Box 11 Working In Biochemistry

Molecular Weight, Molecular Mass, and Their Correct Units

There are two common (and equivalent) ways to describe molecular mass; both are used in this text. The first is molecular weight, or relative molecular mass, denoted Mr. The molecular weight of a substance is defined as the ratio of the mass of a molecule of that substance to one-twelfth the mass of carbon-12 (12C). Since Mr is a ratio, it is dimensionless—it has no associated units. The second is molecular mass, denoted m. This is simply the mass of one molecule, or the molar mass divided by Avogadro's number. The molecular mass, m, is expressed in daltons (abbreviated Da). One dalton is equivalent to one-twelfth the mass of carbon-12; a kilodalton (kDa) is 1,000 daltons; a mega-dalton (MDa) is 1 million daltons.

Consider, for example, a molecule with a mass 1,000 times that of water. We can say of this molecule either Mr = 18,000 or m = 18,000 daltons. We can also describe it as an "18 kDa molecule." However, the expression Mr = 18,000 daltons is incorrect.

Another convenient unit for describing the mass of a single atom or molecule is the atomic mass unit (formerly amu, now commonly denoted u). One atomic mass unit (1 u) is defined as one-twelfth the mass of an atom of carbon-12. Since the experimentally measured mass of an atom of carbon-12 is 1.9926 X 10~23 g, 1 u = 1.6606 X 10~24 g. The atomic mass unit is convenient for describing the mass of a peak observed by mass spectrometry (see Box 3-2).

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