A red-green photochromic bacterial protein as a new contrast agent for improved photoacoustic imaging.

The GAF3 domain of the cyanobacteriochrome Slr1393 from . PCC6803, binding phycocyanobilin as a chromophore, shows photochromicity between two stable, green- and red-absorbing states, characterized by relatively high photoconversion yields. Using nanosecond-pulsed excitation by red or green light, respectively, and suitable cw photoconversion beams, we demonstrate that the light-modulatable photoacoustic waveforms arising from GAF3 can be easily distinguished from background signals originating from non-modulatable competitive absorbers and scattering media. It is demonstrated that this effect can be exploited to identify the position of the photochromic molecule by using as a phantom a cylindrical capillary tube filled with either a GAF3 solution or with an suspension overexpressing GAF3. These properties identify the high potential of GAF3 to be included in the palette of genetically encoded photochromic probes for photoacoustic imaging.