Why is coupling efficiency important?
Coupling efficiency is important because the effects are cumulative during DNA synthesis. The Table below shows the effect of a 1% difference in coupling efficiency and how this influences the amount of full-length product, following the synthesis of oligos of different length. Considering a relatively short oligo of 20 bases, a 1% difference in coupling efficiency can result in a 15% difference, in terms of full-length final product.
The table also shows that the longer an oligo, the lower the yield of full length product that can be expected, due to limitations set by chemistry. Assuming a coupling efficiency of 99% for every single base addition (industry standard is 98.5% in average), the raw product of a 95-mer synthesis would consist of only 38.5% full length oligonucleotide. Separating full length and failure sequences from each other by HPLC purification results in additional loss, so that low yields are a normal matter of fact.
Note that metabion regularly exceeds 99% of coupling efficiency, reaching a coupling efficiency of up to 99,7%, even for long unmodified oligonucleotides.