Protein Determination by UV Absorption
Alastair Aitken and Michèle P. Learmonth
1. Introduction
1.1. Near UV Absorbance (280 nm)
Quantitation of the amount of protein in a solution is possible in a simple spectrometer.
Absorption of radiation in the near UV by proteins depends on the Tyr and Trp
content (and to a very small extent on the amount of Phe and disulfide bonds). Therefore
the A280 varies greatly between different proteins (for a 1 mg/mL solution, from 0
up to 4 [for some tyrosine-rich wool proteins], although most values are in the
range 0.5–1.5 [1]). The advantages of this method are that it is simple, and the sample
is recoverable. The method has some disadvantages, including interference from other
chromophores, and the specific absorption value for a given protein must be determined.
The extinction of nucleic acid in the 280-nm region may be as much as 10 times
that of protein at their same wavelength, and hence, a few percent of nucleic acid can
greatly influence the absorption.
1.2. Far UV Absorbance
The peptide bond absorbs strongly in the far UV with a maximum at about 190 nm.
This very strong absorption of proteins at these wavelengths has been used in protein
determination. Because of the difficulties caused by absorption by oxygen and the low
output of conventional spectrophotometers at this wavelength, measurements are more
conveniently made at 205 nm, where the absorbance is about half that at 190 nm. Most
proteins have extinction coefficients at 205 nm for a 1 mg/mL solution of 30–35 and
between 20 and 24 at 210 nm (2).
Various side chains, including those of Trp, Phe, Tyr, His, Cys, Met, and Arg (in
that descending order), make contributions to the A205 (3).
The advantages of this method include simplicity and sensitivity. As in the method
outlined in Subheading 3.1. the sample is recoverable and in addition there is little
variation in response between different proteins, permitting near-absolute determination
of protein. Disadvantages of this method include the necessity for accurate calibration
of the spectrophotometer in the far UV. Many buffers and other components, such
as heme or pyridoxal groups, absorb strongly in this region.
2. Materials
1. 0.1 M K2SO4 (pH 7.0).
2. 5 mM potassium phosphate buffer, pH 7.0.
3. Nonionic detergent (0.01% Brij 35)
4. Guanidinium-HCl.
5. 0.2-μm Millipore (Watford, UK) filter.
6. UV-visible spectrometer: The hydrogen lamp should be selected for maximum intensity
at the particular wavelength.
7. Cuvets, quartz, for <215 nm.
3. Methods
3.1. Estimation of Protein by Near UV Absorbance (280 nm)
1. A reliable spectrophotometer is necessary. The protein solution must be diluted in the
buffer to a concentration that is well within the accurate range of the instrument (see
Notes 1 and 2).
2. The protein solution to be measured can be in a wide range of buffers, so it is usually no
problem to find one that is appropriate for the protein which may already be in a particular
buffer required for a purification step or assay for enzyme activity, for example (see Notes
3 and 4).
3. Measure the absorbance of the protein solution at 280 nm, using quartz cuvets or cuvets
that are known to be transparent to this wavelength, filled with a volume of solution sufficient
to cover the aperture through which the light beam passes.
4. The value obtained will depend on the path length of the cuvet. If not 1 cm, it must be
adjusted by the appropriate factor. The Beer-Lambert law states that:
A (absorbance) = ε c l (1)
where ε = extinction coefficient, c = concentration in mol/L and l = optical path length in
cm. Therefore, if ε is known, measurement of A gives the concentration directly, ε is
normally quoted for a 1-cm path length.
5. The actual value of UV absorbance for a given protein must be determined by some absolute
method, e.g., calculated from the amino acid composition, which can be determined
by amino acid analysis (4). The UV absorbance for a protein is then calculated according
to the following formula:
A280 (1 mg/mL) = (5690nw + 1280ny + 120nc)/M (2)
where nw, ny, and nc are the numbers of Trp, Tyr, and Cys residues in the polypeptide of
mass M and 5690, 1280 and 120 are the respective extinction coefficients for these residues
(see Note 5).
3.2. Estimation of Protein by Far UV Absorbance
1. The protein solution is diluted with a sodium chloride solution (0.9% w/v) until the absorbance
at 215 nm is <1.5 (see Notes 1 and 6).
2. Alternatively, dilute the sample in another non-UV-absorbing buffer such as 0.1 M K2SO4,
containing 5 mM potassium phosphate buffer adjusted to pH 7.0 (see Note 6).
3. Measure the absorbances at the appropriate wavelengths (either A280 and A205, or A225 and
A215, depending on the formula to be applied), using a spectrometer fitted with a hydrogen
lamp that is accurate at these wavelengths, using quartz cuvets filled with a volume of
solution sufficient to cover the aperture through which the light beam passes (details in
Subheading 3.1.).
4. The A205 for a 1 mg/mL solution of protein (A205
1 mg/mL) can be calculated within ±2%,
according to the empirical formula proposed by Scopes (2) (see Notes 7–10):
A205
1 mg/mL = 27 + 120 (A280/A205) (3)
5. Alternatively, measurements may be made at longer wavelengths (5):
Protein concentration (μg/mL) = 144 (A215 – A225) (4)
The extinction at 225 nm is subtracted from that at 215 nm; the difference multiplied by
144 gives the protein concentration in the sample in μg/mL. With a particular protein under
specific conditions accurate measurements of concentration to within 5 μg/L are possible.
4. Notes
1. It is best to measure absorbances in the range 0.05–1.0 (between 10 and 90% of the incident
radiation). At around 0.3 absorbance (50% absorption), the accuracy is greatest.
2. Bovine serum albumin is frequently used as a protein standard; 1 mg/mL has an A280 of 0.66.
3. If the solution is turbid, the apparent A280 will be increased by light scattering. Filtration
(through a 0.2-μm Millipore filter) or clarification of the solution by centrifugation can be
carried out. For turbid solutions, a convenient approximate correction can be applied by
subtracting the A310 (proteins do not normally absorb at this wavelength unless they contain
particular chromophores) from the A280.
4. At low concentrations, protein can be lost from solution by adsorption on the cuvet; the
high ionic strength helps to prevent this. Inclusion of a nonionic detergent (0.01% Brij 35)
in the buffer may also help to prevent these losses.
5. The presence of nonprotein chromophores (e.g., heme, pyridoxal) can increase A280. If
nucleic acids are present (which absorb strongly at 260 nm), the following formula can be
applied. This gives an accurate estimate of the protein content by removing the contribution
to absorbance by nucleotides at 280 nm, by measuring the A260 which is largely owing
to the latter (6).
Protein (mg/mL) = 1.55 A280 – 0.76 A260 (5)
Other formulae (using similar principles of absorbance differences) employed to determine
protein in the possible presence of nucleic acids are the following (7,8):
Protein (mg/mL) = (A235 – A280)/2.51 (6)
References
1. Kirschenbaum, D. M. (1975) Molar absorptivity and A1%/1 cm values for proteins at
selected wavelengths of the ultraviolet and visible regions. Analyt. Biochem. 68, 465–484.
2. Scopes, R. K. (1974) Measurement of protein by spectrometry at 205 nm. Analyt. Biochem.
59, 277–282.
3. Goldfarb, A. R., Saidel, L. J., and Mosovich, E. (1951) The ultraviolet absorption spectra
of proteins. J. Biol. Chem. 193, 397–404.
4. Gill, S. C. and von Hippel, P. H. (1989) Calculation of protein extinction coefficients from
amino acid sequence data. Analyt. Biochem. 182, 319–326.
5. Waddell, W. J. (1956) A simple UV spectrophotometric method for the determination of
protein. J. Lab. Clin. Med. 48, 311–314.
6. Layne, E. (1957) Spectrophotornetric and turbidimetric methods for measuring proteins.
Meth. Enzymol. 3, 447–454.
7. Whitaker, J. R. and Granum, P. E. (1980) An absolute method for protein determination
based on difference in absorbance at 235 and 280 nm. Analyt. Biochem. 109, 156–159.
8. Kalb, V. F. and Bernlohr, R. W. (1977). A new spectrophotometric assay for protein in cell
extracts. Analyt. Biochem. 82, 362–371.
Protein (mg/mL) = 0.183 A230 – 0.075.8 A260 (7)
6. Protein solutions obey Beer-Lambert’s Law at 215 nm provided the absorbance is <2.0.
7. Strictly speaking, this value applies to the protein in 6 M guanidinium-HCl, but the value
in buffer is generally within 10% of this value, and the relative absorbances in guanidinium-
HCl and buffer can be easily determined by parallel dilutions from a stock solution.
8. Sodium chloride, ammonium sulfate, borate, phosphate, and Tris do not interfere, whereas
0.1M acetate, succinate, citrate, phthalate, and barbiturate show high absorption at 215 nm.
9. The absorption of proteins in the range 215–225 nm is practically independent of pH
between pH values 4–8.
10. The specific extinction coefficient of a number of proteins and peptides at 205 nm and
210 nm (3) has been determined. The average extinction coefficient for a 1 mg/mL solution
of 40 serum proteins at 210 nm is 20.5 ± 0.14. At this wavelength, a protein concentration
of 2 μg/mL gives A = 0.04 (5).
