The spectroscopic properties of the light-harvesting complex of Rhodospirillum rubrum, B873, and a detergent-isolated subunit form, B820, are presented. Absorption and circular dichroism spectra suggest excitonically interacting bacteriochlorophyll alpha (BChl alpha) molecules give B820 its unique spectroscopic properties. Resonance Raman results indicate that BCHl alpha is 5-coordinate in both B820 and B873 but that the interactions with the BChl C2 acetyl in B820 and B873 are different. The reactivity of BChl alpha in B820 in light and oxygen, or NaBH4, suggests that it is exposed to detergent and the aqueous environment. Excited-state lifetimes of the completely dissociated 777-nm-absorbing form [1.98 ns in 4.5% octyl glucoside (OG)], the intermediate subunit B820 (0.72 ns in 0.8% OG), and the in vivo like reassociated B873 (0.39 ns in 0.3% OG) were measured by single-photon counting. The fluorescence decays were exponential when emission was detected at wavelengths longer than 864 nm. An in vivo like B873 complex, as judged by its spectroscopic properties, can be formed from B820 without the presence of a reaction center.