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The Alexa®, or Alexa Fluor®, dyes were introduced by Molecular Probes, Inc. (now part of Life Technologies, Inc.).
A range of Alexa dyes is available, with emission wavelengths spanning the visible spectrum and extending into the infrared region of the spectrum. Some can be used in single molecule studies (Table 1).
Table 1 ⎪ Alexa Fluor dyes
| Name | λmax / nm (absorption) | λmax / nm (emission) | Colour | E at λmax | Φ | τ / ns |
|---|---|---|---|---|---|---|
| Alexa Fluor 350 | 346 | 442 | Blue | 19 000 | - | - |
| Alexa Fluor 405 | 402 | 421 | Blue | 35 000 | - | - |
| Alexa Fluor 430 | 434 | 539 | Green/Yellow | 15 000 | - | - |
| Alexa Fluor 488 | 495 | 519 | Green | 73 000 | 0.92 | 4.1 |
| Alexa Fluor 514 | 518 | 540 | Green | 80 000 | - | - |
| Alexa Fluor 532 | 531 | 554 | Yellow | 81 000 | 0.61 | 2.5 |
| Alexa Fluor 546 | 556 | 573 | Orange | 112 000 | 0.79 | 4.1 |
| Alexa Fluor 555 | 555 | 565 | Orange | 155 000 | 0.10 | 0.3 |
| Alexa Fluor 568 | 578 | 603 | Orange/Red | 88 000 | 0.69 | 3.6 |
| Alexa Fluor 594 | 590 | 617 | Red | 92 000 | 0.66 | 3.6 |
| Alexa Fluor 610 | 612 | 628 | Red | 144 000 | - | - |
| Alexa Fluor 633 | 632 | 647 | Far-red | 159 000 | - | - |
| Alexa Fluor 635 | 633 | 647 | Far-red | 140 000 | - | - |
| Alexa Fluor 647 | 650 | 668 | Far-red | 270 000 | 0.33 | 1.0 |
| Alexa Fluor 660 | 663 | 690 | Near-IR | 132 000 | 0.37 | 1.2 |
| Alexa Fluor 680 | 679 | 702 | Near-IR | 183 000 | 0.36 | 1.2 |
| Alexa Fluor 700 | 702 | 723 | Near-IR | 205 000 | 0.25 | 1.0 |
| Alexa Fluor 750 | 749 | 775 | Near-IR | 290 000 | 0.12 | 0.7 |
| Alexa Fluor 790 | 782 | 805 | Near-IR | 260 000 | - | - |
Not all of the structures of the Alexa dyes are available, but some of the structures are listed in Figure 1.
Figure 1 | Structures of selected Alexa Fluor® dyes (structures of other Alexa Fluor dyes are not published)
The Alexa Fluor dyes, and other dyes that emit at longer wavelengths, towards the red end of the spectrum (> 600 nm) have advantages over dyes that emit at shorter wavelengths in that there is typically less background fluorescence at longer these wavelengths. In experiments that involve cells/cell debris, other chemicals (e.g. buffers) and plastics (e.g. microplates) this can lead to increased sensitivity.
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