AbstractThe present study describes the construction of aRhodobacter sphaeroideslight‐harvesting (LH2) mutant in which the charged residueβSLys23 is changed by site‐directed mutagenesis to a Gin residue, and the characterization of the resulting mutant complex by a range of spectroscopic techniques. In the 77 K absorption spectrum of the mutant, the peak equivalent to the 850 nm peak in the wild‐type membrane is blue‐shifted by approximately 18 nm to 837 nm; except for this blue‐shift, the 77 K. fluorescence excitation and emission spectra and the circular dichroism spectrum of the mutant are very similar to the equivalent spectra from the wild‐type membranes, suggesting that the mutation βLys23 → Gin probably does not cause any major changes in the conformation or aggregation state of these membranes. Possible causes of the 18 nm blue‐shift in the absorption spect