Bioinspired light-driven chloride pump with helical porphyrin channels

The study addresses the challenge of creating an artificial ion channel that mimics the biological light-driven chloride pump function of halorhodopsin (HR). HR employs a chloride-binding chromophore (protonated Schiff base) to absorb light and drive selective, directional Cl− transport across membranes. Despite many artificial light-responsive nanochannels (e.g., TiO2, porphyrin, spiropyran, graphitic carbon nitride, azobenzene), none have achieved specific chloride selectivity and active light-driven Cl− pumping analogous to HR. The key requirements identified are: (1) a photo-receptor to harvest light and generate a driving force, and (2) a specific chloride-selective transport site. Inspired by HR’s Schiff base environment, the authors hypothesize that porphyrin units, with nitrogen-containing conjugated structures, can provide both light responsiveness and a selective interaction site for chloride. The purpose is to align porphyrins into helical channels within a block copolymer matrix to realize a bioinspired light-driven Cl− pump capable of selective transport and up-gradient pumping, thereby advancing bioinspired ion channel systems and optogenetic/energy-conversion applications.

Background highlights include: HR from Halobacterium salinarum is a light-gated chloride channel enabling directional Cl− transport via a protonated Schiff base chromophore. Prior artificial light-responsive channels based on TiO2, porphyrins, spiropyran, graphitic carbon nitride, and azobenzene can undergo photoinduced charge separation or isomerization but lack specific chloride selectivity akin to HR. Structural insights into HR show aggregated Schiff base groups forming selective Cl− transport sites and light-triggered activity. Work on osmotic energy conversion and responsive nanochannels underscores the importance of selective ion transport and channel architecture. The authors build on these insights, leveraging porphyrin’s nitrogenous conjugation (analogous to Schiff base) to introduce both photoreactivity and Cl− affinity, aiming to fill the gap of specific light-driven Cl− pumping in artificial systems.

• Materials and channel fabrication: A porphyrin-cored star block copolymer (p-BCP) was synthesized (details in Supplementary Information). Each polymer chain contains a single porphyrin unit (<1 wt% total). Under synergistic BCP self-assembly and porphyrin π–π stacking, porphyrins aggregate into vertically aligned, high-density helical nanochannels. To repair stacking defects arising from polymer steric effects and chain entanglement, free porphyrin (tetraphenylporphyrin, TPP) was doped at controlled ratios, yielding p-BCP@nTPP membranes (n = number of TPP molecules per p-BCP; n = 0, 2, 4, 8). - Structural characterization: TEM revealed nanocylinder morphology; GI-SAXS confirmed vertically aligned nanocylinders with periodic length ~70.6 nm; cross-sectional SEM showed transmembrane nanocylinders. WAXD indicated porphyrin π–π stacking; with TPP doping, porphyrin d-spacing and FWHM decreased (defect repair), minimal at n=4; at n=8, extra diffraction at 2θ=26.8° indicated TPP aggregation and slight d-spacing increase. Circular dichroism showed positive Cotton effect at 417 nm; UV–vis (Q-band absorption at ~728 nm) and fluorescence (~717 nm) confirmed J-aggregation and helical channel formation. XPS supported minor TPP aggregation at high doping. - Electrochemical measurements: Ionic conductance and selectivity were assessed by I–V curves under symmetric and asymmetric KCl electrolytes using a Keithley 6487. For anion vs cation selectivity, asymmetric gradients up to 50-fold (e.g., 0.5 M/0.01 M) were used; a salt bridge was employed to subtract redox potential/current, and open-circuit diffusion potential (Ediff) was analyzed via the Goldman–Hodgkin–Katz equation to extract selectivity S = DCl/DK. Conductance among different anions (fixed K+ cation) compared KCl, KBr, KF, KHCO3, KH2PO4. - Photoresponse and energy conversion: Light-driven transport was evaluated under varied light intensities (≈50–100.8 mW·cm−2). Time-dependent photocurrent in symmetric electrolytes (optimal 0.1 M KCl) established light-induced diffusion current and potential (UOC, short-circuit current). Power output was measured by connecting external resistances and calculating power density P = I²R/S, identifying maximum at load matching internal resistance. Surface photovoltage spectroscopy (SPV) probed light-induced positive surface charge; zeta potential assessed surface charge; temperature rise due to illumination was checked and deemed negligible. - Pumping against gradients and pH effects: Asymmetric gradients (C/C1 = 2.0 to 3.25) were tested with illumination on the low-concentration side to probe up-gradient Cl− pumping. pH dependence was studied; acidic conditions increased baseline protonation/Cl− selectivity but reduced light-driven force; alkaline conditions reduced Cl− selectivity yet enabled pumping against larger gradients. - Theory and simulation: DFT calculations modeled Cl− migration between adjacent porphyrins and estimated relative free energy barriers, comparing p-BCP vs p-BCP@4TPP. Continuum Poisson–Nernst–Planck (PNP) simulations (steady-state) quantified light-induced charge redistribution effects on potential profiles and Cl− concentration along a single channel under symmetric and asymmetric conditions (e.g., 2.0-fold gradient). Diffusion coefficients used: DK = 1.96×10−9 m²·s−1, DCl = 2.03×10−9 m²·s−1; temperature 298 K; dielectric constant 80.

• Helical porphyrin channels with defect repair: Doping small amounts of TPP into p-BCP decreased porphyrin d-spacing and WAXD FWHM, indicating repaired stacking; optimal at n=4 (p-BCP@4TPP). Excessive doping (n=8) led to TPP aggregation (2θ=26.8°) and slight d-spacing increase. GI-SAXS verified preserved nanocylinder periodicity (~70.6 nm). CD at 417 nm and J-aggregate signatures (UV–vis ~728 nm, fluorescence ~717 nm) confirmed helical porphyrin channels. - Highest conductance at optimal doping: Among p-BCP@nTPP (n=0,2,4,8), p-BCP@4TPP exhibited the highest ionic conductance under symmetric KCl. DFT showed the lowest relative free energy barrier for Cl− migration with p-BCP@4TPP, consistent with experiments. - Specific Cl− selectivity: Under a 50-fold gradient (0.5 M/0.01 M), the diffusion potential indicated a Cl− over K+ selectivity ratio S = DCl/DK ≈ −18.6 (sign from equation usage), evidencing much faster Cl− transport than K+. Cl− showed stronger affinity to porphyrin than K+. Across anions (KCl, KBr, KF, KHCO3, KH2PO4), conductance and power density were highest for KCl, demonstrating specific Cl− selectivity over Br−, F−, HCO3−, and H2PO4−. - Light-driven transport and energy conversion: In symmetric 0.1 M KCl, illumination induced directional Cl− migration and measurable diffusion current and potential. Maximum photocurrent density reached 0.68 mA·cm−2 for p-BCP@4TPP at 100.8 mW·cm−2; increased linearly with light intensity (≈0.15 → 0.68 mA·cm−2). Maximum power density was 56.0 mW·m−2 at load matching. Light-to-electric conversion efficiency was reported as −0.27%. SPV confirmed light-induced positive surface charge; simulated potential profiles showed an illumination-induced axial potential drop. Optimal concentration for photocurrent was 0.1 M KCl; at higher concentrations, shorter Debye length (~0.3 nm at 1.0 M) reduced selectivity and photocurrent. - Active pumping against gradients: With illumination on the low-concentration side, Cl− transport was driven against the concentration gradient, inverting diffusion potential/current under 2-fold gradients and still achieving up to a 3-fold gradient (beyond 3.25-fold, concentration force dominated and transport followed the gradient). Simulations showed light-induced surface charge redistribution inverting the concentration profile along the channel. - pH effects: Acidic conditions increased porphyrin protonation and Cl− selectivity; light-driven force was weaker, enabling up-gradient pumping against ~2.0-fold gradients. Under alkaline conditions, lower baseline Cl− selectivity but larger light-driven force enabled up-gradient pumping against a ~4.0-fold gradient. Overall, the system demonstrates a bioinspired light-driven chloride pump with high selectivity and tunable photoresponse.

The study demonstrates that aligning a minimal number of porphyrin units into helical channels within a block copolymer matrix and repairing stacking defects by limited TPP doping provides the dual functionality required to mimic HR: a photo-receptive scaffold and specific Cl− transport sites. Optimal doping (n=4) minimizes stacking defects, lowers Cl− migration barriers, and maximizes ionic conductance. Upon illumination, charge separation within porphyrins generates an asymmetric surface charge distribution across the membrane, establishing an internal electric field that enriches Cl− on the illuminated side. This drives directional Cl− migration, producing measurable diffusion currents and potentials even under symmetric electrolytes, thereby converting light energy to electrical output. The channels’ strong Cl− affinity confers selectivity over cations and other anions, enabling preferential transport and higher power output with KCl. Crucially, the light-induced internal field can overcome concentration gradients, achieving up-gradient pumping (up to about 3-fold, with pH-dependent modulation). Simulations (PNP) and DFT corroborate the mechanistic picture by revealing reduced energy barriers and illumination-induced potential and concentration redistributions along single channels. These findings address the research goal of realizing a light-driven chloride pump with specific Cl− selectivity, advancing bioinspired ion-channel design and iontronic energy conversion.

This work introduces a bioinspired artificial light-driven chloride pump based on helical porphyrin channels self-assembled in a block copolymer, with defect repair via minimal TPP doping. The optimized membrane (p-BCP@4TPP) exhibits specific Cl− selectivity over cations and competing anions, high photocurrent density (0.68 mA·cm−2), and notable power density (56.0 mW·m−2), and can pump Cl− against up to ~3-fold concentration gradients (pH-tunable). The mechanism centers on light-induced surface charge redistribution and a built-in electric field across the membrane. These results provide a general design strategy using aggregated photoactive groups to construct responsive ion channels for optogenetics and solar-to-ionic/electrical energy conversion. Future research could optimize porphyrin content and arrangement to enhance efficiency, explore long-term stability and cycling under diverse electrolytes and pH, integrate with flexible iontronic devices, extend the concept to other selective ions, and scale membranes for practical energy harvesting.

• Conversion efficiency remains low (reported −0.27%), indicating room for optimization of photophysical coupling and channel architecture. - Excessive TPP doping causes porphyrin aggregation and worsened stacking (n=8), narrowing the processing window. - Selectivity and photocurrent decrease at high ionic strengths (short Debye length), constraining operation within an optimal concentration range (~0.1 M KCl). - Demonstrations focus on KCl and selected anions; broader ion chemistries and real-world electrolytes need validation. - Performance depends on light intensity and pH; environmental variations may affect stability and reproducibility. - The study employs membrane-level measurements; in vivo or biomimetic bilayer integration and long-term durability under cycling were not addressed.