Example Data Table
| Input |
Example Value |
Expected Meaning |
| Sequence |
ACDEFGHIK |
Nine standard amino acid residues |
| Mass type |
Monoisotopic |
High resolution residue mass mode |
| Disulfide bonds |
0 |
No hydrogen loss from cystine formation |
| Terminal water |
Yes |
Adds one water molecule to the residue total |
| Approximate result |
1018.45419 g/mol |
Base monomer mass for the example sequence |
Formula Used
Protein molar mass equals the sum of residue masses plus terminal water, selected modifications,
labels, tags, salts, and adducts. Disulfide bonds subtract two hydrogen atoms per bond.
Monomer mass = residue total + terminal water + modification total - disulfide deduction.
Oligomer mass = monomer mass x number of copies.
m/z = (oligomer mass + charge x proton mass) / charge.
Molarity = corrected concentration in g/L divided by molar mass in g/mol.
Extinction coefficient = 5500 x Trp + 1490 x Tyr + 125 x disulfide bonds.
How To Use This Calculator
Paste a one letter protein sequence into the sequence box.
Select average mass for general work, or monoisotopic mass for exact mass work.
Keep terminal water enabled for a complete neutral protein chain.
Add modification masses, labels, tags, glycans, salts, and disulfide bonds as needed.
Enter concentration and volume when you need molarity, moles, molecules, or sample mass.
Press calculate. The result appears above the form and below the header.
Use CSV or PDF buttons to save the current calculation.
Understanding Protein Molar Mass
Protein molar mass tells how many grams one mole of a protein weighs. It depends on the amino acid sequence.
Each residue has a known mass. During peptide bond formation, water is lost between residues. Many calculators
handle this by using residue masses. Then one terminal water molecule is added for the complete chain.
Why Sequence Details Matter
Small sequence changes can shift mass quickly. A single tryptophan adds much more mass than glycine. Cysteine
pairs can form disulfide bonds. Each disulfide bond removes two hydrogen atoms. Tags, labels, salts, and
chemical groups also change the final value. This tool lets you include these changes directly. It can calculate
average mass or monoisotopic mass. Average mass is useful for routine chemistry work. Monoisotopic mass is useful
for high resolution mass analysis.
Practical Laboratory Use
A molar mass result helps prepare solutions. It converts mass concentration into molarity. For example, a protein
at one milligram per milliliter can be converted into micromolar units. This is helpful for enzyme assays, binding
tests, and dilution planning. The calculator also estimates the A280 extinction coefficient. That estimate uses
counts of tryptophan, tyrosine, and cystine. It gives a fast check for absorbance based concentration work.
Reading The Output
The result separates base sequence mass, terminal water, modifications, and deductions. This makes the calculation
easier to audit. The charged ion estimate is useful when planning mass spectrometry checks. The oligomer option helps
when a protein works as a dimer, trimer, or larger complex. The CSV and PDF buttons save the current result. They are
useful for lab notes, reports, and class assignments.
Good Input Habits
Paste only one letter amino acid codes when possible. Remove numbering, spaces, and extra comments. Review any invalid
letters shown after calculation. Enter custom mass changes with signs when needed. Use positive values for added groups.
Use negative values for losses. For best reporting, record the mass type, sequence source, and every chosen option.
Helpful Limitations
The result is an estimate, not a sequence confirmation. It cannot detect cleavage, oxidation, missed residues, or sample
impurities unless you enter those changes. Compare results with experimental data. Keep units consistent. Save outputs
only after checking each input field carefully.
FAQs
What is protein molar mass?
Protein molar mass is the mass of one mole of protein molecules. It is usually reported in grams per mole.
Should I use average or monoisotopic mass?
Use average mass for routine chemistry and solution work. Use monoisotopic mass when comparing exact masses from high resolution instruments.
Why is terminal water added?
Residue masses represent amino acids after peptide bond formation. A complete protein chain still has terminal groups, so one water molecule is commonly added.
How are disulfide bonds handled?
Each disulfide bond removes two hydrogen atoms. The calculator subtracts that mass from the protein total for each entered bond.
Can I add custom modifications?
Yes. Enter positive mass for added groups and negative mass for losses. You can also add tags, labels, glycans, salts, and adducts.
What does the m/z result mean?
The m/z value estimates mass to charge ratio for the selected charge state. It is helpful for basic mass spectrometry planning.
How is molarity calculated?
The calculator converts corrected mg/mL into g/L, then divides by molar mass. It also shows micromolar concentration.
Can this replace experimental measurement?
No. It is a calculation tool. Confirm final protein identity, purity, cleavage, oxidation, and modifications with suitable laboratory methods.