Protein pI, MW and Extinction Coefficient Calculator

Results

Three Letter Sequence:
One Letter Sequence:
Sequence Length:
Molecular Weight:
Molecular Formula:
Theoretical pl:
Gravy:
Net Charge at pH 7.4:
Extinction Coefficient:
Abs 0.1% (=1 g/l): assuming all pairs of Cys residues form cystines
Extinction Coefficient:
Abs 0.1% (=1 g/l): assuming all Cys residues are reduced
Amino Acid Composition: :%  
What is the isoelectric point (pI) of a protein?
Proteins are amphoteric electrolytes composed of amino acids that contain positively charged amino groups (-NH3+) and negatively charged carboxyl groups (-COO-). The charge state of a protein molecule varies with pH. The isoelectric point (pI) is defined as the specific pH at which the net electrical charge of the protein is zero.
What is protein molecular weight (MW)?
Proteins are macromolecules in which numerous amino acid residues are linked by peptide bonds— covalent bonds formed via dehydration condensation between the α-carboxyl group of one amino acid and the α-amino group of another. The molecular weight (MW) of a protein is the sum of the molecular weights of its constituent amino acid residues and represents a fundamental physicochemical parameter. The standard unit is the Dalton (Da, or D), defined as 1/12 the mass of a carbon-12 atom (1 Da = 1/NA g, where N is Avogadro's constant). As a general approximation, protein molecular weight can be estimated by multiplying the number of amino acid residues by 110.
What is the molar extinction coefficient of a protein?
The protein extinction coefficient (also known as the molar extinction coefficient, ε) represents the absorbance of a 1 M protein solution measured at a specific wavelength using a 1 cm pathlength. Its conventional units are M-1 cm-1 or L mol-1 cm-1. This parameter quantifies the capacity of a protein to absorb light at a given wavelength and is critical for bioanalytical applications, including protein concentration determination, purity assessment, and the study of protein–molecule interactions.
What should be considered when using recombinant proteins for SPR assays?
  1. 1. Purity and homogeneity: Protein purity should exceed 90% with no visible aggregates. Higher sample purity reduces non-specific binding and background signal.
  2. 2. Buffer composition: Carefully review the protein buffer formulation, paying particular attention to the presence of glycerol, imidazole, reducing agents, DMSO, or surfactants, as these may interfere with chip immobilization or binding kinetics.
  3. 3. Affinity tags: Confirm whether the protein carries a His-tag or other affinity tag, as this dictates the optimal immobilization strategy.

MCE also provides SPR detection services. For further details, please refer to our service page: Surface Plasmon Resonance (SPR) Assay Services.

The 20 Proteinogenic Amino Acids

Nomenclature, Single-Letter/Three-Letter Codes, and Molecular Weights

Amino Acid 3-Letter Code 1-Letter Code Average MW (Da)
Glycine Gly G 75.07
Alanine Ala A 89.09
Leucine Leu L 131.17
Isoleucine Ile I 131.17
Valine Val V 117.15
Cysteine Cys C 121.16
Methionine Met M 149.21
Serine Ser S 105.09
Threonine Thr T 119.12
Aspartic acid Asp D 133.10
Amino Acid 3-Letter Code 1-Letter Code Average MW (Da)
Glutamic acid Glu E 147.13
Asparagine Asn N 132.12
Glutamine Gln Q 146.14
Arginine Arg R 174.20
Lysine Lys K 146.19
Histidine His H 155.15
Proline Pro P 115.13
Phenylalanine Phe F 165.19
Tyrosine Tyr Y 181.19
Tryptophan Trp W 204.23