Alexa modification is a post synthesis conjugation to a primary amino group thus an additional modification with an amino group is required. A C6 or C12 amino group can be placed at the 5' or for the 3' end a C3 or C7 amino and for internal positions an amino modified base is used, e.g Amino dT C6.
Alexa Fluor NHS Esters is a set of fluorescent dyes that span the entire visible electromagnetics spectrum, as well as some of the near-IR. The absorbance range is 346-749 nm, and the emission range is 442-775 nm. Generally speaking, Alexa Fluor dyes are brighter, chemically more stable, and less pH-sensitive than other fluorescent dyes commonly used to label oligonucleotides (1). Because they currently only are in the form of NHS esters, oligos first must be synthesized with an Amino Linker modification (either at the ends or internally). The appropriate Alexa Fluor NHS ester is then manually attached to the oligo through the amino group in a separate reaction post-synthesis. The list of currently available dyes includes Alexa 350, -405, -430, -488, -500, -514, -532, -546, -555, -568, -594, -610, -633, -647, -660, -680, -700, -750, with the number indicating the appropriate absorbance wavelength for the particular dye. The molecular structure shown above is for Alexa 488, as an example; note that not all Alexa molecular structures are available. Alexa Fluors are suitable for a variety of in vitro and in vivo applications. However, for in vivo experiments, users should note that Alexa Fluor 350/405, being “blue” dyes, require higher-energy excitations than the other. Users of these particular dyes should confirm that the higher-energy required for excitation does not damage the relevant cells or tissues being used in the in vivo experiments.
Royalty charges are additional for Alexa dyes.>
1. Panchuk-Voloshina, N., Haugland, R.P., Bishop-Stewart, J., Bhalgat, M.K., Millard, P.J., Mao, F., Leung, W-Y., Haugland, R.P. Alexa Dyes, a Series of New Fluorescent Dyes that Yield Exceptionally Bright, Photostable Conjugates. J. Histochem. Cytochem. (1999), 47: 1179-1188.