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Flexible sugar and acyclic nucleic acid chemistries for duplex destabilization, Flexible sugar and acyclic nucleic acid chemistries for duplex destabilization, specificity tuning, siRNA off-target reduction, aptamer engineering, and advanced probe and oligonucleotide design.
Flexible nucleic acid analogs are designed to increase local conformational freedom rather than lock the sugar into a rigid geometry. The best-known example is unlocked nucleic acid (UNA), an acyclic RNA analog in which the bond between the 2′ and 3′ carbon atoms of the ribose ring is absent.
UNA generally lowers duplex melting temperature in a position- and sequence-dependent manner. This controlled destabilization can be useful when an oligonucleotide binds too strongly, when mismatch discrimination must be improved, or when siRNA seed-region interactions need to be weakened to reduce miRNA-like off-target activity.
Bio-Synthesis supports standard UNA monomers and can evaluate modified UNA derivatives, specialty base-functionalized UNA analogs and customer-specified flexible or acyclic nucleic acid building blocks for research applications.
UNA can lower duplex Tm without changing the base-pairing sequence.
Strategic placement can increase mismatch or allele discrimination.
UNA can reduce passenger-strand loading and seed-mediated off-target activity.
Acyclic geometry enables conformational and structure-function studies.
Design principle: UNA is not an affinity enhancer. It is primarily a duplex-tuning and destabilizing modification. Placement should be selected according to the desired thermodynamic and biological effect.
Choose the design objective to see how UNA or a related flexible analog may be used and what tradeoffs should be reviewed.
Use one or a small number of UNA residues when a duplex is too stable or when deliberate local destabilization is needed. The effect is additive but strongly depends on sequence and position.
Tm Effect
Placement
Mechanism
Risk
Too many UNA residues can substantially reduce target binding.
Local flexibility can weaken binding to mismatched targets more than to the intended target when the UNA residue is placed strategically near a SNP, mutation or related-sequence difference.
Benefit
Affinity
Use
Review
Destabilizing the matched duplex too much can reduce assay sensitivity.
UNA can be placed in the guide seed region to reduce miRNA-like off-target interactions or at the passenger-strand 5′ end to discourage passenger-strand loading into RISC.
Potency
Incorrect placement can reduce on-target silencing.
UNA can alter local folding, loop geometry and conformational dynamics in aptamers, G-quadruplexes and i-motif structures. The effect is highly position-dependent and should be evaluated experimentally.
Effect
Testing
A single flexible residue can change the entire folded architecture.
UNA can reduce excessive probe affinity, improve mismatch discrimination or tune the melting behavior of probes that are too GC-rich or over-stabilized by other modifications.
Combination
UNA cannot rescue a poorly positioned probe or unsuitable amplicon.
Because UNA removes the ribose C2′–C3′ bond, it can be used to investigate how local sugar flexibility affects duplexes, hairpins, quadruplexes, i-motifs and other nucleic acid structures.
Readout
Design
Results from one structural motif may not transfer to another.
Select an analog to review its structural concept, thermodynamic effect, common applications and design considerations.
UNA is an acyclic RNA analog lacking the bond between the ribose C2′ and C3′ atoms. It increases local flexibility and generally lowers duplex melting temperature in a position-dependent manner.
Flexibility
Best Known For
MUNA includes methylated or methoxylated UNA derivatives designed to expand the chemical and biological properties of the UNA scaffold, particularly for siRNA optimization.
Status
Piperazino-functionalized UNA introduces a basic heterocycle onto the flexible UNA scaffold, providing a platform for altered charge, conjugation and structure-function studies.
Charge
Pyrene-functionalized UNA combines a flexible acyclic sugar analog with an aromatic fluorescent base substituent for structure, fluorescence and molecular-recognition studies.
4-Thio-UNA combines the flexible UNA sugar framework with a sulfur-modified uracil base, enabling specialized photochemical, structural and aptamer investigations.
Base
This table provides relative design guidance. Specialty derivatives require project-specific feasibility and analytical review.
Important: UNA and related flexible analogs usually reduce or redistribute duplex stability. They should not be grouped with constrained affinity-enhancing chemistries such as LNA/BNA, ENA or cEt.
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UNA is often most useful when it balances another stabilizing modification or tunes a specific region of an otherwise conventional oligonucleotide.
Balances local destabilization and affinity enhancement in probes, aptamers and hybrid designs.
Supports stability while preserving local flexibility or RNase H-compatible gap designs.
Useful for structure-sensitive probes, aptamers and hybridization studies.
Emerging strategy for seed-region tuning and off-target mitigation.
Expected Tm change, mismatch behavior, synthesis feasibility, purification, analytical QC and combination-modification compatibility.
Flexible nucleic acid projects require controlled synthesis, purification, analytical review and project-specific documentation.
Bio-Synthesis supports standard UNA and specialty flexible nucleic acid analog projects with controlled synthesis, purification, analytical QC, documentation and project-specific packaging.
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