| Color† | Absorb (nm) |
Emit (nm) |
MM (g/mol) |
ε (cm−1M−1) |
Quantum Yield [1] | |
|---|---|---|---|---|---|---|
| Alexa Fluor 350 | blue | 346 | 442 | 410 | 19,000 | - |
| — 405 | violet | 401 | 421 | 1028 | 34,000 | - |
| — 430 | green | 434 | 541 | 702 | 16,000 | - |
| — 488 | cyan-green | 495 | 519 | 643 | 71,000 | 0.92 |
| — 500 | green | 502 | 525 | 700 | 71,000 | - |
| — 514 | green | 517 | 542 | 714 | 80,000 | - |
| — 532 | green | 532 | 554 | 721 | 81,000 | 0.61 |
| — 546 | yellow | 556 | 573 | 1079 | 104,000 | 0.79 |
| — 555 | yellow-green | 555 | 565 | ~1250 | 150,000 | 0.1 |
| — 568 | orange | 578 | 603 | 792 | 91,300 | 0.69 |
| — 594 | orange-red | 590 | 617 | 820 | 90,000 | 0.66 |
| — 610 | red | 612 | 628 | 1172 | 138,000 | - |
| — 633 | red | 632 | 647 | ~1200 | 100,000 | - |
| — 647 | red | 650 | 665 | ~1300 | 239,000 | 0.33 |
| — 660 | red | 663 | 690 | ~1100 | 132,000 | 0.37 |
| — 680 | red | 679 | 702 | ~1150 | 184,000 | 0.36 |
| — 700 | red | 702 | 723 | ~1400 | 192,000 | 0.25 |
| — 750 | red | 749 | 775 | ~1300 | 240,000 | 0.12 |
| † = approximate color of the emission spectrum ε = extinction coefficient |
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The Alexa Fluor family of fluorescent dyes is produced by Molecular Probes, a subsidiary of Invitrogen. Alexa Fluor dyes are typically used as cell and tissue labels in fluorescence microscopy and cell biology.[2]
The excitation and emission spectra of the Alexa Fluor series cover the visible spectrum and extend into the infrared.[3] The individual members of the family are numbered according roughly to their excitation maxima (in nm).
Alexa Fluor dyes are synthesized through sulfonation of coumarin, rhodamine, xanthene (such as fluorescein), and cyanine dyes. Sulfonation makes Alexa Fluor dyes negatively charged and hydrophilic. Alexa Fluor dyes are generally more stable, brighter, and less pH-sensitive than common dyes (e.g. fluorescein, rhodamine) of comparable excitation and emission,[4] and to some extent the newer cyanine series.[5] However, they are also more expensive. They are patented by Invitrogen (which acquired the company that developed the Alexa dyes, Molecular Probes).
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The Alexa Fluor dyes were named after Alex Haugland, son of the founders of Molecular Probes, Richard and Rosaria Haugland. The Marina Blue dye was named after their daughter, Marina.
While extinction coefficients are known (see the table above), quantum yields and life times are not. Comparisons with other dyes should be considered depending on the conditions (technique) used and performance (signal, background, stability) needed.
The Alexa series dyes are less pH-sensitive and more photostable than the original dyes (fluorescein, rhodamine, etc.) from which they were synthesized. Brightness comparisons are also generally favorable. Comparisons with other dyes are less consistent, and also even more delicate, depending on the conditions (technique) used. A third party has compared AlexaFluor647 with Cy5 (similar wavelength), conjugated to DNA.[6] This study found that Cy5 is brighter, but less photostable than Alexa 647. Other providers claim better brightness of photostability (i.e. AlexaFluor488 compared to Dylight488)[7] and Fluoprobes488.[8] These findings have been both confirmed and contested in scientific literature marking the difficulty in quantitatively comparing dyes. AlexaFluors remain excellent dyes in many cases, but empirical testing is needed for optimal results in each application.
Similar lines of fluorescent dyes provide an alternative to the AlexaFluor Dyes (see also the list in Category:Fluorescent dyes).
Labeled Nucleic acids as well as labeled proteins are key features for multiple biological applications. In most cases the amount of labeled sample is minimal and the labeling efficiency has to be quantified spectrophotometrically prior to the experiment. Specialized photometer, like the NanoPhotometer[9], offer the possibility to determine the DNA, RNA or protein concentration of a sample as well as the incorporation of Alexa Flour dyes with submicroliter volumes (starting with 0.3 µl). In addition, due to the reduction of the optical pathlength with the NanoPhotometer samples are diluted automatically in comparison to standard cuvette measurements. The respective virtual dilution factors are considered by the software of the instrument. Because the measurements are processed with undiluted samples, the reproducibility of the results is very high and if desired, samples can be retrieved after the measurement for further processing.