Loading Determination of DMTr‐substituted Resins for Large‐scale Oligonucleotide Synthesis

The loading (i.e., substitution) of solid supports for oligonucleotide synthesis is an important parameter in large‐scale manufacturing of oligonucleotides. Several key process parameters are dependent on the substitution of the solid support, including the number of phosphoramidite nucleoside equivalents used in the coupling step. For dimethoxytrityl (DMTr)–loaded solid supports, the substitution of the resin is determined by quantitatively cleaving the DMTr protecting group from the resin under acidic conditions and then analyzing the DMTr cation extinction by UV/vis spectroscopy. The spectrometric measurement can be performed at 409 nm or the global extinction maximum of 510 nm. The substitution is then calculated based on the Lambert‐Beer law analogously to the substitution determination of Fmoc‐substituted resins. Below, the determination of the molar extinction coefficient at 510 nm in a solution of 10% dichloroacetic acid in toluene and subsequent determination of the DMTr loading of DMTr‐substituted resins is reported.