Two-photon conversion of a bacterial phytochrome.

Area of Science:

• Optogenetics
• Biophysics
• Molecular Biology

Background:

• Photoreceptors mediate light-dependent biological responses and are key tools in optogenetics.
• Phytochromes, a class of photoreceptors, shift between red light (Pr) and far-red light (Pfr) states, extending optogenetics into the near-infrared spectrum.
• Current near-infrared optogenetics faces limitations due to poor light penetration through bone and skull.

Purpose of the Study:

• To investigate the activation of bacterial phytochromes using femtosecond lasers in the 1 μm wavelength range.
• To overcome the challenge of light penetration through dense tissues for optogenetic applications.
• To explore two-photon absorption as a mechanism for phytochrome activation.

Main Methods:

• Utilized quantum chemical calculations to predict two-photon absorption cross sections of bacterial phytochromes.
• Experimentally demonstrated phytochrome activation using femtosecond laser pulses at wavelengths from 1170 to 1450 nm.
• Measured the photoreversible Pr ↔ Pfr conversion efficiency across different wavelengths.

Main Results:

• Bacterial phytochromes exhibit significant two-photon absorption cross sections.
• Photoreversible Pr ↔ Pfr conversion was successfully driven by two-photon absorption between 1170 and 1450 nm.
• Optimal Pfr yield was observed between 1170 and 1280 nm, with a sharp decline beyond 1300 nm.

Conclusions:

• Two-photon activation of bacterial phytochromes provides a novel approach for optogenetics.
• This method enables unprecedented light penetration through bone, skull, and soft tissues.
• The findings lay the groundwork for enhanced spatial resolution in optogenetics and deep-tissue applications.