What the results mean
- Monoisotopic mass: the mass using only the most abundant isotope of each element (¹²C, ¹H, ¹⁴N, ¹⁶O, ³²S). Compare it with high-resolution mass spectrometry data.
- Average mass: weighted across natural isotope abundances. This is the "molecular weight" used for weighing out material.
- m/z values: where the protonated ions [M+nH]ⁿ⁺ appear in an electrospray mass spectrum. See Mass spectrometry explained.
- Net charge at pH 7 and pI: estimated with the Henderson–Hasselbalch equation and the EMBOSS pKa set (N-terminus 8.6, C-terminus 3.6, K 10.8, R 12.5, H 6.5, D 3.9, E 4.1, C 8.5, Y 10.1). Real values depend on sequence context and conditions.
- Extinction coefficient at 280 nm: predicted from tryptophan, tyrosine and cystine content using the method of Pace et al. Peptides without W or Y absorb weakly at 280 nm.
Notes: the calculator assumes free termini unless modified, and the 20 standard L-amino acids. Disulfide bonds are capped at the number of available cysteine pairs. The calculation runs entirely in your browser; nothing is sent anywhere.
Sources and further reading
- Pace CN, Vajdos F, Fee L, Grimsley G, Gray T. How to measure and predict the molar absorption coefficient of a protein. Protein Sci 1995;4:2411–2423. doi:10.1002/pro.5560041120 · PMID: 8563639
- Steen H, Mann M. The ABC’s (and XYZ’s) of peptide sequencing. Nat Rev Mol Cell Biol 2004;5:699–711. doi:10.1038/nrm1468 · PMID: 15340378