The influence of hydrogen bonds on the electronic structure of light-harvesting complexes from photosynthetic bacteria.

The influence of hydrogen bonds on the electronic structure of the light-harvesting I complex from Rhodobacter sphaeroides has been examined by site-directed mutagenesis, steady-state optical spectroscopy, and Fourier-transform resonance Raman spectroscopy. Shifts of 4-23 nm in the Q(y) absorption band were observed in seven mutants with single or double changes at Leu alpha44, Trp alpha43, and Trp beta48. Resonance Raman spectra were consistent with the loss of a hydrogen bond with the alteration of either Trp alpha43 or Trp beta48 to Phe. However, when the Trp alpha43 to Phe alteration is combined with Leu alpha44 to Tyr, the spectra show that the loss of the hydrogen bond to alpha43 is compensated by the addition of a new hydrogen bond to Tyr alpha44. Comparison of the absorption and vibrational spectra of the seven mutants suggests that changes in the absorption spectra can be interpreted as being due to both structural and hydrogen-bonding changes. To model these changes, the structural and hydrogen bond changes are considered to be independent of each other. The calculated shifts agree within 1 nm of the observed values. Excellent agreement is also found assuming that the structural changes arise from rotations of the C3-acetyl group conformation and hydrogen bonding. These results provide the basis for a simple model that describes the effect of hydrogen bonds on the electronic structures of the wild-type and mutant light-harvesting I complexes and also is applicable for the light-harvesting II and light-harvesting III complexes. Other possible effects of the mutations, such as changes in the disorder of the environment of the bacteriochlorophylls, are discussed.