted in the original input. FASTA headers, modified residues and ambiguous codes are not supported.

Average mass is the sum of SIB/ExPASy average residue masses plus one water molecule (18.01524 Da). The model has free, unblocked ends and includes no disulfide bonds, modifications or other chains. The insulin A example is an isolated, unmodified chain.

Charge uses independent Henderson–Hasselbalch sites: the first residue supplies the N-terminal pKa, the last supplies the C-terminal pKa, and each ionizing side chain adds its contribution. The values follow OpenStax Table 26.1 for free amino acids. Peptide terminal pKas can differ; folding and local interactions also affect real charge. This is a teaching approximation, not a calibrated protein pI prediction. The pI is a bracketed zero of model charge in pH 0–14. Acidic side-chain sites include weak acids Cys and Tyr; the class does not imply a negative charge at every pH.

Kyte–Doolittle scores describe relative hydropathy. The mean uses the whole sequence; the curve averages only complete windows. Even windows are centered between residues. A window longer than the sequence produces no curve. These values do not establish a membrane-spanning segment or a three-dimensional structure.

Analyze after editing the sequence, encoding or window. The pH controls then update charge and its marker while mass, pI and composition remain fixed. The tool has no saved session state.