*3.1. Oligonucleotides and DOCs in this Study*

In the first step, we designed and synthesized 13-, 17-, and 22-mer oligonucleotide conjugates with three dodecyl chains (Figure 1a). The structures and nucleotide sequences of the oligomers used in this study are given in Figure 1. While planning the synthesis of oligonucleotides with non-nucleoside units bearing dodecyl chains, we considered that the free terminal hydroxyethyl group of this unit can cause degradation of the modified oligonucleotide during the deprotection step in basic aqueous solutions [40,41]. Therefore, we added an extra thymidylate unit at 3- - (FAM-17-D (Figure S3)) or 5- -end (D-13, D-13PG, D-17, D-17PG, D-17-FAM (Figure S4), D-22PG) of corresponding oligomers to overcome this problem. Two phosphoryl guanidine [37,39] (PG) modifications were introduced at 3- -ends of D-13PG, D-17PG (Figure S5), and D-22PG oligomers to enhance their stability in serum (Figure 1a,c).

**Figure 1.** The sequences of oligonucleotides and their conjugates studied in this work (**a**), the structure of "like-a-brush" dodecyl-containing non-nucleoside backbone part of dodecyl oligonucleotide conjugates (DOC) (**b**). All oligonucleotides are deoxy. D-13PG, D-17PG, FAM-D-17PG, D-22PG are oligodeoxynucleotides partially substituted with phosphoryl guanidine groups (PG); \* indicated a position of PG modification, R1 is the linker with the terminal amino group protecting 3- -end of indicated oligomers from nuclease degradation (**c**). Here, 6-carboxyfluorescein (FAM) is represented 6-carboxyfluorescein residue (See Section 2.3). During post-chromatographic purification, dodecyl-containing oligonucleotides (especially D-17, D-17PG, and FAM-D-17PG) tended to aggregate, which resulted in significant salting-out of the conjugates at millimolar range concentrations used in the experiments.
