Structural Assessment of the Bacteriochlorophyll d Stacking in Chlorosomes from a C. tepidum Mutant with MAS NMR Spectroscopy
Magic angle spinning (MAS) solidstate NMR spectroscopy was used to investigate the stacking of bacteriochlorophylls (BChl) in the bchQRU mutant of the green sulfur bacterium C. tepidum. This mutant produces [8-Et, 12-Me] BChl d instead of the BChl c in the wild type. Using uniformly 13C enriched bchQRU chlorosomes, a 13C and 1H resonance assignment of the BChl d ring was made using two-dimensional 13C−13C homonuclear and 1H−13C heteronuclear MAS NMR dipolar correlation experiments. The aggregation shifts are largest for the 21−H3, 121−H3, 31−H, and 5-H, which are shifted upfield by −3.3, −2.6, −3.7, and −2.0 ppm, respectively. A comparison of the bchQRU chlorosomes with aggregation shifts for the wild type and chlorin models forming dense aggregates reveals parallel stacking of the [8-Et, 12-Me]BChl d, which is more dense and much more homogeneously ordered than for the BChl c in the wild type. However, the structure is less dense than for the Cd-chlorin models which lack the 31-Me.
KeywordsChlorosomes bchQRU mutant BChl d MAS NMR spectroscopy
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