Fractional mass values of large molecules

From the standpoint of relative molecular mass (RMM) determination by mass spectrometry, the average relative molecular masses of organic molecules in the range over 2000 daltons are significantly influenced both by the fractional masses of the constituent elements and by natural variations in isotopic abundance. Differences between monoisotopic RMM and relative atomic mass (RAM) based RMM tend characteristically to differ between major classes of biomolecules primarily because of molar carbon content, the difference between polypeptides and nucleic acids being about 4 daltons at M_r = 25 000. Considering terrestrial sources alone, variations in the isotopic abundance of carbon lead to differences of about 10–25 ppm in average molecular mass, which is significant with respect to present mass measurement capability in the region up to several thousand daltons. Comments are made concerning the calculation of average RMM values horn isotopic probability profiles versus those from RAMs.