3'- spacers are often used as an alternative to 3'-phosphate as blockers; the most common is the C3 spacer. In general, a spacer is incorporated to add a distance between the oligo and a modifier. Spacers can be hydrophilic (e.g., the HEG spacer) or hydrophobic (C3 or C6 spacers).
The C9 spacer (Figure 1) consists of a triethylene glycol chain, formed by 9 atoms, 6 carbons and 3 oxygens. For its chemical nature, this spacer is slightly more hydrophilic than C3 and C12 spacers.
Figure 1. Structure of the C9 spacer (R = sequence, B = base).
metabion offers the C9 spacer in its standard DNA portfolio: