Nucleic acid analogues are compounds structurally similar (analog) to naturally occurring RNA and DNA, used in medicine and in molecular biology research. Nucleic acids are chains of nucleotides, which are composed of three parts: a phosphate backbone, a pucker-shaped pentose sugar, either ribose or deoxyribose, and one of four nucleobases. An analogue may have any of these altered. Typically the analogue nucleobases confer, among other things, different base pairing and base stacking proprieties. Examples include universal bases, which can pair with all four canon bases, and phosphate-sugar backbone analogues such as PNA, which affect the properties of the chain (PNA can even form a triple helix). [1]
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Medicine
Several nucleoside analogues are used as antiviral or anticancer agents. The viral polymerase incorporates these compounds with non-canon bases. These compounds are activated in the cells by being converted into nucleotides, they are administered as nucleosides since charged nucleotides cannot easily cross cell membranes.
Molecular biology
Nucleic acid analogues are used in molecular biology for several purposes:
- As a tool to detect particular sequences
- As a tool with resistance to RNA hydrolysis
- As a tool for another purpose, such as sequencing
- Naturally occurring, such as in tRNA
- Investigation of the mechanisms used by enzyme, such as an Enzyme inhibitor
- Investigation of possible scenarios of the origin of life
- Investigation of the structural features of nucleic acids
- Investigation of the possible alternatives to the natural system in Synthetic biology
Backbone analogues
Hydrolysis resistant RNA-analogues
To overcome the fact that ribose's 2' hydroxy group that reacts with the phosphate linked 3' hydroxy group (RNA is too unstable to be used or synthesised reliably), a ribose anologue is used. The most common RNA analogues are locked nucleic acid (LNA), morpholino, and peptide nucleic acid (PNA). These oligonucleotides differ as they have a different backbone sugar but still bind according to Watson and Crick pairing with RNA or DNA, but are immune to nuclease activity (They generally cannot be enzymatically synthesized and can only be produced synthetically).
Other notable analogues used as tools
Dideoxynucleotides are used in sequencing . These nucleotide triphosphates possess a non-canonical sugar, dideoxyribose, which lacks the 3' hydroxyl group normally present in DNA and therefore cannot bond with the next base. The lack of the 3' hydroxyl group terminates the chain reaction as the DNA polymerases mistake it for a regular deoxyribonucleotide. Another chain terminating analogue that lacks a 3' hydroxyl and mimics adenosine is called cordycepin. Cordycepin is an anticancer drug that targets RNA replication. Another analogue in sequencing is a nucleobase analogue, 7-deaza-GTP and is used to sequence CG rich regions, instead 7-deaza-ATP is called tubercidin, an antibiotic.
Precursors to the RNA world
RNA may be too complex to be the first nucleic acid, so before the RNA world several simpler nucleic acids which differ in the backbone, such as TNA and GNA and PNA, have been offered as candidates for the first nucleic acids.
Base analogues
Nucleobase structure and nomenclature
Natural bases are divided into two classes depending on their structure: pyrimidine (an heterocyclic aromatic six-membered ring with nitrogen atoms in position 1 and 3) and purine (a pyrimidine (numeration inverted) fused with an imidazole ring, a five-membered ring with 2 nitrogen atoms separated by one carbon (meta), 7,9). Their main proprieties are base pairing, resulting form 2 or 3 hydrogen bonds between keto (EWG) and amino (EDG) functional groups, and base stacking, caused by the attraction of the delocalized pi electron clouds of the aromatic ring structure.
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| Purine | Pyrimidine |
Fluorophores
Commonly fluorophores (such as rhodamine or fluorescein) are linked to the ring linked to the sugar (in para) via a flexible arm, presumably extruding from the major groove of the helix. Due to low processivity of the nucleotides linked to bulky adducts such as florophores by taq polymerases , the sequence is typically copied using a nucleotide with an arm and later coupled with a reactive fluorophore (indirect labelling):
- amine reactive: Aminoallyl nucleotide contain a primary amine group on a linker that reacts with the amino-reactive dye such as a cyanine or Alexa Fluor dyes, which contain a reactive leaving group, such as a succinimidyl ester (NHS). (base pairing amino groups are not affected).
- thiol reactive: thiol containing nucleotides reacts with the fluorophore linked to a reactive leaving group, such as a maleimide.
- biotin linked nucleotides rely on the same indirect labelling principle (+ fluorescent streptavidin) and are used in Affymetrix DNAchips.
Fluorophores find a variety of uses in medicine and biochemistry.
Natural non-canon bases
In a cell, there are several noncanon bases present: CpG islands in DNA (are often methylated), all eukaryotic mRNA (capped with a methyl-7-guanosine), and several bases of rRNAs (are methylated). Often, tRNAs are heavily modified postranscriptionally in order to improve their conformation or base pairing in particular in/near the anticodon: inosine can base pair with C, U, and even with A, whereas thiouridine (with A) is more specific than uracil (with a purine). Other common tRNA base modifications are pseudorindine (which gives its name to the TΨC loop), dihydrouridine (which does not stack as it is not aromatic), queosine, wyosine and so forth. Nevertheless these are all modifications to normal bases and are not placed by a polymerase.
Base-pairing
Canonical bases may have either a keto or amino group on the carbons surrounding the nitrogen atom furthest away from the glycosidic bond, which allows them to base pair (Watson-Crick base pairing) via hydrogen bonds (amine with keto, purine with pyrimidine). A and T are amine only and keto only, while C and G are mixed (inverted in respect to each other).
| Natural basepairs | |
|---|---|
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| A GC basepair: purine keto/amine forms three
intermolecular hydrogen bonds with pyrimidine amine/keto |
An AT basepair: purine amine/- forms two
intermolecular hydrogen bonds with pyrimidine keto/keto |
The precise reason why there are only four nucleotides is debated, but there are several unused possibilities. Furthermore adenine is not the most stable choice for base pairing: in Cyanophage S-2L diaminopurine (DAP) is used instead of adenine (host evasion). Diaminopurine basepairs perfectly with thymine as it is identical to adenine but has an amine group at position 2 forming 3 intramolecular hydrogen bonds, eliminating the major difference between the two types of basepairs (Weak:A-T and Strong:C-G). This improved stability affects protein binding ineractions which rely on those differences. Other combination include,
- isoguanosine and isocytosine, which have their amino and keto inverted, (not used probably as tautomers are problematic for base pairing, but isoG and isoG can be amplified correctly with PCR even in the presence of the 4 canon bases) [2]
- diaminopyrimidine and a xanthine (not used as xanthine is a deamination product)
| Unused basepair arrangements | ||
|---|---|---|
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| A DAP-T base: purine amine/amine forms three
intermolecular hydrogen bonds with pyrimidine keto/keto |
An X-DAY base: purine keto/keto forms three
intermolecular hydrogen bonds with pyrimidine amine/amine |
A iG-iC base: purine amine/keto forms three
intermolecular hydrogen bonds with pyrimidine keto/amine |
However correct DNA structure can form even when the bases are not paired via hydrogen bonding, as studies have shown using DNA isosteres (analogues with same number of atoms), such as the thymine analogue 2,4-difluorotoluene (F) or the adenine analogue 4-methylbenzimidazole (Z).
Other noteworthy basepairs:
- Several fluorescent bases have also been made, such as the 2-amino-6-(2-thienyl)purine and pyrrole-2-carbaldehyde base pair. [3]
- Metal coordinated bases, such as two 2,6-bis(ethylthiomethyl)pyridine (SPy) with a silver ion or pyridine-2,6-dicarboxamide (Dipam) and a mondentate pyridine (Py) with a copper ion [4].
- Universal bases may pair indiscriminately with any other base, but generally lower the melting temperature of the sequence considerably, examples include 2'-deoxyinosine derivatives, nitroazole analogues and hydrophobic aromatic non-hydrogen bonding bases (strong stacking effects). These are used as proof of concept and are not generally utilised in degenerate primers (which are a mixture of primers).
- The numbers of possible base pairs is doubled when xDNA is considered. xDNA contains expanded bases, in which a benzene ring has been added, which may pair with canon bases, resulting in four possible base-pairs (8 bases:xA-T,xT-A,xC-G,xG-C, 16 bases if the unused arrangements are used). Another form of benzene added bases is yDNA, in which the base is widened by the benzene[5].
| Novel basepairs with special proprieties | |||
|---|---|---|---|
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| A F-Z base: methylbenzimidazole does not form intermolecular
hydrogen bonds with toluene F/F |
An S-Pa base: purine thienyl/amine forms three intermolecular
hydrogen bonds with pyrrole -/carbaldehyde |
An xA-T base: same bonding as A-T |
Metal Base Pairs
In metal base-pairing, the Watson-Crick hydrogen bonds are replaced by the interaction between a metal ion with nucleosides acting as ligands. The possible geometries of the metal that would allow for duplex formation with two bidentate nucleosides around a central metal atom are: tetrahedral, dodecahedral, and square planar. Metal-complexing with DNA can occur by the formation of non-canonical base pairs from natural nucleobases with participation by metal ions and also by the exchanging the hydrogen atoms that are part of the Watson-Crick base pairing by metal ions[6]. Introduction of metal ions into a DNA duplex has shown to have potential magnetic[7], conducting properties[8] , as well as increased stability[9].
Metal complexing has been shown to occur between natural nucleobases. A well documented example is the formation of T-Hg-T, which involves two deportonated thymine nucleobases that are brought together by Hg2+ and forms a connected metal-base pair[10]. This motif does not accommodate stacked Hg2+ in a duplex due to an intrastrand hairpin formation process that is favored over duplex formation [11]. Two thymines across from each other in a duplex do not form a Watson-Crick base pair in a duplex; this is an example where a Watson-Crick basepair mismatch is stabilized by the formation of the metal-base pair. Another example of a metal complexing to natural nucleobases is the formation of A-Zn-T and G-Zn-C at high pH; Co+2 and Ni+2 also form these complexes. These are Watson-Crick base pairs where the divalent cation in coordinated to the nucleobases. The exact binding is debated[12].
A large variety of artificial nucleobases have been developed for use as metal base pairs. These modified nucleobases exhibit tunable electronic properties, sizes, and binding affinities that can be optimized for a specific metal. For, example a nucleoside modified with a pyridine-2,6-dicarboxylate has shown to bind tightly to Cu2+, whereas other divalent ions are only loosely bound. The tridentate character contributes to this selectivity. The fourth coordination site on the copper is saturated by an oppositely arranged pyridine nucleobase [13]. The asymmetric metal base pairing system is orthogonal to the Watson-Crick base pairs. Another example of an artificial nucleobase is that with hydroxypyridone nucleobases, which are able to bind Cu2+ inside the DNA duplex. Five consecutive copper-hydroxypyridone base pairs were incorporated into a double strand, which were flanked by only one natural nucleobase on both ends. EPR data showed that the distance between copper centers was estimated to be 3.7 ± 0.1 Å, while a natural B-type DNA duplex is only slighly larger (3.4 Å)[14]. The appeal for stacking metal ions inside a DNA duplex is the hope to obtain nanoscopic self-assembling metal wires, though this has not been realized yet.
See also
- Molecular biology
- Genetics
- Synthetic biology
- Oligonucleotide synthesis
- Expanded genetic code
- Nucleobase, nucleoside, nucleotide, and nucleic acid
- Biotin, fluorophore and dark quencher
- Ribozyme
References
- ^ Petersson B et al. Crystal structure of a partly self-complementary peptide nucleic acid (PNA) oligomer showing a duplex-triplex network. J Am Chem Soc. 2005 Feb 9;127(5):1424-30.
- ^ Johnson SC et al. A third base pair for the polymerase chain reaction: inserting isoC and isoG. Nucleic Acids Res. 2004 Mar 29;32(6):1937-41.
- ^ Kimoto M et al. Fluorescent probing for RNA molecules by an unnatural base-pair system. Nucleic Acids Res. 2007;35(16):5360-9.
- ^ Zimmermann N et al. A second-generation copper(II)-mediated metallo-DNA-base pair. Bioorg Chem. 2004 Feb;32(1):13-25.
- ^ Liu H et al. (ET Kool Lab)[1]. A four-base paired genetic helix with expanded size. Science. 2003 Oct 31;302(5646):868-71
- ^ S. D. Wettig, D. O. Wood.m J.S. Lee.J. Inorg. Biochem. 2003, 94, 94-99
- ^ H. Zhang, A. Calzolari, R. Di Felice. J. Phys. Chem. B 2005, 109, 15345-15348.
- ^ P. Aich, R. J. S. Skinner, S. D. Wettig, R. P. Steer, J. S. Lee.Biomol. Struct. Dyn. 2002, 20, 93-98.
- ^ G. H. Clever, K. Polborn, T. Carell, Angew. Chem. Int. Ed.2005, 117, 7370-7374
- ^ E. Buncel, C. Boone, H. Joly, R. Kumar, A. R. J. Norris, Inorg. Biochem.198525, 61-73
- ^ A. Ono, H. Togashi, Angew. Chem.2004, 43, 4300-4302
- ^ E. Meggers, P. L. Holland, W. B. Tolman, F. E. Romesberg, P. G. Schultz. J. Am. Chem. Soc.2000122, 10714-10715
- ^ J. S. Lee, R. J. S. Skinner, L. J. P. Latimer, R. S. Reid. Biochem. Cell Biol.199371, 162-168
- ^ K. Tanaka, A. Tengeiji, T. Kato, N. Toyama, M. Shionoya. Science2003, 299, 1212-1213
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