[TIBS - February 1983 - p. 49]

H.B.F. Dixon

(Chairman, Nomenclature Committee of IUB)

Department of Biochemistry, Tennis Court Road, Cambridge CB2 1QW, UK

SIR: Although schools have long been using ‘relative molecular mass’ (Mr), most biochemical papers continue to call this quantity ‘molecular weight’, a term possibly inappropriate for a quantity that is not a weight. One reason is presumably the length of the preferred term, but the confusion is particularly harmful to students just learning to express ideas with exactness. Shortening ‘relative molecular mass’ to ‘em-ar’ in conversion may partly meet this objection, but the quantity Mr has the further disadvantage that, since it is a pure number and so has no units, it cannot be used with multiplicative prefixes like kilo and mega. As there are other ways of expressing information on molecular mass, they should be considered.

First of these is molar mass (M or Mm). It is particularly useful in calculations because its units (g mol^-1, kg mol^-1, etc.) allow checking of the units and dimensions at every step. Thus 1 mg of protein of molar mass 50 kg mol^-1 is clearly 1/50 m mol, or 20 nmol. Students may therefore be advised to convert other expressions into molar mass for calculation, to allow easy checking.

Second, there is expression of the molecular mass; mass is one of the seven quantities on which the units of SI are based, and it has the symbol m. IUB has approved, through its nomenclature committee, the name ‘dalton’ (symbol: Da) as an alternative to the cumbersome earlier name of "atomic mass unit" (symbol: u) for one-twelfth of the mass of the atom ¹²C. So a molecular mass may be 180 Da (as for glucose) and a ribosomal mass 2.6 MDa.

In formal contexts it may be necessary to state a quantity by name before giving its value by number and unit. Nevertheless the number and unit are often enough to indicate the quantity: we do not specify ‘a time of’ before saying ‘two hours’. So ‘a 55-kDa protein’ will usually be clear enough without writing ‘a protein of molecular mass 55 kDa’, and such adjectival use may be one of the main values of the dalton, if this form is thought nearer than ‘a 55-kg mol^-1 protein’. Similarly a scale of 10 kDa, 20 kDa, etc. is largely self-explanatory; the practice of 10K, 20K, etc. used in some journals, although not in much danger of suggesting temperatures, is confusing to those in nearby fields, especially if they know that the symbol for kilo always precedes the symbol for a unit, and is a lower-case ‘k’.

So authors are free to choose, according to their taste and the material they are presenting, whether to use relative molecular mass (a pure number), or molar mass (g mol^-1), or molecular mass (Da), with the same number in each case. Let us not put pressure on them to used one rather than another, but let us urge them to keep to normal conventions within each form of expression.