Molecular mass

Molecular masses are calculated from the atomic masses of each nuclide present in the molecule, while molar masses and relative molecular masses (molecular weights) are calculated from the standard atomic weights 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 molar mass of 18.0153(3) g/mol, but individual water molecules have molecular masses between 18.010 564 6863(15) Da (1H16O) and 22.027 7364(9) Da (2H18O). Atomic and molecular masses are usually reported in daltons, which is defined in terms of the mass of the isotope 12C (carbon-12). However, the name unified atomic mass unit (u) is still used in common practice. Relative atomic and molecular masses as defined are dimensionless. Molar masses when expressed in g/mol have almost identical numerical values as relative atomic and molecular masses. For example, the molar mass and molecular mass of methane, whose molecular formula is CH4, are calculated respectively as follows: 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. == Determination ==

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. This also differs subtly from the molecular mass in that the choice of isotopes is defined and thus is a single specific molecular mass out of the (perhaps many) possibilities. The masses used to compute the monoisotopic molecular mass are found in a table of isotopic masses and are not found in a typical periodic table. The average molecular mass is often used for larger molecules, since molecules with many atoms are often 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 in a typical periodic table. The average molecular mass of a very small sample, however, might differ 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 and nucleic acids at the single-molecule level. The technique is based on interferometric scattered light microscopy. 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 can also be used to measure sample homogeneity, to detect protein oligomerisation states, and to identify complex macromolecular assemblies (ribosomes, GroEL, AAV) and protein interactions such as protein-protein interactions. Mass photometry can accurately measure molecular mass over a wide range of molecular masses (40 kDa – 5 MDa). Hydrodynamic methods To a first approximation, the basis for determination of molecular mass according to Mark–Houwink relations 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). The apparent hydrodynamic size can then be used to approximate molecular mass using a series of macromolecule-specific standards. 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. == See also ==