Although sulforhodamine 101 acid chloride (also called Texas Red^®) is the most popular labeling reagent of sulfonyl chlorides, it is quite unstable in water, especially at the higher pH required for reactions with aliphatic amines. Texas Red^® reacts with both aliphatic amines and aromatic amines indiscriminately. In addition, the labeling efficiency of Texas Red^® is extremely low compared to dye succinimidyl esters. California Red™ SE is a succinimidyl ester. It is an excellent replacement for Texas Red^®. California Red™ reacts with amine compounds, such as amino acids, peptides and proteins, to give bright red fluorescent conjugates that are extremely stable. Compared to Texas Red^®, California Red™ has much higher labeling efficiency, and more importantly, the resulted conjugates are more fluorescent than the corresponding Texas Red^® conjugates for long peptides and oligonucleotides. The conjugates of California Red™ have the identical excitation and emission wavelengths to those of Texas Red^®. Our in-house studies indicated that California Red™ is more stable than Texas Red^® under the same labeling conditions.

SunRed™ has even better water solubility than Texas Red^®, Texas Red^®-X and California Red™. It is extremely useful for labeling hydrophobic peptides and oligonucleotides that are often poorly labeled by Texas Red^® or Texas Red^®-X. The conjugates of hydrophobic peptides and oligonucleotides with Texas Red^® are difficult to use for measuring biological activities due to their poor water solubility.