Literature Recipes For Calculating Extinction Coefficients and Other Physical Properties of Short Oligonucleotides and DNA.
Web Pages for Order Custom Oligonucleotides and Polynucleotides.

Compiled by Carlos E. Crespo-Hernández
Draft: 12/8/02

Preface

The following report is intended as a general guide on the calculation of various useful physical properties of the DNA biopolymers. Emphasis was given to the available methods for calculated the extinction coefficient (EC) and concentration (C) of oligonicleotides, and single and double stranded DNA at 260 nm. In addition, web pages to order custom-made oligonucleotides and to automatically determined the physical properties of oligonicleotides, such as ECs, MW, C, and melting temperature were included.

A. The Schepartz Lab Biopolymer Calculator

recommended by Science Magazine, 22 May, 1998

Taken from: http://paris.chem.yale.edu/extinct.html

Notes on use

The calculator has been revised as of 7/21/98. Note that I have substantially changed the code for calculations, and though I have tested most of the functions for accuracy, I can't promise that all the functions are working properly-if you notice something funny, please email me and let me know. You can compare with the old version at the OLD FORM. Thanks.

This form can be used to calculate the molecular weight, the extinction coefficient, the concentration, and the melting temperature of a single stranded nucleic acid.

New as of 21 July is the ability to enter degenerate nucleic acid sequences. Using the standard degenerate code (N=ACGT, S=CG, W=AT, M=AC, K=GT, R=AG, Y=CT, V=ACG, H=ACT, D=AGT, B=GCT, replacing T with U for RNA), enter your sequence normally, and click on the degenerate DNA check box. All the numbers calculated, including mass, extinction coefficient, concentrations, and melting are for an average of the possibilities (ie W means a 50/50 ratio of A and T).

Additionally, you can enter a protein sequence, and the form will calculate the molecular weight (using average isotopic mass), extinction coefficient, the concentration, and the formal charge. The calculator will accept X as an unknown amino acid, and will provide a mass based on the average of all 20 amino acids, not average statistical weighting in proteins. Charge and absorbance are not altered.

For both sets, either nucleotide or amino acid composition and percentages are calculated.

All numbers on the form are calculated independently, so if you are only interested in one set of numbers, the other default values will not affect your results.

The numbers on calculated by this page are believed to be accurate, but you should know that anything you find on the web is not guaranteed. We use them and believe them, but that's not to say there aren't errors. Please let me know if you find any.

The source code (written in perl) and html scripts for this program can be obtained free for academic users and at a modest price for companies. Contact rodgers@paris.chem.yale.edu for information.

References:

CRC Handbook of Biochemistry and Molecular Biology, 3rd ed.
G.P. Fasman, ed. Nucleic Acids, Volume I, p.581

Molecular Cloning, A Laboratory Manual, 2nd ed.
J. Sambrook, E.F. Fritsch, T. Maniatis, ed. Volume II, p.11.46

The Encyclopedia of Molecular Biology, 1st ed.
J. Kendrew, ed. Hybridization, p. 503-506

Also see references for melting temperatures or extinction coefficients, both at Genosys (http://www.sigma-genosys.co.uk/oligos/frameset.html).

B. Oligo Extinction Coefficient Calculator

Taken from: http://www.scripps.edu/mb/gottesfeld/ExtCoeff.html

This form can be used in two different ways. You can either enter your sequence (lower/upper case, spaces OK) or enter the number of each nucleotide (in addition to the absorbance, pathlength*, sample volume of stock measured, total volume in cuvette*, and total volume of your stock). If you choose to enter the number of each nucleotide, be sure that the sequence textbox is empty (the 'Clear Form' button will ensure this). Also, make sure to choose whether your oligo is DNA (the default) or RNA. Finally, the date does not appear upon loading, however it will show up once you click the 'Calculate' button.

*Defaults have been set for these values, but they can be changed.

^§Extinction coefficients for individual NMPs are based on:
A = 15.4 mM^-1 cm^-1
C = 7.4 mM^-1 cm^-1
G = 11.8 mM^-1 cm^-1
T = 9.6 mM^-1 cm^-1
U = 9.9 mM^-1 cm^-1

T_m calculation for DNA oligo is based on the following equation:

T_m = 81.5° + 0.41°(%GC) - 675°/length of oligo

C. Calculation procedure for extinction (absorption) coefficient of DNA

Taken from: http://www.owczarzy.net/emethod.htm

Method is based on paper Cantor C.R., Warshaw M.M., Shapiro, H, Biopolymers, 9, 1059-1077 (1970). The data are also shown at Handbook of Biochemistry and Molecular Biology, Volume 1: Nucleic Acids, Fasman, G.D. editor, page 589, 3rd edition, CRC Press, 1975. Extinction coefficient at 260 nm, 25 degrees of Celsius and neutral pH for the single-strand DNA is determined by the nearest-neighbor method. For example, the extinction coefficient of oligomer 5'-ATGCTTC-3' is

The errors are probably no more than 4%.

D. Extinction coefficients for common polynucleotides

Taken from: http://www1.amershambiosciences.com

References

1. Chamberlin, M. J., Fed. Proc. 24, 1446 (1965).
2. Tso, P. O. et al., Biochemistry 5, 4153 (1966).
3. Wells, R. D. et al., J. Mol. Biol. 54, 465 (1970).
4. Inman, R. B. et al., J. Mol. Biol. 8, 452 (1964).
5. Inman, R. B. et al., J. Mol. Biol. 5, 172 (1962).
6. Grant, R. C., JACS 90, 4474 (1968).
7. Amersham Biosciences, unpublished results.

E. Calculating oligonucleotide concentrations

The molar concentration of a oligonucleotide can be calculated based on the absorbance of the primer at 260 nm (A₂₆₀) and the molar extinction coefficient for the oligonucleotide at this wavelength. The molar extinction coefficient for the oligonucleotide can be calculated by knowing the sequence of the oligonucleotide and then summing the molar extinction values for the individual bases which comprise the oligonucleotide. The individual bases have the following molar extinction coefficients at 260 nm:

• A 1.0 molar solution of dT has a value of 8,400 absorbance units at 260 nm.
• A 1.0 molar solution of dA has a value of 15,200 absorbance units at 260 nm.
• A 1.0 molar solution of dG has a value of 12,010 absorbance units at 260 nm.
• A 1.0 molar solution of dC has a value of 7050 absorbance units at 260 nm.

For example, the oligonucleotide 5' TAGC 3' would have a molar extinction coefficient of 42,660 at 260 nm. Likewise, a 10 M solution of this oligonucleotide would give an absorbence of 0.427 at 260 nm.

Taken from: Henderson JT, Benight AS, Hanlon S, A semi-micromethod for the determination of the extinction coefficients of duplex and single stranded DNA. Analytical Biochemistry 1992 Feb 14;201 (1):17-29

F. Calculations Amounts Of Nucleic Acids

Taken from: http://omrf.ouhsc.edu/~frank/dnamole.html

O.D. Units:
The "absorbance" of a chemical is a product of its (concentration) x (optical path length) x (extinction coefficient, E). Nucleic acids have a peak absorbance in the ultraviolet range at about 260 nm. When the spectrophotometer has a path length of 1 cm, absorbance = "optical density" (O.D.), and O.D. = E x concentration. Extinction coefficients vary with the type of nucleic acid. Double stranded DNA (dsDNA) has an E = 20 g^-1cm^-1L). Depending on the reference you read, the E for single stranded DNA (ssDNA) is 20 or 30 g^-1cm^-1L), while E for RNA is 25 g^-1cm^-1L).

Extinction coefficients can be used to estimate the concentration of a sample dissolved in a known aqueous volume, or to calculate the number of grams or moles of nucleic acid according to the following formulas:

Determining Concentrations:
1 A260 O.D. Unit for dsDNA = 50 µg/ml
1 A260 O.D. Unit for ssDNA = 33 or 50 µg/ml
1 A260 O.D. Unit for RNA = 40 µg/ml

Examples:
A 1:50 dilution of dsDNA gives an A₂₆₀ = 0.063. [DNA] = 0.063x50/20 = 0.16 mg/ml. This is for a 1 cm path length. The path length of the 5 µl cuvette is only 0.5 mm = 1/20 cm. Therefore multiplication by 20 to give a 1 cm path length and then dividing by 20 for the extinction coefficient cancel each other. So for that cuvette, [DNA] = (A₂₆₀ - A₃₂₀) x dilution factor. The A₃₂₀ is used to subtract absorbance due to particles in suspension in this small chamber.

Determining Moles:
For ssDNA oligonucleotides, an estimate of the number of moles can be obtained by using an approximate mw of 350 Daltons/nucleotide. Accordingly, 1 O.D. Unit ssDNA = 33 x 10^-6/(350 x length) moles = 94.3/length nmoles

• An O.D. of 1.0 corresponds roughly to:
• 10-mer 10 nmoles
• 20-mer 5 nmoles
• 50-mer 2 nmoles
• 100-mer 1 nmoles
• Also, since 1 O.D. Unit = 33µg, then 1 µg = 2.86/length nmoles.
• A precise estimate of the molecular weight of dephosphorylated oligos can be calculated using the formula:
  
• mw = [(#A's x 312.2)+(#G's x 328.2)+(#C's x 288.2)+(#T's x 302.2) - 61.0]¹
  
• For phosphorylated oligos:
  
• mw = [(#A's x 312.2)+(#G's x 328.2)+(#C's x 288.2)+(#T's x 302.2) + 17.0]¹


• For long (at least 30bp) dsDNA
• approximate mw = 700 Daltons/base pair; mw of 1 kb is about 7x10⁵ Daltons (649 Daltons/bp is more accurate, but harder to remember)
• # µg in a pmole = length x 700 g/mole x 10^-12 moles/pmole x 10⁶µg/g
• = length x (7 x 10^-4) µg/pmole


• Multiply pmoles by 2 for pmoles 5' ends for dsDNA.
• ex: 1 kb DNA is about 0.7µg/pmole
• 1 µg of a 1 kb DNA = 1.52 pmol; 3.03 (pmol ends)
• 1 pmole of 1 kb DNA = 0.66 µg


• MW RNA: length x 350 (341 is more accurate)



• 1. Other Useful Conversions for Oligonucleotides
• Oligonucleotide length to molecular weight
• 350 x length = molecular weight
• Mass to µmoles
• µg / (350 x length) = µmoles
• (µg x 10⁶) / (350 x length) = pmoles
• pMoles to µg
• pmoles x length x 350 /10⁶ = µg
• µg to pmoles
• µg x 10⁶/(length x 350)



• MW of oligonucleotide	pMoles 3' ends per microgram of DNA
  10 x 106	0.05
  1 x 106	0.50
  0.1 x 106	5.0
  0.01x 106	50.0



• Melting Temperature Formulas (14-70 bases):
• Tm = 81.5°C + 16.6(log[Na]) + 0.41(fraction G+C) - 600/L
• where
• Na = [monovalent cations] usually 50-60 mM
• L = primer length
• fraction G+C = G+C content of primer
• Also, Tm (approx) = 4°C x(fraction G+C) + 2°C x(fraction A+T)


• Oligonucleotide Stock Concentrations for Amplifications:²
• mls to dissolve oligo = Total ODs (or A260) / (E x molar concentration needed)
• Count the numbers of each base in the oligo. Compute E
• = [#A's x (16,000)] + [#G's x (12,000)] + [#C's x (7,000)] + [#T's x (9,600)] + [#I's x (12,000)]


• Make your oligonucleotide solution more concentrated than needed so that when you add it to a working solution, the final concentration will be correct.
• Ex:   18-mer with 2(A) + 6(G) + 8(C) + 2(T).
• E = 2(16,000) + 6(12,000) + 8(7,000) + 2(9,600) = 179,200
• Total ODs in tube = 11
• Concentrations needed = 5x10^-4 = 0.5 mM
• mls needed to dissolve = 11/(179,600 x 5x10^-4) = 0.123 ml


• Citations:
• 1. Genosys Corp.
• 2. Oligo, Etc catalog, p.14

2. Spectrophotometric Quantitation of DNA or RNA

Spectrophotometric measurements of nucleic acid solutions are typically taken at wavelengths of 260 nm and 280 nm. The A₂₆₀ reading is used to determine the concentration of nucleic acid in solution. For a solution with an A₂₆₀ = 1.0, the following approximations hold:

1 A₂₆₀ unit of dsDNA = 50 µg/ml
1 A₂₆₀ unit of ssDNA = 37 µg/ml*
1 A₂₆₀ unit of ssRNA = 40 µg/ml

* For oligonucleotides, an A₂₆₀ of 1.0 represents anywhere from 20 to 33 µg/ml with the actual conversion factor dependent on the length and base sequence of the oligonucleotide (1).

The ratio between measurements at 260 nm and 280 nm provides an indication of the purity of a nucleic acid. In solution, pure DNA and RNA typically have A₂₆₀/A₂₈₀ ratios of 1.8 and 2.0, respectively. If the absorbance ratio is significantly less than the values above, the nucleic acid is probably contaminated with protein or phenol. Accurate quantification of a contaminated nucleic acid is not feasible without prior purification, and the efficacy of this can be established by the A₂₆₀ /A₂₈₀ ratio.

Most of the lyophilized polynucleotides are sold as A₂₆₀ units (2). For an approximation of quantity, use the conversion factors provided above to convert the A₂₆₀ units into micrograms ¾ which must be known if a certain concentration is desired.

¹ For a more accurate approximation, refer to Borer in the Handbook of Biochemistry and Molecular Biology, 3rd edition (G.D. Fasman, ed.) CRC Press, Cleveland, OH, page 589 (1975).

² Unit definition: One unit is that quantity of oligonucleotide or polynucleotide which has an absorbance of 1.0 at a given wavelength when dissolved in 1 ml of buffer and measured in a  1 cm cuvette at 20 °C. The wavelength at which the absorbance is measured is printed on the Certificate of Analysis which accompanies the product. For nucleic acids, typically an absorbance is taken at 260 nm in 20 mM sodium phosphate  (pH 7.0), 0.1 M NaCl.

G. Quantification by UV Absorbance

Introduction

Oligonucleotides are most accurately and conveniently quantified by measuring the UV absorbance at 260 nm of a sample with a spectrophotometer. According to the Beer-Lambert law: A = eCl

A = absorbance ( OD unit )

e = molar extinction coefficient ( M-1 cm-1 )

C = concentration ( M )

l = path length (cm), typically 1 cm

The conditions are defined at a specific wavelength, temperature and buffer, all of which influence "e."

Definition

One OD unit is the amount of oligonucleotide which when dissolved in 1 ml of water results in an absorbance of 1 when measured at 260 nm in a 1 cm path-length cuvette. For 1 OD, the actual concentration can range from 39 µg/ml for a homopolymer of C, to 20 µg/ml for a homopolymer of A.

Commonly, for practical experiments, 1 OD unit represents approximately 33 µg of single-stranded DNA or RNA with an equal mixture of the four bases.

Calculation of Concentration

According to the Beer-Lambert, the concentration of an oligo can be easily calculated once you know the absorbance of this oligo and its molar extinction coefficient.

The OD value is obtained thanks to a proprietary robot which handles and calculates the total OD of your oligo.

The purine and pyrimidine bases of DNA and RNA strongly absorb light with maxima near 260 nm. A useful approximation is e = 10,000 OD/mmole for each of the four bases. However the bicyclic purines, deoxyadenosine and deoxyguanosine, absorb more strongly (higher extinction coefficients) than the monocyclic pyrimidines, deoxycytidine and deoxythymidine.

The precise molar extinction coefficient of each base is listed in Table 1 (units: OD/mmole)

TABLE 1

Base stacking interactions in single strand oligos interfere, however, with the proper 'e'value of each base. The extinction coefficient value used in our calculation e (OD/mmole), is automatically obtained by the Proligo Primers & Probes SPARC workstation and is based on the exact nucleotide composition of each oligonucleotide.

For example, the extinction coefficient of ACGT is the sum of the pairwise extinction coefficients:

e (ACGT) = 2 x (e AC + e CG + e GT) - e C - e G

The pairwise values are listed in TABLE 2 (units: OD/mmole)

TABLE 2

The measured OD and calculated extinction coefficient are automatically converted into oligonucleotide concentration (µM) by the workstation, by using the Beer Lambert law.

To obtain a concentration in µg/µl, the workstation calculates the molecular weight of the oligo according to its sequence (cf Table 3 for the molecular weights of each base), by using the following formula:

Conc. µg/µl = conc. (M) x MW

TABLE 3

CONCLUSION

The following information is provided on the label of each oligonucleotide produced by Proligo Primers&Probes:

• Extinction coefficient value (OD/µmole)
• Concentration in µM
• Concentration in µg/µl

Commonly, for practical experiments, 1 OD unit represents approximately 33 µg of single-stranded DNA or RNA with an equal mixture of the four bases.

Go to Oligo Calculation now for immediate calculation of the MW, epsilon value, or conversion data between OD - nmoles - mg!

Taken from:

Proligo webpage

H. Custom Oligos - UV Quantitation of DNA

http://www.sigma-genosys.com/oligo_uvquant.asp

DNA absorbs ultraviolet light due to its highly conjugated nature. DNA may thus be easily quantitated in a UV spectrometer. Typically, 1 OD260 (i.e. a solution having an absorbance of one unit at 260 nm with a path length of 1 cm) is said to correspond to a concentration of 30-37 ug per ml. This is for single stranded DNA with an equal mixture of each of the four bases. RNA or double stranded DNA have values of 40 and 50 mg per ml, respectively. For work with DNA longer than probes and primers, these assumptions are valid because the constituent bases are usually fairly evenly represented. With oligos, however, the base composition may be highly skewed. For instance, 1 OD of a sequence of all cytidines would correspond to 39.4 ug/ml and 1 OD of a sequence of all adenosites would correspond to 22.8 ug/ml. Since the composition and sequence of the oligo are usually known, this information may be used to calculate individualized values for more accurate quantitation.

Values are calculated by either of two similar methods. The following equations is the simplest:

1.  ug                      1    

    -- = --------------------------------------------

    ml         (15400A+7400C+11500G+8700T)

             -------------------------------

              1000*(312A+288C+328G+303T-61)

A, C, G, and T are simply the number of each base in the oligo. This equation simply averages the extinction coefficients (in L/mol-Table 1) of each base and divides by the molecular weight (g/mol).

Table 1

The previous equation ignores the interactions of adjacent bases and a more accurate equation includes nearest neighbor interactions:  

2.   ug                           1    

     -- = --------------------------------------------------

     ml          2*Sum(Eab)-(E2+E3+...+E(n-2)+E(n-1))

                --------------------------------------

                   1000*(312A+288C+328G+303T-61)

Where sum(Eab) is the sum of the pairwise extinction coefficients (Table 2) and the E values are the normal single extinction coefficients for each individual base. (Please note that the first and last bases'extinction coefficients, E₁ and E₂, are not subtracted.) The pairwise values can be found in Table 2. Both of these equations give good results. The difference between them is rarely more than a few ug per mil.

Table 2.

Sigma-Genosys uses equation 2 to determine the extinction coefficient of your oligo.

Examples

For the sequence AGAG, equation (1) gives:  

Equation 1 Gives:

        ug                         1        

        -- = ------------------------------------------  

        ml               (15400*2+11500*2)  

                      ---------------------  

                      1000*(312*2+328*2-61)  

           =     22.7 

Equation 2 Gives:

        ug                        1        

        -- = ------------------------------------------  

        ml       2(12500+12600+12500)-(11500+15400)  

               --------------------------------------  

                       1000*(312*2+328*2-61)  

           =     25.4 

Conclusion

In our example, the results are similar to each other and quite different from the all-purpose value of 30 mg/ml. The degree of accuracy offered by the above equations may not be needed in normal circumstances. Where highly precise quantitation is required, they will yield excellent results.

Sigma-Genosys Guaranteed Yields

Please see the Sigma-Genosys Yields tech sheet for a listing of the yield guarantee for various oligos ordered from Sigma- Genosys.

I. http://www.idtdna.com/program/catalog/Custom_Gene_Synt hesis.asp

Custom Gene Synthesis

     *New Low Price*

Synthetic Double-Stranded DNA Cloned into a Plasmid and Sequence Verified

IDT’s Gene Synthesis Service Includes:

1. Design consultation with IDT scientists
2. Codon optimization
3. Gene assembly (including oligo synthesis, PAGE purification, subunit annealing, ligation, cloning, DNA sequencing and mutagenesis when necessary)

Specifics:

J. http://www.keydna.com/productsAndServices/microplateOligos.

BioSource International, Inc has expanded capabilities for manufacturing microplate-formatted oligos in its Camarillo, California facilities. Intensive efforts have been made to ensure the same high quality and rapid turnaround that our customers have enjoyed over the past 8 years. Highly trained, capable personnel are now carefully monitoring orders electronically generated through our website, or emailed to dna@biosource.com.

BioSource continues the tradition of offering a broad range of scales of synthesis (0.05-100 micromole), purifications (desalted, cartridge, HPLC and PAGE), and all commercially available modifications, reporters and quenchers. Microwell plate-formatted oligos can be either dried or resuspended to desired concentration and frozen.

Certificates accompany each plated-product and detail individual oligo molecular weights, total concentration, picomoles/OD, extinction coefficients, melting temperatures and location confirmation in the plate, in addition to all customer-supplied information.

Mass spec analysis is performed on random oligos to confirm quality and can be performed on entire plate as requested.

Please contact us with inquiries at 1-800-242-0607 or email your orders to plates@biosource.com

K. http://musom.marshall.edu/genomics/40new.htm

MU DNA Core Facility Price List

Custom DNA Synthesis Price List ---Updated December 12, 1999

* There will be a $50.00 setup charge for 10.0 µMole scale.

Shipping Rates:

Please contact Don Primerano for volume discounts and new customer promotions.

These prices are for the synthesis of oligonucleotides carried out using the four standard bases: dA, dC, dG, T, and Inosine. There are no set-up charges , hidden charges, or extra charges for mixed sites. There is no minimum volume order required. All oligos are fully deprotected, lyophilized, and ready for use. You will recieve a custom DNA Synthesis report for each oligo ordered. This report includes extinction coefficients, Tm, Td, base composition, and quantitation value of oligo (expresed in A₂₆₀ Units).

Others web pages for order oligonucleotides

L.

• Amersham Biosciences Inc. Product Info
• Gene Therapy Systems Inc. Product Info
• Lofstrand Labs Ltd.
• Monomer Sciences Inc.
• New England Biolabs Inc. Product Info
• QIAGEN Operon
• Roche Applied Science
• United States Biological Product Info
• USB Corp. Product Info

* All companies listed are featured in the BioSupplyNet database; those listed above have multiple links to the web from the BioSupplyNet directory.

• Annovis Inc. Product Info
• BioSource International Inc. Product Info
• Calbiochem-Novabiochem Corp.
• Cambio Ltd.
• Midland Certified Reagent Co. Product Info
• Sigma-Aldrich
• Sigma-Genosys
• Vangard International Inc. Product Info
• Zytex (India) Pvt. Ltd - Arun& Co.

M.

• BioNexus Inc./ABP Product Info
• Biosearch Technologies, Inc. Product Info
• bioWORLD Product Info
• Cell & Molecular Technologies Inc.
• Chemicon International Inc. Product Info
• DNAengine, Inc.
• Epoch Biosciences Product Info
• Gene Therapy Systems Inc. Product Info
• GenoMechanix, LLC
• Lofstrand Labs Ltd.
• Monomer Sciences Inc.
• Paragon BioServices Inc.
• Primm Labs Inc.
• QIAGEN Operon
• Rabbit & Rodent Diagnostic Associates
• Rabbitt & Rodent Diagnostics Associates
• United States Biological Product Info

* All companies listed are featured in the BioSupplyNet database; those listed above have multiple links to the web from the BioSupplyNet directory.

• AerBio Ltd.
• Ana-Gen Technologies Inc.
• Annovis Inc. Product Info
• Bartlett-Williams Scientific Resources
• Bio-Synthesis Inc.
• Bio/Can Scientific Inc.
• BioSource International Inc. Product Info
• CHIMERx Product Info
• Commonwealth Biotechnologies Inc.
• Dharmacon Research
• DNA International Inc.
• EMP Biotech GmbH
• Entelechon GmbH
• Gene Link Inc. Product Info
• GENSET OLIGOS
• Integrated DNA Technologies Inc.
• INTERACTIVA The Virtual Laboratory
• ISC BioExpress Product Info
• Life Sciences Inc.
• Macromolecular Resources
• Midland Certified Reagent Co. Product Info
• Molecula Research Laboratories
• Molecular Biology Resources
• MWG-Biotech
• Oligos Etc. Inc.
• Retrogen Inc.
• Sigma-Genosys
• TriLink BioTechnologies, Inc. Product Info
• Xeragon AG

O.

http://www1.a mershambiosciences.com/aptrix/upp01077.nsf/Content/Products?OpenDocument&parentid=41193

P.

http://www.genosys.com/oligos/frameset.html

Q.

http://www.idtdna.com/program/catalog/Custom_Gene_Synt hesis.asp

R.

http://www.operon.com/

S.

http://bpf.med.harvard.edu/Pages/techs/O/IDT/I DT_Catalog_July_2000.html
This page include modification with 2-aminopurine!

T.

http://proligo2.proligo.com/Ordering/order_frame.html