R-Phycoerythrin （R-PE） is isolated from red algae。 Its primary absorption peak is at 565 nm with secondary peaks at 496 and 545 nm。 The relative prominence of the secondary peaks varies significantly among R-PEs from different species。 R-PE has three types of subunits: ? （~20，000 daltons）， ? （~20，000 daltons） and ? （~30，000 daltons）。 The molecular weight of intact R-PE has been found to be about 240，000 daltons， and a subunit structure of （??）6 ? has been determined。 The ? subunit of R-PE contains only the phycoerythrobilin （PEB） chromophore， while ? and ? subunits contain both PEB and phycourobilin （PUB）。 Variability in the absorption spectra of R-PEs from various species reflects differences in the PEB/PUB ratio of the subunits。 R-PE and closely related B-PE are the most intensely fluorescent phycobiliproteins， with quantum efficiencies probably in excess of 90%， and its orange fluorescence is readily visible by eye in any moderately concentrated solution。 Our RPE-iFluor 750 Tandem is an excellent replacement to RPE-Cy7 and RPE-Alexa Fluor 750 Tandem due to their similar spectral properties。 In general， our RPE Tandems have less residual RPE fluorescence， and are more photostable （in some cases） than RPE-Cy7 and RPE-Alexa Fluor 750 Tandems。
