Structural Biochemistry 14. Permethylation of Nucleosides

Prominent molecular ions are generally observed in the field ionization (FI) mass spectra of unprotected nucleosides.² In the one exception so far observed, that of guanosine, we found the simple methyl derivative, N²,N²-dimethylguanosine, to afford an easily detectible molecular ion. The electron impact (EI) mass spectrum of N²,N²-dimethylguanosine also displayed a molecular ion and suggested that nucleoside methyl derivatives might be more easily studied by EI methods. For this purpose and the more important objective of developing methods for sequencing small nucleic acid units by computer assisted³ FI-EI mass spectrometry, a program was initiated (1967) to explore permethylation of nucleosides. We believe protection by permethylation to be superior to pertrimethylsilylation and acetylation principally for reasons involving molecular weight and stability. Concurrently it was anticipated that such nucleoside methylation studies would afford routes to partially mathylated nucleosides of value in characterizing such components of virus and cellular DNA and RNA⁴. Subsequently, using variations of the methanol-DCCI,⁵ diazomethane with various Lewis acids,⁶ methyl iodide in, for example, dimethylsulfoxide,⁷ methyl iodide-sodium hydride in dimethylformamide,⁸ methyl iodide-silver oxides⁹ and methyl iodidemethylsulfinyl carbanion in dimethylsulfoxide,¹⁰ techniques were evaluated but none was found to provide permethyl nucleosides in high yield.¹¹ The latter two methods have, however, been applied to methylating nucleosides for EI mass spectral investigations.¹²