Spurious circular dichroism signals with intact cells of heliobacteria

Abstract

Strong anomalous circular dichroism (CD) signals were observed with suspensions of intact cells of the heliobacteria Heliobacillus mobilis and Heliobacterium chlorum. They were an order of magnitude larger than with isolated membranes at 6 K and changed dramatically with temperature. From the fact that suspensions of intact cells of heliobacteria showed linear dichroism (LD), it was concluded that the cells were oriented. The signals were proven to be artifacts, caused by coupling between LD and strain linear birefringence together with a smaller direct contribution of LD. The same effects were observed with oriented membrane fragments. The orientation of the cells was caused by gravity and could be eliminated by suspending the cells in an isopycnic solution. This way the real CD of intact cells could be measured. As expected, this spectrum was very similar to that of membrane fragments.