The molecular mass (m) is the mass of a given molecule: it is measured in daltons (Da or u).[1][2] Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quantity relative molecular mass, as defined by IUPAC, is the ratio of the mass of a molecule to the unified atomic mass unit (also known as the dalton) and is unitless. The molecular mass and relative molecular mass are distinct from but related to the molar mass. The molar mass is defined as the mass of a given substance divided by the amount of a substance and is expressed in g/mol. That makes the molar mass an average of many particles or molecules, and the molecular mass the mass of one specific particle or molecule. The molar mass is usually the more appropriate figure when dealing with macroscopic (weigh-able) quantities of a substance.
The definition of molecular weight is most authoritatively synonymous with relative molecular mass; however, in common practice, it is highly variable. When the molecular weight is used with the units Da or u, it is frequently as a weighted average similar to the molar mass but with different units. In molecular biology, the mass of macromolecules is referred to as their molecular weight and is expressed in kDa, although the numerical value is often approximate and representative of an average.
The terms molecular mass, molecular weight, and molar mass are often used interchangeably in areas of science where distinguishing between them is unhelpful. In other areas of science, the distinction is crucial. The molecular mass is more commonly used when referring to the mass of a single or specific well-defined molecule and less commonly than molecular weight when referring to a weighted average of a sample. Prior to the 2019 redefinition of SI base units quantities expressed in daltons (Da or u) were by definition numerically equivalent to otherwise identical quantities expressed in the units g/mol and were thus strictly numerically interchangeable. After the 20 May 2019 redefinition of units, this relationship is only nearly equivalent.
The molecular mass of small to medium size molecules, measured by mass spectrometry, can be used to determine the composition of elements in the molecule. The molecular masses of macromolecules, such as proteins, can also be determined by mass spectrometry; however, methods based on viscosity and light-scattering are also used to determine molecular mass when crystallographic or mass spectrometric data are not available.
Molecular masses are calculated from the atomic masses of each nuclide present in the molecule, while relative molecular masses are calculated from the standard atomic weights[3] of each element. The standard atomic weight takes into account the isotopic distribution of the element in a given sample (usually assumed to be "normal"). For example, water has a relative molecular mass of 18.0153(3), but individual water molecules have molecular masses which range between 18.010 564 6863(15) Da (1H 216O) and 22.027 7364(9) Da (2H 218O).
Atomic and molecular masses are usually reported in daltons which is defined relative to the mass of the isotope12C (carbon 12). Relative atomic and molecular mass values as defined are dimensionless. However, the "unit" Dalton is still used in common practice. For example, the relative molecular mass and molecular mass of methane, whose molecular formula is CH4, are calculated respectively as follows:
Relative molecular mass of CH4
Standard atomic weight
Number
Total molecular weight (dimensionless)
C
12.011
1
12.011
H
1.008
4
4.032
CH4
16.043
Molecular mass of 12C1H4
Nuclide mass
Number
Total molecular mass (Da or u)
12C
12.00
1
12.00
1H
1.007825
4
4.0313
CH4
16.0313
The uncertainty in molecular mass reflects variance (error) in measurement not the natural variance in isotopic abundances across the globe. In high-resolution mass spectrometry the mass isotopomers 12C1H4 and 13C1H4 are observed as distinct molecules, with molecular masses of approximately 16.031 Da and 17.035 Da, respectively. The intensity of the mass-spectrometry peaks is proportional to the isotopic abundances in the molecular species. 12C 2H 1H3 can also be observed with molecular mass of 17 Da.
In mass spectrometry, the molecular mass of a small molecule is usually reported as the monoisotopic mass, that is, the mass of the molecule containing only the most common isotope of each element. Note that this also differs subtly from the molecular mass in that the choice of isotopes is defined and thus is a single specific molecular mass of the many possibilities. The masses used to compute the monoisotopic molecular mass are found on a table of isotopic masses and are not found on a typical periodic table. The average molecular mass is often used for larger molecules since molecules with many atoms are unlikely to be composed exclusively of the most abundant isotope of each element. A theoretical average molecular mass can be calculated using the standard atomic weights found on a typical periodic table, since there is likely to be a statistical distribution of atoms representing the isotopes throughout the molecule. The average molecular mass of a sample, however, usually differs substantially from this since a single sample average is not the same as the average of many geographically distributed samples.
Mass photometry
Mass photometry (MP) is a rapid, in-solution, label-free method of obtaining the molecular mass of proteins, lipids, sugars & nucleic acids at the single-molecule level. The technique is based on interferometric scattered light microscopy.[4] Contrast from scattered light by a single binding event at the interface between the protein solution and glass slide is detected and is linearly proportional to the mass of the molecule. This technique is also capable of measuring sample homogeneity,[5] detecting protein oligomerisation state, characterisation of complex macromolecular assemblies (ribosomes, GroEL, AAV) and protein interactions such as protein-protein interactions.[6] Mass photometry can measure molecular mass to an accurate degree over a wide range of molecular masses (40kDa – 5MDa).
Hydrodynamic methods
To a first approximation, the basis for determination of molecular mass according to Mark–Houwink relations[7] is the fact that the intrinsic viscosity of solutions (or suspensions) of macromolecules depends on volumetric proportion of the dispersed particles in a particular solvent. Specifically, the hydrodynamic size as related to molecular mass depends on a conversion factor, describing the shape of a particular molecule. This allows the apparent molecular mass to be described from a range of techniques sensitive to hydrodynamic effects, including DLS, SEC (also known as GPC when the eluent is an organic solvent), viscometry, and diffusion ordered nuclear magnetic resonance spectroscopy (DOSY).[8] The apparent hydrodynamic size can then be used to approximate molecular mass using a series of macromolecule-specific standards.[9] As this requires calibration, it's frequently described as a "relative" molecular mass determination method.
Static light scattering
It is also possible to determine absolute molecular mass directly from light scattering, traditionally using the Zimm method. This can be accomplished either via classical static light scattering or via multi-angle light scattering detectors. Molecular masses determined by this method do not require calibration, hence the term "absolute". The only external measurement required is refractive index increment, which describes the change in refractive index with concentration.
^Mohr, Peter J.; Taylor, Barry N.; Newell, David B. (2011). "CODATA Recommended Values of the Fundamental Physical Constants: 2010". Database developed by J. Baker, M. Douma, and S. Kotochigova. National Institute of Standards and Technology, Gaithersburg, MD 20899.
^"Atomic Weights and Isotopic Compositions for All Elements". NIST. Retrieved 2007-10-14.
^Sonn-Segev, A., Belacic, K., Bodrug, T. et al. Quantifying the heterogeneity of macromolecular machines by mass photometry. Nat Commun 11, 1772 (2020). https://doi.org/10.1038/s41467-020-15642-w
^Soltermman et al. Quantifying protein-protein interactions by molecular counting using mass photometry. Angew. Chem Int Ed, 2020, 59(27), 10774-10779
^Paul, Hiemenz C., and Lodge P. Timothy. Polymer Chemistry. Second ed. Boca Raton: CRC P, 2007. 336, 338–339.
^Johnson Jr., C. S. (1999). "Diffusion ordered nuclear magnetic resonance spectroscopy: principles and applications". Progress in Nuclear Magnetic Resonance Spectroscopy. 34 (3–4): 203–256. doi:10.1016/S0079-6565(99)00003-5.
^Neufeld, R.; Stalke, D. (2015). "Accurate Molecular Weight Determination of Small Molecules via DOSY-NMR by Using External Calibration Curves with Normalized Diffusion Coefficients" (PDF). Chem. Sci.6 (6): 3354–3364. doi:10.1039/C5SC00670H. PMC5656982. PMID 29142693.
External links
A Free Android application for molecular and reciprocal weight calculation of any chemical formula
Stoichiometry Add-In for Microsoft Excel for calculation of molecular weights, reaction coefficients and stoichiometry.
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Not to be confused with Molar mass or Mass number The molecular mass m is the mass of a given molecule it is measured in daltons Da or u 1 2 Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element The related quantity relative molecular mass as defined by IUPAC is the ratio of the mass of a molecule to the unified atomic mass unit also known as the dalton and is unitless The molecular mass and relative molecular mass are distinct from but related to the molar mass The molar mass is defined as the mass of a given substance divided by the amount of a substance and is expressed in g mol That makes the molar mass an average of many particles or molecules and the molecular mass the mass of one specific particle or molecule The molar mass is usually the more appropriate figure when dealing with macroscopic weigh able quantities of a substance The definition of molecular weight is most authoritatively synonymous with relative molecular mass however in common practice it is highly variable When the molecular weight is used with the units Da or u it is frequently as a weighted average similar to the molar mass but with different units In molecular biology the mass of macromolecules is referred to as their molecular weight and is expressed in kDa although the numerical value is often approximate and representative of an average The terms molecular mass molecular weight and molar mass are often used interchangeably in areas of science where distinguishing between them is unhelpful In other areas of science the distinction is crucial The molecular mass is more commonly used when referring to the mass of a single or specific well defined molecule and less commonly than molecular weight when referring to a weighted average of a sample Prior to the 2019 redefinition of SI base units quantities expressed in daltons Da or u were by definition numerically equivalent to otherwise identical quantities expressed in the units g mol and were thus strictly numerically interchangeable After the 20 May 2019 redefinition of units this relationship is only nearly equivalent The molecular mass of small to medium size molecules measured by mass spectrometry can be used to determine the composition of elements in the molecule The molecular masses of macromolecules such as proteins can also be determined by mass spectrometry however methods based on viscosity and light scattering are also used to determine molecular mass when crystallographic or mass spectrometric data are not available Contents 1 Calculation 2 Determination 2 1 Mass spectrometry 2 2 Mass photometry 2 3 Hydrodynamic methods 2 4 Static light scattering 3 See also 4 References 5 External linksCalculation EditMolecular masses are calculated from the atomic masses of each nuclide present in the molecule while relative molecular masses are calculated from the standard atomic weights 3 of each element The standard atomic weight takes into account the isotopic distribution of the element in a given sample usually assumed to be normal For example water has a relative molecular mass of 18 0153 3 but individual water molecules have molecular masses which range between 18 010 564 6863 15 Da 1H2 16O and 22 027 7364 9 Da 2H2 18O Atomic and molecular masses are usually reported in daltons which is defined relative to the mass of the isotope 12C carbon 12 Relative atomic and molecular mass values as defined are dimensionless However the unit Dalton is still used in common practice For example the relative molecular mass and molecular mass of methane whose molecular formula is CH4 are calculated respectively as follows Relative molecular mass of CH4Standard atomic weight Number Total molecular weight dimensionless C 12 011 1 12 011H 1 008 4 4 032CH4 16 043Molecular mass of 12C1H4Nuclide mass Number Total molecular mass Da or u 12C 12 00 1 12 001H 1 007825 4 4 0313CH4 16 0313The uncertainty in molecular mass reflects variance error in measurement not the natural variance in isotopic abundances across the globe In high resolution mass spectrometry the mass isotopomers 12C1H4 and 13C1H4 are observed as distinct molecules with molecular masses of approximately 16 031 Da and 17 035 Da respectively The intensity of the mass spectrometry peaks is proportional to the isotopic abundances in the molecular species 12C 2H 1H3 can also be observed with molecular mass of 17 Da Determination EditMass spectrometry Edit Main article Mass spectrometry In mass spectrometry the molecular mass of a small molecule is usually reported as the monoisotopic mass that is the mass of the molecule containing only the most common isotope of each element Note that this also differs subtly from the molecular mass in that the choice of isotopes is defined and thus is a single specific molecular mass of the many possibilities The masses used to compute the monoisotopic molecular mass are found on a table of isotopic masses and are not found on a typical periodic table The average molecular mass is often used for larger molecules since molecules with many atoms are unlikely to be composed exclusively of the most abundant isotope of each element A theoretical average molecular mass can be calculated using the standard atomic weights found on a typical periodic table since there is likely to be a statistical distribution of atoms representing the isotopes throughout the molecule The average molecular mass of a sample however usually differs substantially from this since a single sample average is not the same as the average of many geographically distributed samples Mass photometry Edit Mass photometry MP is a rapid in solution label free method of obtaining the molecular mass of proteins lipids sugars amp nucleic acids at the single molecule level The technique is based on interferometric scattered light microscopy 4 Contrast from scattered light by a single binding event at the interface between the protein solution and glass slide is detected and is linearly proportional to the mass of the molecule This technique is also capable of measuring sample homogeneity 5 detecting protein oligomerisation state characterisation of complex macromolecular assemblies ribosomes GroEL AAV and protein interactions such as protein protein interactions 6 Mass photometry can measure molecular mass to an accurate degree over a wide range of molecular masses 40kDa 5MDa Hydrodynamic methods Edit To a first approximation the basis for determination of molecular mass according to Mark Houwink relations 7 is the fact that the intrinsic viscosity of solutions or suspensions of macromolecules depends on volumetric proportion of the dispersed particles in a particular solvent Specifically the hydrodynamic size as related to molecular mass depends on a conversion factor describing the shape of a particular molecule This allows the apparent molecular mass to be described from a range of techniques sensitive to hydrodynamic effects including DLS SEC also known as GPC when the eluent is an organic solvent viscometry and diffusion ordered nuclear magnetic resonance spectroscopy DOSY 8 The apparent hydrodynamic size can then be used to approximate molecular mass using a series of macromolecule specific standards 9 As this requires calibration it s frequently described as a relative molecular mass determination method Static light scattering Edit It is also possible to determine absolute molecular mass directly from light scattering traditionally using the Zimm method This can be accomplished either via classical static light scattering or via multi angle light scattering detectors Molecular masses determined by this method do not require calibration hence the term absolute The only external measurement required is refractive index increment which describes the change in refractive index with concentration See also EditCryoscopy and cryoscopic constant Ebullioscopy and ebullioscopic constant Dumas method of molecular weight determination Francois Marie Raoult Standard atomic weight Mass number Absolute molar mass Molar mass distribution Dalton unit SDS PAGEReferences Edit International Bureau of Weights and Measures 2006 The International System of Units SI PDF 8th ed p 126 ISBN 92 822 2213 6 archived PDF from the original on 2021 06 04 retrieved 2021 12 16 Mohr Peter J Taylor Barry N Newell David B 2011 CODATA Recommended Values of the Fundamental Physical Constants 2010 Database developed by J Baker M Douma and S Kotochigova National Institute of Standards and Technology Gaithersburg MD 20899 Atomic Weights and Isotopic Compositions for All Elements NIST Retrieved 2007 10 14 Young et al 2018 Quantitative imaging of single biological macromolecules Science 360 423 427 DOI https doi org 10 1126 science aar5839 Sonn Segev A Belacic K Bodrug T et al Quantifying the heterogeneity of macromolecular machines by mass photometry Nat Commun 11 1772 2020 https doi org 10 1038 s41467 020 15642 w Soltermman et al Quantifying protein protein interactions by molecular counting using mass photometry Angew Chem Int Ed 2020 59 27 10774 10779 Paul Hiemenz C and Lodge P Timothy Polymer Chemistry Second ed Boca Raton CRC P 2007 336 338 339 Johnson Jr C S 1999 Diffusion ordered nuclear magnetic resonance spectroscopy principles and applications Progress in Nuclear Magnetic Resonance Spectroscopy 34 3 4 203 256 doi 10 1016 S0079 6565 99 00003 5 Neufeld R Stalke D 2015 Accurate Molecular Weight Determination of Small Molecules via DOSY NMR by Using External Calibration Curves with Normalized Diffusion Coefficients PDF Chem Sci 6 6 3354 3364 doi 10 1039 C5SC00670H PMC 5656982 PMID 29142693 External links EditA Free Android application for molecular and reciprocal weight calculation of any chemical formula Stoichiometry Add In for Microsoft Excel for calculation of molecular weights reaction coefficients and stoichiometry Portal Physics Retrieved from https en wikipedia org w index php title Molecular mass amp oldid 1133124328, wikipedia, wiki, book, books, library,
