Enter your protein or peptide sequence in the Sequence Format of your choice, choose your N-terminus and C-terminus Modifications, and the Amino Acid Molecular Weight Calculator gives you the molecular weight of your peptide alongside the total amino acid count, average residue weight, and extinction coefficient at 280nm.
Molecular Weight
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Total Amino Acids
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Average Residue Weight
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Extinction Coefficient (280nm)
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This calculator can be used to determine the molecular weight of proteins, peptides, and amino acid sequences. It's useful for protein analysis, gel electrophoresis planning, mass spectrometry verification, and biochemical research applications.
Peptide molecular weight is calculated by summing the molecular weights of all constituent amino acids, subtracting water molecules lost during peptide bond formation, and adding terminal group weights. The formula is: MW = Σ(amino acid weights) - (n-1) × 18.015 + terminal modifications.
The calculator parses your input sequence, identifies each amino acid, looks up standard atomic weights, accounts for peptide bond formation (water loss), and includes any specified terminal modifications. It provides both total molecular weight and detailed composition analysis.
The calculator supports both single-letter amino acid codes (A, C, D, E, etc.) and three-letter codes (Ala, Cys, Asp, Glu, etc.). Single-letter format is most commonly used and recommended for longer sequences.
Terminal modifications significantly impact molecular weight. N-terminus modifications like acetylation (+43 Da) or formylation (+29 Da), and C-terminus modifications like amidation or esterification can add substantial mass to your peptide or protein.
The extinction coefficient at 280nm indicates how strongly your protein absorbs UV light, primarily due to tryptophan and tyrosine residues. This value is crucial for determining protein concentration using UV spectrophotometry.
Currently, the calculator handles standard proteinogenic amino acids and common N/C-terminal modifications. For non-standard amino acids or post-translational modifications, you may need specialized software or manual calculations.