Trehalose: The Sugar That Freezes Photosynthesis in Place to Reveal Its Secrets

Deep inside every leaf and every photosynthetic bacterium, electrons race through molecular machinery with astonishing precision. Now, researchers have shown that an ordinary-looking sugar — trehalose, the same disaccharide that lets desert plants and tardigrades survive near-total desiccation — can act as a molecular freeze-frame, holding photosynthetic protein complexes still long enough to reveal how their electron transfer reactions actually work. A new review published in Frontiers in Photobiology by Mahir Mamedov, Georgy Milanovsky, Liya Vitukhnovskaya and Alexey Semenov of the A.N. Belozersky Institute of Physical-Chemical Biology at Lomonosov Moscow State University, together with the N.N. Semenov Federal Research Center for Chemical Physics, brings together evidence from three very different photosynthetic systems and makes a striking case: trehalose is not merely a preservative, but a powerful experimental tool for dissecting the physics of biological charge transfer.

The logic behind the approach is elegant. Trehalose, a natural disaccharide composed of two glucose units, is classified as a kosmotrope — a molecule that orders the water surrounding it. Some organisms overproduce trehalose to survive drying out, preserving their internal structures through vitrification, the formation of an amorphous, glass-like matrix. When photosynthetic proteins are embedded in such a glass, their native conformations are retained while their conformational mobility is dramatically suppressed. This gives researchers a unique handle on a long-standing question in biophysics: how much does the physical flexing of a protein control the speed and efficiency of the electrons moving through it? Because photosynthetic complexes carry out multistep charge transfer reactions spanning timescales from subpicoseconds to seconds — including forward and backward electron transfer as well as proton uptake and release — they are ideal testbeds for answering that question.

The review’s first case study concerns photosystem II, the multisubunit complex embedded in the thylakoid membranes of plants, algae and cyanobacteria that catalyzes light-driven water oxidation. The plant PS II monomer is a molecular colossus: roughly 20 protein subunits, about 25 integral lipids, 35 chlorophyll a molecules, 2 pheophytins, 12 carotenoids, 3 plastoquinones, the Mn4Ca cluster and a chloride ion. The team focused on Mn-depleted PS II core complexes, in which the water-oxidizing cluster has been removed. In this modified state, the redox-active tyrosine YZ can still donate an electron to the photo-oxidized primary donor P680+, but it is not efficiently re-reduced, making the system an ideal model for probing how hydration and conformational dynamics regulate electron transfer without the confounding influence of continuous electron input from water splitting.

Using time-resolved optical spectroscopy at 820 nanometers, which tracks the redox state of P680, the researchers compared PS II complexes in aqueous solution with complexes embedded in trehalose glasses equilibrated at controlled relative humidity levels ranging from 6 to 74 percent. In solution at pH 6.5, more than 90 percent of P680+ decay is driven by the fast, physiologically relevant electron transfer from YZ, which occurs within 20 to 250 nanoseconds in intact complexes and slows to 1 to 10 microseconds in Mn-depleted samples. The remaining fraction reflects slower charge recombination with the reduced primary quinone acceptor QA−. But when the complexes were embedded in trehalose and dehydrated to 11 percent relative humidity — corresponding to roughly one water molecule per trehalose molecule — the overall decay lifetime stretched by more than an order of magnitude, and the balance shifted dramatically toward the recombination pathway.

The most striking finding was that dehydration suppressed the YZ-mediated pathway in roughly half of the PS II population. At a residual water content of about four water molecules per trehalose molecule, the two decay pathways contributed equally, and further dehydration produced almost no additional change — evidence that a subpopulation of complexes had become trapped in a nonfunctional conformational state. All kinetic components slowed by a factor of three to five, reflecting what the authors describe as conformational freezing of the complex. Remarkably, dissolving the trehalose matrix restored the original kinetics completely, demonstrating that the interaction is fully reversible. The interpretation is that oxidation of YZ by P680+ is a dynamically coupled process requiring subtle repositioning of the tyrosine, rearrangement of its hydrogen bond to His190, and reorganization of the surrounding water network — all motions that a dry trehalose glass renders inaccessible.

The second system shifts from a glassy matrix to a hybrid photoelectrochemical device. Photosystem I, prized for its high stability, long-lived charge-separated states and well-defined cofactor arrangement, has long attracted attention for semi-artificial solar energy conversion. Yet immobilizing PS I on electrodes — whether titanium dioxide, fluorine-doped tin oxide, gold, graphene or carbon nanotubes — often disrupts the protein’s delicate hydration shell, causing denaturation and loss of activity. The Moscow team tackled this by depositing PS I from the cyanobacterium Synechocystis sp. PCC 6803 onto a nitrocellulose membrane filter sandwiched between two indium tin oxide electrodes, and measuring the light-induced photovoltage directly under continuous illumination.

Without trehalose, the photovoltage rose rapidly during the first 20 seconds of illumination but then decayed to about one-third of its maximum amplitude over roughly 400 seconds. Adding trehalose to the PS I suspension before adsorption transformed the response: the signal rose rapidly and then remained stable over 200 seconds of continuous illumination. Sucrose, despite producing similar viscosity, did not replicate the effect — pointing to trehalose’s unique physicochemical properties rather than a simple viscous damping mechanism. The practical stakes became clear in storage tests. After just 24 hours at room temperature in the dark, the photovoltage amplitude collapsed by roughly ninefold, apparently due to desiccation of the protein’s microenvironment. Reinjecting a small volume of fresh buffer restored about 80 percent of the original response.

Controlled humidity proved to be the decisive variable. When the ITO|PS I–MF|ITO assembly was stored at a relative air humidity of about 65 percent — maintained with a saturated ammonium nitrate solution at 22 degrees Celsius — in the presence of trehalose and the ascorbate/DCPIP redox couple, the photovoltage amplitude declined by only about 15 percent after seven days, with a more substantial drop by day 14. Reinjection of fresh buffer, even without added redox mediators, recovered the signal to near-initial levels. By contrast, storage under ambient laboratory conditions led to almost complete signal loss within 24 hours. The redox mediators themselves also retained functional activity over these extended incubation periods. The authors conclude that maintaining water balance is critically important, because it governs the mobility and diffusion of redox mediators and, consequently, the efficiency of trans-electrode electron transfer — a systematic exploration of humidity across the 33 to 85 percent range now appears promising for maximizing the operational lifetime of such devices at room temperature.

The third system brought a surprise. In bacterial chromatophores — closed membrane vesicles from the purple bacterium Cereibacter sphaeroides suspended in aqueous solution — trehalose slowed the electrogenic protonation of the secondary quinone acceptor QB− to QBH2 by about 25 percent, while leaving the reduction of the oxidized primary donor P870+ by endogenous cytochrome c2 untouched. This selectivity makes mechanistic sense: protonation of QB− is known to involve conformational changes in both the protein and the quinone itself, whereas cytochrome-mediated reduction of P870+ requires no significant structural rearrangement. Even more strikingly, trehalose slowed electron transfer through the high-potential branch of the cytochrome bc1 complex — the rate-limiting step of which is the large-scale movement of the Rieske iron-sulfur protein between the quinol-oxidizing site and cytochrome c1 — stretching the half-time of the antimycin-sensitive kinetic phase from 2.8 milliseconds to 12.6 milliseconds at 0.8 molar trehalose. Because the affected sites lie close to the inner surface of the chromatophore membrane, the implication is that trehalose actually penetrates the vesicles, a finding confirmed by colorimetric measurement of the sugar inside. The membrane, it turns out, is permeable to trehalose — possibly via an as-yet-unidentified nonspecific sugar transporter.

Taken together, the three systems reveal a common thread: trehalose confines protein conformational mobility, prevents damage and degradation at room temperature, and enables prolonged storage of photosynthetic complexes at high trehalose-to-water ratios. But the details differ in instructive ways. PS II retains substantially more bound water within dehydrated trehalose matrices than PS I does — likely several thousand additional water molecules per complex, corresponding to at least a complete first hydration shell — a result inconsistent with the classical water-replacement hypothesis and supportive instead of models based on water entrapment and anchorage. And while viscous agents like glycerol slow charge recombination in bacterial reaction centers in accordance with modified Kramers theory, severe dehydration in detergent films accelerates it, because the conformational changes that stabilize the charge-separated state are blocked. No such stabilization-associated motions have been identified in PS II, which explains why dehydration produces the opposite kinetic effect there. Beyond the fundamental insights, the work points toward a practical vision: trehalose-stabilized photosynthetic complexes embedded in biohybrid devices could underpin sustainable, ecologically safe solar energy converters, bioelectronics and sensors — all built from nature’s own light-harvesting machinery, held perfectly still by a sugar.

Subject of Research: Effects of trehalose on electron transfer in photosystem II, photosystem I and bacterial chromatophores

Article Title: Trehalose as a tool for studying the molecular mechanisms of electron transfer in photosynthetic systems

Article References: Mamedov, M., Milanovsky, G., Vitukhnovskaya, L., & Semenov, A. (2026). Trehalose as a tool for studying the molecular mechanisms of electron transfer in photosynthetic systems. Frontiers in Photobiology, 4, Article 1875984. https://doi.org/10.3389/fphbi.2026.1875984

Image Credits: AI Generated

DOI: 10.3389/fphbi.2026.1875984

Keywords: trehalose, photosystem II, photosystem I, electron transfer, chromatophores, photovoltage, protein dynamics, biohybrid solar cells, photosynthesis, biophysics, hydration, cytochrome bc1