SPURIOUS CIRCULAR-DICHROISM SIGNALS WITH INTACT-CELLS OF HELIOBACTERIA

Strong anomalous circular dichroism (CD) signals were observed with su spensions of intact cells of the heliobacteria Heliobacillus mobilis a nd Heliobacterium chlorum. They were an order of magnitude larger than with isolated membranes at 6 K and changed dramatically with temperat ure. From the fact that suspensions of intact cells of heliobacteria s howed linear dichroism (LD), it was concluded that the cells were orie nted. The signals were proven to be artifacts, caused by coupling betw een LD and strain linear birefringence together with a smaller direct contribution of LD. The same effects were observed with oriented membr ane 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, t his spectrum was very similar to that of membrane fragments.