Determination of oligonucleotide composition from mass spectrometrically measured molecular weight

Extensive calculations for molecular mass versus subunit composition have been made for oligonucleotides from RNA and DNA to determine the extent to which base compositions might be derived from mass spectrometrically determined molecular weights. In the absence of compositional constraints (e.g., any numbers of A, U, G, C), measurement of molecular weight leads to only modest restrictions in allowable number of base compositions; however, if the compositional value for any one residue is known, such as from selective chemical modification or enzymatic cleavage, the number of allowable base compositions becomes unexpectedly low. For example, hydrolysis of RNA by ribonuclease T₁ produces oligonucleotides for which G=1, for which all base compositions can be uniquely specified up to the 14-mer level, solely by measurement of mass to within ±0,01%. The effects of methylation, phosphorylation state of nucleotide termini, and knowledge of chain length on the determination of subunit composition are discussed.