MAGNETIC CIRCULAR DICHROISM OF BACTERIOCHLOROPHYLL a IN SOLUTION AND IN A PROTEIN

Abstract— The magnetic circular dichroism (MCD) (300–850nm) of the bacteriochlorophyll (Bchl) a‐protein from the green photosynthetic bacterium Prosthecochloris aestuarii 2 K is qualitatively similar to the MCD of Bchl a in methanol and ether solution. This result implies that the transition dipole of the lowest energy electronic transition (near 800 nm) is roughly perpendicular to the transition dipole of the next higher electronic band (near 600nm) for Bchl a molecules in the protein just as it is for molecules in solution. This result provides no support for the recent proposal that interactions with the protein rotates the direction of the transition dipoles of the 800nm band of all the Bchl a molecules in the protein by 90°. While a rotation of the 800nm transition dipoles cannot be rigorously excluded, it would be necessary for the postulated perturbation to rotate the transition dipoles of both the 800 and 600nm bands by 90°. In a broader sense, any postulated perturbation would have to be shown to leave both the absorption spectrum and the MCD largely unaffected. MCD is a more sensitive test than absorption spectroscopy for perturbations of electronic states and changes in the relative orientation of transitions, because it depends on both the magnitudes and directions of at least two electric and one magnetic transition dipole.