Name: rapid colorimetric assays to qualitatively distinguish rna and dna in biomolecular samples
Uploaded: 2013-02-04T09:51:00
Description: Biochemical experimentation generally requires accurate knowledge, at an early stage, of the nucleic acid, protein, and other biomolecular components in ...

This work was funded by the University of Virginia and the Jeffress Memorial Trust (J-971). We thank L. Columbus, K. Jain, and P. Randolph for helpful discussions and critical reading of the manuscript.

1. Benedict's Assay for Reducing Sugars
<ol>
 <li>Prepare a suitable quantity of Benedict's reagent - 940 mM anhydrous sodium carbonate, 588 mM sodium citrate dehydrate, 68 mM copper (II) sulfate pentahydrate. This reagent can be stored at room temperature (RT) for at least six months with no noticeable change in reactivity.</li>
 <li>The above reagent is 6x. Thus, for 600 &mu;l reactions, add 100 &mu;l of Benedict's reagent to a clean 1.5 ml microcentrifuge tube (e.g., Eppendorf brand), per sample to be assay...

<ol>
	<li>De Mey, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparison+of+DNA+and+RNA+quantification+methods+suitable+for+parameter+estimation+in+metabolic+modeling+of+microorganisms.">Comparison of DNA and RNA quantification methods suitable for parameter estimation in metabolic modeling of microorganisms.</a> Anal. Biochem. 353, 198-203 (2006).</li><li>Desjardins, P., Conklin, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=NanoDrop+Microvolume+Quantitation+of+Nucleic+Acids.">NanoDrop Microvolume Quantitation of Nucleic Acids.</a> J. Vis. Exp. (45), e2565 (2010).</li><li>Ausubel, F. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Short Protocols in Molecular Biology: A Compendium of Methods from Current Protocols in Molecular Biology. , (1999).</li><li>Voet, D., Voet, J. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Biochemistry. , (2011).</li><li>Adams, R. L. P., Knowler, J. T., Leader, D. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> The Biochemistry of the Nucleic Acids. , (1986).</li><li>Rice, P. A., Correll, C. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Protein-nucleic acid interactions: Structural biology. , (2008).</li><li>Bowman, J. C., Lenz, T. K., Hud, N. V., Williams, L. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cations+in+charge:+Magnesium+ions+in+RNA+folding+and+catalysis.">Cations in charge: Magnesium ions in RNA folding and catalysis.</a> Curr. Opin. Struct. Biol. , (2012).</li><li>Balandina, A., Kamashev, D., Rouviere-Yaniv, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+bacterial+histone-like+protein+HU+specifically+recognizes+similar+structures+in+all+nucleic+acids.+DNA,+RNA,+and+their+hybrids.">The bacterial histone-like protein HU specifically recognizes similar structures in all nucleic acids. DNA, RNA, and their hybrids.</a> J. Biol. Chem. 277, 27622-27628 (1074).</li><li>Graslund, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Protein+production+and+purification.">Protein production and purification.</a> Nat. Methods. 5, 135-146 (2008).</li><li>Pagano, J. M., Clingman, C. C., Ryder, S. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantitative+approaches+to+monitor+protein-nucleic+acid+interactions+using+fluorescent+probes.">Quantitative approaches to monitor protein-nucleic acid interactions using fluorescent probes.</a> RNA. 17, 14-20 (2011).</li><li>Benedict, S. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+reagent+for+the+detection+of+reducing+sugars.">A reagent for the detection of reducing sugars.</a> J. Biol. Chem. 277, e5 (1908).</li><li>Endo, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+simultaneous+estimation+method+of+DNA+and+RNA+by+the+orcinol+reaction+and+a+study+on+the+reaction+mechanism.">A simultaneous estimation method of DNA and RNA by the orcinol reaction and a study on the reaction mechanism.</a> J. Biochem. 67, 629-633 (1970).</li><li>Almog, R., Shirey, T. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+modified+orcinol+test+for+the+specific+determination+of+RNA.">A modified orcinol test for the specific determination of RNA.</a> Anal. Biochem. 91, 130-137 (1978).</li><li>Dische, Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=New+color+reactions+for+determination+of+sugars+in+polysaccharides.">New color reactions for determination of sugars in polysaccharides.</a> Methods Biochem. Anal. 2, 313-358 (1955).</li><li>Burton, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+study+of+the+conditions+and+mechanism+of+the+diphenylamine+reaction+for+the+colorimetric+estimation+of+deoxyribonucleic+acid.">A study of the conditions and mechanism of the diphenylamine reaction for the colorimetric estimation of deoxyribonucleic acid.</a> Biochemical Journal. 62, 315-323 (1956).</li><li>Vogel, J., Luisi, B. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hfq+and+its+constellation+of+RNA.">Hfq and its constellation of RNA.</a> Nat. Rev. Microbiol. 9, 578-589 (2011).</li><li>Deckert, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Protein+composition+and+electron+microscopy+structure+of+affinity-purified+human+spliceosomal+B+complexes+isolated+under+physiological+conditions.">Protein composition and electron microscopy structure of affinity-purified human spliceosomal B complexes isolated under physiological conditions.</a> Mol. Cell Biol. 26, 5528-5543 (2006).</li><li>Stevens, S. W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Composition+and+functional+characterization+of+the+yeast+spliceosomal+penta-snRNP.">Composition and functional characterization of the yeast spliceosomal penta-snRNP.</a> Mol. Cell. 9, 31-44 (2002).</li><li>Sapan, C. V., Lundblad, R. L., Price, N. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Colorimetric+protein+assay+techniques.">Colorimetric protein assay techniques.</a> Biotechnol. Appl. Biochem. 29 (Pt. 2), 99-108 (1999).</li><li>Chomczynski, P., Sacchi, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+single-step+method+of+RNA+isolation+by+acid+guanidinium+thiocyanate-phenol-chloroform+extraction:+twenty-something+years+on.">The single-step method of RNA isolation by acid guanidinium thiocyanate-phenol-chloroform extraction: twenty-something years on.</a> Nat. Protoc. 1, 581-585 (2006).</li></ol>

The colorimetric assays presented here offer a simple approach to rapidly assess the chemical nature of biomolecular mixtures, such as are encountered when purifying proteins, RNAs or complexes from whole-cell lysate in preparation for further studies. As structural biology pursues more native-like assemblies, progressively greater challenges, such as sample heterogeneity, will be posed by the intricate and multi-component complexes. Supramolecular assemblies are often only marginally stable, and their successful isolati...

Much of cell biology occurs via molecular interactions involving DNA and RNA.4 These naturally occurring nucleic acids (NAs) interact with one another,5 with proteins,6 and with a host of small-molecule compounds and ligands in vivo (e.g., divalent cations7). The interactions may be short- or long-lived (kinetically), may range from high to moderate to low affinity (thermodynamic strength), and can also exhibit substantial variation in chemical properties and specificity - some associations are quite specific (e.g., DNA&middot;&middot;&middot;transcription factors, RNA&middot;&middot;&middot;splicing factors), while other interactions are necessarily far more generic (e.g., DNA&middot;&middot;&middot;bacterial histone-like HU proteins8). Non-specific interactions with NAs can have practical consequences for in vitro experiments involving mixtures of biomolecules, as it is possible, and even likely, that some NAs will associate with the biomolecules of interest, at least under some subset of the experimental conditions being used (ionic strength, solution pH, etc).
Consider, for instance, production of a protein of interest (POI) via heterologous over-expression of the recombinant protein in Escherichia coli cell culture; such a procedure is routinely performed in virtually any structural biology lab.9 In preparing for further experiments, such as biochemical/biophysical characterization, crystallization, etc., initial efforts generally focus on obtaining a sufficient quantity of the POI in as pure a form as possible, ideally as a chemically homogeneous and biophysically monodisperse specimen. After disruption of host cells, the early stages of a typical purification workflow aim to isolate the POI from E. coli proteins, nucleic acids, cell wall debris, and other components of the cellular lysate. However, host NAs may co-purify with the POI for several physicochemical reasons - a highly basic POI may non-specifically pull-down host DNA/RNA; the POI may have a generic NA-binding activity (e.g., the aforementioned HU); the POI may be a fairly specific NA-binding protein but exhibit cross-reactivity with host RNAs or DNAs; host NAs may interact with a chromatography matrix and thereby simply co-elute with the POI; and so on. Indiscriminate, high-affinity binding of host NAs to a POI can pose a vexing problem because the NA impurities will likely interfere with downstream experiments (e.g., fluorescence anisotropy assays of POI&bull;RNA binding10). Alternatively, unanticipated POI&middot;&middot;&middot;NA associations also can be viewed fortuitously, as such interactions illuminate the POI's nucleic acid-binding capacity. Either way, whether NAs are key components or contaminants, one must first quantify and identify the type (DNA, RNA) of co-purifying NAs in preparation for downstream experiments.
Several analytical methods exist for detecting and quantitating NAs in a sample. Most of the available methods are fundamentally either spectrophotometric (e.g., A260 absorbance values and A260/A280 ratios), fluorometric (e.g., binding of thiazole orange or other fluorescent dyes to NA), or colorimetric (susceptibility of nucleosides to chemical reactions that yield chromophores absorbing in the uv-vis region of the electromagnetic spectrum), as recently described by De Mey et al.1 However, the crucial step of identifying the type of polynucleotide as RNA or DNA is beyond the scope of many of these quantitation approaches. Here we provide a set of colorimetric assays to rapidly identify the types of NA components in a proteinaceous sample.
The protocols described here can be efficiently executed without additional steps of isolating the potential NA impurities, and rely on Benedict's assay for reducing sugars11, the orcinol assay for pentoses12,13, and diphenylamine reactions14,15 of 2'-deoxypentoses (Figures 1 and 2). The Benedict's test (Figure 2a) utilizes the ability of the linear, open-chain (aldehyde) form of an aldose sugar to reduce Cu2+, with concomitant oxidation of the sugar's carbonyl to a carboxylate moiety and production of Cu2O as an insoluble red precipitate. This reaction will test positive with free reducing sugars such as aldoses and ketoses (which convert to the corresponding aldoses via enediol intermediates), but not with pentose sugars that are locked into cyclic form as part of the covalent backbone of a DNA or RNA polynucleotide. Because of the minimalistic requirement of a free hemiacetal functionality, other compounds that could test positive in this assay - and therefore act as potential interferents - include &alpha;-hydroxy-ketones and short oligosaccharides (e.g. the disaccharide maltose). Both the Bial's orcinol (Figure 2b) and Dische's diphenylamine (Figure 2c) reactions are based on initial destruction of the polynucleotide backbone, via depurination of the nucleoside and further acid- or base-catalyzed hydrolysis of the parent nucleotides, to yield furan-2-carbaldehyde (furfural) derivatives; these derivatives then react with either a polyhydroxy phenol such as orcinol (Bial's) or diphenylamine (Dische's) reagents to form colored condensation products of largely unknown chemical structure. The DNA versus RNA specificity of the Dische's assay stems from the fact that the pentose sugar must be 2'-deoxygenated in order to be susceptible to oxidation to &omega;-hydroxylevulinyl aldehyde, which further reacts with diphenylamine under acidic conditions to yield a bright blue condensate (Figure 2c). Using the streamlined protocols described here, we have found that these sugar-specific colorimetric reactions can differentiate between RNA and DNA, and will also indicate the presence of free reducing sugars such as glucose, fructose, or ribose in a biomolecular sample.

<table border="1">
 <tbody>
 <tr>
 <td>Reagent or equipment</td>
 <td>Supplier/company</td>
 <td>Catalog number</td>
 <td>Comments, notes</td>
 </tr>
 <tr>
 <td>Anhydrous sodium carbonate</td>
 <td>Fisher Scientific</td>
 <td>S263</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Sodium citrate dihydrate</td>
 <td>Sigma</td>
 <td>S-4641</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Copper (II) sulfate pentahydrate</td>
 <td>VWR</td>
 <td>VW3312-2</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Orcinol monohydrate</td>
 <td>Sigma-Aldrich</td>
 <td>O1875</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Concentrated HCl</td>
 <td>VWR</td>
 <td>BDH3030</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Ferric chloride hexahydrate</td>
 <td>Sigma</td>
 <td>F-2877</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Diphenylamine</td>
 <td>Aldrich</td>
 <td>112763</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Glacial acetic acid</td>
 <td>Fisher Scientific</td>
 <td>A28</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Sulfuric acid</td>
 <td>Sigma-Aldrich</td>
 <td>258105</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Ethanol</td>
 <td>Koptec</td>
 <td>V1101</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Ribose</td>
 <td>Sigma</td>
 <td>R-7500</td>
 <td>prep at 1% w/v in H2O</td>
 </tr>
 <tr>
 <td>Ribonucleic acid from baker's yeast (S. cerevisiae)</td>
 <td>Sigma</td>
 <td>R6750</td>
 <td>prep at 10 mg/ml in H2O; store at -20 &deg;C</td>
 </tr>
 <tr>
 <td>Deoxyribonucleic acid (sodium salt), from calf thymus</td>
 <td>Sigma</td>
 <td>D1501</td>
 <td>prep at 10 mg/ml in H2O; store at 4 &deg;C</td>
 </tr>
 <tr>
 <td>&nbsp;</td>
 <td>&nbsp;</td>
 <td>&nbsp;</td>
 <td>
 Reagents, Equipment &amp; Safety
 Materials are listed in the following table in the order in which they appear in the Protocol section. Unless otherwise noted (above), all reagents can be stored at ambient room temperature and lighting. For any items not listed below (e.g., microcentrifuge tubes), the usual make / model / variety found in a standard biochemical laboratory can be used (e.g., Eppendorf brand 1.5 ml microfuge tubes). Standard plastic microfuge tubes should be used for steps involving centrifugation (e.g., to sediment particulate material near the end of each protocol). No particular material is preferable, as long as the tubes can be sealed; the typical polypropylene tubes found in biochemistry laboratories work well. In terms of safety concerns and waste disposal, standard laboratory precautions (safety glasses, fume hoods) should be exercised in pre-paring, working with, and disposing of solutions containing concentrated acetic, hydrochloric, or sulfuric acids. For organic reagents such as orcinol or diphenylamine, nitrile gloves are preferable to the common latex (natural rubber) variety.</td>
 </tr>
 </tbody>
</table>

rapid colorimetric assays to qualitatively distinguish rna and dna in biomolecular samples

Biochemical experimentation generally requires accurate knowledge, at an early stage, of the nucleic acid, protein, and other biomolecular components in potentially heterogeneous specimens. Nucleic acids can be detected via several established approaches, including analytical methods that are spectrophotometric (e.g., A260), fluorometric (e.g., binding of fluorescent dyes), or colorimetric (nucleoside-specific chromogenic chemical reactions).1 Though it cannot readily distinguish RNA from DNA, the A260/A280 ratio is commonly employed, as it offers a simple and rapid2 assessment of the relative content of nucleic acid, which absorbs predominantly near 260 nm and protein, which absorbs primarily near 280 nm. Ratios &lt; 0.8 are taken as indicative of 'pure' protein specimens, while pure nucleic acid (NA) is characterized by ratios &gt; 1.53.
However, there are scenarios in which the protein/NA content cannot be as clearly or reliably inferred from simple uv-vis spectrophotometric measurements. For instance, (i) samples may contain one or more proteins which are relatively devoid of the aromatic amino acids responsible for absorption at &asymp;280 nm (Trp, Tyr, Phe), as is the case with some small RNA-binding proteins, and (ii) samples can exhibit intermediate A260/A280 ratios (~0.8 &lt; ~1.5), where the protein/NA content is far less clear and may even reflect some high-affinity association between the protein and NA components. For such scenarios, we describe herein a suite of colorimetric assays to rapidly distinguish RNA, DNA, and reducing sugars in a potentially mixed sample of biomolecules. The methods rely on the differential sensitivity of pentoses and other carbohydrates to Benedict's, Bial's (orcinol), and Dische's (diphenylamine) reagents; the streamlined protocols can be completed in a matter of minutes, without any additional steps of having to isolate the components. The assays can be performed in parallel to differentiate between RNA and DNA, as well as indicate the presence of free reducing sugars such as glucose, fructose, and ribose (Figure 1).

Results are shown in Figure 3 for application of these colorimetric assays to known reference compounds. Representative qualitative data are shown for the Benedict's (a), Bial's orcinol (b), and Dische's diphenylamine (c) assays, and standard curves for these three assays are shown in Figure 4. In panels 3(a-c), the left panels show positive/negative control experiments using suitably reactive/unreactive analytes; these visual results are shown in situ, in the cuvettes as descri...

A suite of colorimetric assays is described for rapidly distinguishing protein, RNA, DNA, and reducing sugars in potentially heterogeneous biomolecular samples.

Rapid Colorimetric Assays to Qualitatively Distinguish RNA and DNA in Biomolecular Samples

Biochemical experimentation generally requires accurate knowledge, at an early stage, of the nucleic acid, protein, and other biomolecular components in ...

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