Bioinspired light-driven chloride pump with helical porphyrin channels

Membrane-embedded protein ion pumps and channels regulate physiological activities by controlling ion transport across cell membranes. Halorhodopsin (HR), a light-gated chloride channel from *Halobacterium salinarum*, functions as a sensory photoreceptor via light-driven chloride ion transport. The HR structure reveals that the Schiff base group, crucial for light absorption and transport, aggregates within the ion channel. Understanding HR's structural characteristics and molecular mechanisms could inspire the development of smart materials for applications such as bioinspired ion channel systems for energy conversion and high-performance optogenetics. Currently, no artificial ion channels achieve light-driven chloride ion pumping like HR. To mimic this, an artificial light-driven chloride pump must have a photoreceptor and a specific chloride-selective site. While materials like TiO₂, porphyrin, spiropyran, graphitic carbon nitride, and azobenzene are used in light-responsive channels, they lack chloride selectivity. This research aims to address this gap by using porphyrin, with its nitrogen-containing conjugated structure, to create a light-responsive chloride ion transport system. The strategy involves aligning porphyrin within helical channels, utilizing the cavity for chloride transport to achieve a light-driven chloride ion pump.

The literature extensively covers natural light-driven ion pumps and channels like Halorhodopsin (HR), highlighting their importance in biological processes. Several studies delve into the structural and mechanistic details of HR, pinpointing the role of the Schiff base in light absorption and ion selectivity. However, the creation of artificial light-driven chloride pumps remains a significant challenge. While progress has been made in developing light-responsive channels using various materials, the lack of specific chloride selectivity has hindered the creation of an effective artificial analog to HR. Existing artificial light-responsive channels often employ materials like TiO₂, porphyrins, spiropyran, and azobenzene, but these lack the key chloride-binding site found in HR. This paper builds upon the understanding of HR's mechanism and aims to overcome limitations in existing artificial systems by incorporating porphyrin to achieve both light responsiveness and chloride selectivity.

The researchers synthesized a porphyrin-cored block copolymer (*p*-BCP) and introduced free porphyrins (TPP) to repair defects in the self-assembled helical channels. The synthesis of *p*-BCP is detailed in the supplementary information, along with chemical characterizations (Supplementary Figs. 1–4). The mass content of porphyrin in *p*-BCP is less than 1%, yet it self-assembles into a high-density helical array due to π–π stacking (Fig. 1c, d). To reduce defects caused by steric hindrance, varying amounts of TPP were doped into the *p*-BCP system (*p*-BCP@*n*TPP, where *n* represents the TPP doping number per *p*-BCP). GI-SAXS and SAXS confirmed that TPP doping didn't alter the periodic nanocylinder structure (Fig. 1d, Supplementary Fig. 6), with a periodic length of ~70.6 nm, consistent with the TEM image. WAXD analysis showed that TPP doping reduced the *d*-spacing and FWHM of porphyrin aggregation, indicating defect repair (Fig. 1e, Supplementary Fig. 8, Supplementary Table 1). Circular dichroism (CD) spectroscopy confirmed the helical structure of the channels (Fig. 1f), attributed to porphyrin J-aggregation (Supplementary Figs. 13, 14). Density functional theory (DFT) calculations were used to investigate the specific chloride ion selectivity of the channels, comparing relative free energy barriers for Cl⁻ transport (Supplementary Fig. 15, Supplementary Data 1). Electrochemical measurements, including current-voltage (I-V) curves under symmetric and asymmetric solutions, were used to assess the ionic conductance and Cl⁻ selectivity (Fig. 2b, c, Supplementary Figs. 16, 17). The Cl⁻ selectivity was quantified using the Goldman-Hodgkin-Katz voltage equation (Eq. 1). The light-driven ion pump was characterized by measuring photocurrents under different light intensities and concentration gradients (Fig. 4b-e, Supplementary Figs. 32-35), as well as the power density generated (Fig. 4c, Supplementary Fig. 35). Numerical simulations, based on Poisson and Nernst-Planck (PNP) equations (Eqs. 2-4), were used to model the ion transport under light irradiation and different concentration gradients (Fig. 4f, Fig. 5f, Supplementary Figs. 42, 43). Detailed descriptions of characterization techniques (TEM, SEM, GI-SAXS, WAXD, CD, UV-vis, fluorescence, XPS, electrochemical measurements) are provided in the Methods section and Supplementary Information.

The study successfully fabricated a bioinspired light-driven chloride pump using a helical porphyrin channel array. The *p*-BCP@4TPP membrane, with optimal TPP doping, exhibited the highest ionic conductance and chloride selectivity. DFT calculations supported the experimental findings, showing the lowest free energy barrier for chloride transport in this optimized structure. The membrane showed excellent chloride selectivity over cations (K⁺) and other anions (F⁻, Br⁻, H₂PO₄⁻, HCO₃⁻). Under light irradiation, chloride ions migrated directionally, even without an external potential or concentration gradient, demonstrating light-driven ion pumping. The light-driven chloride pump generated a high diffusion current density of 0.68 mA·cm⁻² under symmetric conditions, reaching a power density of 56.0 mW·m⁻². Significantly, chloride ions could migrate against a 3-fold concentration gradient under light irradiation, indicating active ion pumping. The mechanism of light-driven ion transport was attributed to light-induced charge redistribution on the membrane surface, creating an electric field that drives directional chloride ion movement. The effects of light intensity and concentration gradients were systematically investigated, demonstrating a direct correlation between light intensity and photocurrent density. Simulations based on Poisson and Nernst-Planck equations corroborated the experimental observations, providing a semi-quantitative understanding of the light-driven ion transport and concentration gradient reversal.

This study successfully demonstrates an artificial light-driven chloride pump, mirroring the functionality of the natural halorhodopsin protein. The use of a porphyrin-based helical channel array provides both the light sensitivity and chloride selectivity essential for this type of pump. The high performance achieved, including current density and power generation, makes this artificial system a promising candidate for applications in energy conversion and optogenetics. The findings significantly advance the field of bioinspired materials and nanofluidics, demonstrating a successful strategy for mimicking complex biological functions in an artificial system. The ability to actively pump ions against a concentration gradient opens new possibilities for designing responsive and energy-efficient devices. The successful integration of DFT calculations with experimental results validates the proposed mechanism of light-driven ion transport, providing a deeper understanding of the underlying principles. Future research could explore the scalability of this system for larger-scale applications, and investigate different porphyrin structures and dopants to further enhance performance and explore other ion transport mechanisms.

This research successfully created a bio-inspired light-driven chloride pump using a helical porphyrin channel array. The system exhibited high Cl⁻ selectivity, a significant diffusion current density (0.68 mA·cm⁻²), and the ability to pump Cl⁻ against a 3-fold concentration gradient. This achievement holds promise for applications in bioinspired responsive ion channels and high-efficiency solar energy conversion. Future research should focus on optimizing the system for improved efficiency and exploring its potential in diverse technological applications.

The current study focuses on chloride ion transport. Further investigations are needed to explore the system's performance with other ions. The power conversion efficiency of −0.27% is relatively low, indicating the need for optimization to improve energy harvesting. The long-term stability and durability of the artificial pump under continuous operation also require further assessment. The model used for simulation simplifies the complex interactions within the nanochannel. More sophisticated models might be needed for a more accurate understanding of the ion transport.