How the molecular weight is calculated
The average molecular weight is the sum of the average masses of each amino-acid residue plus one water molecule (18.015 Da) for the free N- and C-termini. Average masses account for the natural abundance of isotopes, which is what SDS-PAGE and most biochemical calculations use.
Post-translational modifications, disulfide bonds and signal-peptide cleavage are not included, so the mass of a mature protein can differ from the value calculated from its sequence.
Extinction coefficient
The molar extinction coefficient at 280 nm is estimated from the number of tryptophan, tyrosine and cystine residues (Pace et al., 1995), assuming all cysteines form disulfide bonds. Divide the absorbance at 280 nm by this value to estimate protein concentration.
GRAVY
The grand average of hydropathy (GRAVY) is the mean Kyte–Doolittle hydropathy value over all residues. Positive values indicate a hydrophobic protein, negative values a hydrophilic one.