James Bond Molecule bag

I saw this cool bag over at io9.com  I had to get a larger image to actually see what the picture is.  It shows the gun barrel shot seen at the start of the James Bond movies, with a molecule of some sort.  Being a geek, I had to figure out just what the molecule is.

Over at eMolecules.com I was able to draw the structure in the editor and do a search.  The compound is called Sulforhodamine 101 acid chloride.  eMolecules provides a little info on the compound as well as links to chemical suppliers and listings in PubChem.

I just want to know what it's used for, so a quick search on Wikipedia turns up two listings:  one for Sulforhodamine 101 and another for Texas Red.  These are related compounds that are primarily used as biological dyes.  They are highly colored (red, in case you couldn't guess) because of the large amount of conjugation, and as it turns out they are also highly fluorescent. 

The difference between Texas Red (the acid chloride) and just plain sulforhodamine is the acid chloride has a chlorine in place of one of the OH groups.  This makes it very reactive with proteins especially - an NH2 on the protein acts as a nucleophile to replace the chlorine - leaving the protein with a bright red label attached to it.  Texas Red is especially convenient in that the acid chloride will also react with water, and the resulting compound is water soluble and can be rinsed away.  This means that only the dye molecules that are attached to the protein remain - excess dye can be washed away.  Other methods of labeling proteins aren't always so helpful - any excess dye may remain and obscure what you are interested in seeing.