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 QM y absorption band were observed in seven mutants with single or double changes at Leu α44, Trp α43, and Trp β48. Resonance Raman spectra were consistent with the loss of a hydrogen bond with the alteration of either Trp α43 or Trp β48 to Phe. However, when the Trp α43 to Phe alteration is combined with Leu α44 to Tyr, the spectra show that the loss of the hydrogen bond to α43 is compensated by the addition of a new hydrogen bond to Tyr α44. 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.
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