Bacteriopheophytin (Bphe) is the first kinetically and spectrally resolved electron carrier in the electron transfer pathway1,2 of the bacterial photosynthetic reaction centre (RC), the only membrane protein whose structure has been determined to atomic resolution3–5. The two Bphes in the RC are not equivalent spectrally, and only the long-wavelength-absorbing Bphe, which is associated with the L subunit (BpheL ) 6, is active in electron transfer. There has been considerable speculation as to whether the spectroscopic red shift of BpheL, and its exclusive involvement in electron transfer can be attributed to its interaction with a glutamic acid residue (L104) found within hydrogen-bonding distance to this Bphe (for examples see refs 7–10). Here we report the results of changing this glutamic acid residue to leucine, glutamine and lysine in RCs of Rhodobacter capsulatus. Low-temperature absorption spectra of these genetically modified RCs indicate that the glutamic acid residue at L104 is largely responsible for the spectroscopic red shift of the photoactive Bphe. However, time-resolved optical measurements indicate that the GluL104 -> Leu RC maintains the same high quantum yield of charge separation observed in wild-type by using only the electron transfer pathway associated with the L subunit. These results indicate that the GluL104-BpheLinteraction is not the dominant contributor to the directionality of electron transfer in RCs.