Purpose and underlying formula
The GST‑Fusion Protein A₂₈₀ Concentration Calculator determines protein concentration in mg · mL⁻¹ from an A₂₈₀ absorbance measurement. It derives the extinction coefficient (ε₂₈₀) and molecular weight (MW) directly from the supplied one‑letter amino‑acid sequence. The calculation follows the Beer‑Lambert relationship c = A₂₈₀·MW / (ε·l), where l is the cuvette path length. The GST fusion A280 calculator shows the step‑by‑step computation for a user‑provided sequence.
All computations run 100 % client‑side, so the sequence and measurements never leave the browser. Because the tool operates locally, it works offline after the initial page load. The service is free and requires no registration. Users can immediately see the headline result and the supporting figures as they type.
Using the calculator
To start, paste a one‑letter protein or peptide sequence into the input field. The calculator automatically extracts the number of cystine (disulfide) residues, calculates ε₂₈₀, and determines MW in daltons. After entering the measured A₂₈₀ value, the tool displays the molar concentration (M) and the converted mg · mL⁻¹ value. A worked example is provided that substitutes the user’s numbers into the formula step by step.
The example on the page uses a sequence with an ε₂₈₀ of 1490 and an MW of 8305.48588 g mol⁻¹. With an A₂₈₀ reading, the tool computes a molar concentration of 6.711 × 10⁻⁴ mol L⁻¹, which converts to 5.574 mg mL⁻¹. The result appears as “5.5742 mg/mL” and the corresponding protein concentration is shown as 0.00067114 M. Users can copy the numeric result or the ready‑made citation snippet.
Interpreting results and pitfalls
The extinction coefficient depends on the oxidation state of cystine residues; using a reduced ε in place of the oxidized value will underestimate concentration. Likewise, the calculated isoelectric point (pI) from sequence may differ from experimental pI, affecting ion‑exchange strategies. Post‑translational modifications are not accounted for, so users should adjust ε manually if modifications are present. Ignoring these factors can lead to significant errors in downstream purification or assay planning.
The calculator explicitly warns about common mistakes such as forgetting cystine contributions to ε₂₈₀ and assuming sequence‑derived pI equals the measured value. By displaying the full worked example, the tool lets users verify each intermediate step before accepting the final figure. This transparency helps prevent costly miscalculations in protein quantitation. Researchers often embed the tool’s output in methods sections because the calculation is reproducible and documented.
Citation and reproducibility
The page provides a one‑line “cite this tool” snippet that includes the tool name, site, and year, suitable for methods sections of publications. Because the computation is deterministic and performed locally, the same input sequence and A₂₈₀ value will always yield identical results on any compatible browser. The disclaimer notes that the calculator offers estimates and should be verified with professional judgment. Linking to the calculator from protocols or SOPs is common practice among protein biochemists and structural biologists.
All figures are estimates; users are advised to confirm concentrations with orthogonal techniques when high accuracy is required. The tool’s privacy model—no data upload—makes it appropriate for confidential or proprietary sequences. Its free, sign‑up‑free nature encourages wide adoption in academic and industrial labs. The calculator therefore serves as a quick, transparent reference for A₂₈₀‑based protein quantitation.
