**5. Conclusions**

To summarize, we designed and conveniently synthesized DOCs containing three "like-a-brush" lipophilic chains. Phosphoryl guanidine modifications were introduced at the 3 end of some DOCs to prolong the plasma exposure. In the current work, we present the investigation of self-assembling and cell-penetrating features of these conjugates. DLS, AFM, and TEM techniques showed self-association of dodecyl oligonucleotide conjugates into spherical micellar particles and their nanosized aggregates (<200 nm). These structures are highly bound by serum albumin, which can increase their circulation half-life and bioavailability. It was found that DOCs and their duplexes can penetrate HepG2 cells by mimicking the spherical architecture or anchoring nonspecifically to the membranes both in the absence (with high efficacy) and in the presence of serum albumin (with reduced efficacy). These results, along with recently obtained data [33] indicate a strong potential to consider these conjugates as essential nanomaterials to develop nucleic acid delivery tools for biomedical applications.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/10/10/1948/s1, Figure S1: The structure of 3- -FAM CPG used in this work, Figure S2: Comparative electrophoretic mobilities of oligonucleotides and DOCs, Figures S3–S5: The structures of FAM-17-D, D-17-FAM and FAM-D-17PG conjugates, Figure S6: Typical MALDI TOF and ESI mass spectra of the conjugates, Figure S7,S8: Self-assembly of 17-mer DOC by Nile Red binding assay, Figure S9: Dynamic light scattering measurements, Figure S10: Possible structures of DOCs self-dimers, Figures S11–S16: AFM additional images, Figures S17,S18: TEM additional images, Figure S19: BSA binding with D-13PG conjugate, Figure S20: Fluorescence quenching additional image, Figure S21: Absorbance spectra of FAM-D-17, FAM-17-D, D-17-FAM, Figure S22: The stability of FAM-D-17PG conjugate and FAM-17- /D-17PG duplex used for transfection in 10% FBS, Table S1: Experimental and theoretical molecular masses of the DOCs.

**Author Contributions:** Methodology, resources, investigation, formal analysis, visualization, validation, data curation, writing—original draft preparation, funding acquisition, A.S.P.; investigation, formal analysis, visualization, writing—original draft preparation (flow cytometry and confocal microscopy imaging), funding acquisition, I.S.D.; resources, formal analysis, visualization, M.S.K.; investigation, formal analysis, visualization, A.E.G.; data validation, project administration, funding acquisition, I.A.P.; conceptualization, methodology, data validation, supervision, writing—review and editing, D.V.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research and APC was funded by the Russian Science Foundation (Project #19-15-00217). Fluorescence quenching experiments and AFM were carried out by A.S.P. and supported by Russian State funded budget project of ICBFM SB RAS # 0245-2019-0002.

**Acknowledgments:** The authors acknowledge Marat F. Kasakin, leading engineer of the Core Facility of Mass Spectrometric Analysis (ICBFM SB RAS) for recording of the MALDI TOF and ESI mass spectra. Authors thank Georgiy Yu. Shevelev (Laboratory of Synthetic Biology, ICBFM SB RAS) for useful technical assistance during AFM investigations followed by helpful discussions.

**Conflicts of Interest:** The authors declare no conflict of interest.
