Phytochrome photoreceptors mediate light responses in plants and in many microorganisms. Here we report studies using 1 H-13 C magic-angle spinning NMR spectroscopy of the sensor module of cyanobacterial phytochrome Cph1. Two isoforms of the red-light absorbing Pr ground state are identified. Conclusive evidence that photoisomerization occurs at the C15-methine bridge leading to a β-facial disposition of the ring D is presented. In the far-red-light absorbing Pfr state, strong hydrogen-bonding interactions of the D-ring carbonyl group to Tyr-263 and of N24 to Asp-207 hold the chromophore in a tensed conformation. Signaling is triggered when Asp-207 is released from its salt bridge to Arg-472, probably inducing conformational changes in the tongue region. A second signal route is initiated by partner swapping of the B-ring propionate between Arg-254 and Arg-222.chromophore-protein interaction | signal transduction | solid-state NMR | photomorphogenesis P hytochrome was first demonstrated in plants as a red-lightdependent photoreceptor regulating numerous photomorphogenic processes (1, 2). Phytochromes are, however, also now known in photosynthetic prokaryotes including cyanobacteria (3, 4), nonphotosynthetic bacteria (5), and fungi (6). Generally, the phytochrome apoprotein binds an open-chain tetrapyrrole as a chromophore (7,8) to form the red-light absorbing Pr ground state (λ max ≈ 658 nm in case of cyanobacterial phytchrome Cph1 from Synechocystis 6803). Red light absorption photoactivates the molecule to form the photoactivated far-red-light absorbing Pfr state (λ max ≈ 702 nm for Cph1) via a series of intermediates (8-10). Photoactivation is thought to be initiated by a double bond isomerization of the chromophore (10, 11). Early NMR spectroscopic studies on proteolytic phytochrome fragments (12, 13) indicated that this isomerization occurs at the C15═C16 double bond (for numbering, see Fig. 1A), a geometrical change in line with vibrational spectroscopic investigations (14-16) and results from recent 13 C solid-state NMR (17, 18) in which the most significant changes during the light-triggered conversions are confined to rings C and D. Exact geometries of the chromophore in the Pr state have been resolved as periplanar ZZZssa configurations in bacteriophytochromes from Deinococcus radiodurans (19) and Rhodopseudomonas palustris (20) as well as in the more plant-phytochrome-like Cph1 from the cyanobacterium Synechocystis 6803 (21). On the other hand, the crystal structure of the unusual bacteriophytochrome PaBphP Pseudomonas aeruginosa (22) whose ground state is Pfr shows a ZZEssa conformation, consistent with the expected primary photochemistry at the C15═C16 double bond (Fig. 1 B vs. C). Very recently, however, Ulijasz et al. presented structural simulations based on liquid NMR data of a 20-kDa GAF (cGMP phosphodiesterase/ adenylyl cyclase/FhlA) domain fragment of "SyB-Cph1" phytochrome from the thermotolerant cyanobacterium Synechococcus OSB′ (23). Surprisingly, they concluded that photoisomerization occ...

Polyethylene terephthalate (PET) is the most important mass‐produced thermoplastic polyester used as a packaging material. Recently, thermophilic polyester hydrolases such as TfCut2 from Thermobifida fusca have emerged as promising biocatalysts for an eco‐friendly PET recycling process. In this study, postconsumer PET food packaging containers are treated with TfCut2 and show weight losses of more than 50% after 96 h of incubation at 70 °C. Differential scanning calorimetry analysis indicates that the high linear degradation rates observed in the first 72 h of incubation is due to the high hydrolysis susceptibility of the mobile amorphous fraction (MAF) of PET. The physical aging process of PET occurring at 70 °C is shown to gradually convert MAF to polymer microstructures with limited accessibility to enzymatic hydrolysis. Analysis of the chain‐length distribution of degraded PET by nuclear magnetic resonance spectroscopy reveals that MAF is rapidly hydrolyzed via a combinatorial exo‐ and endo‐type degradation mechanism whereas the remaining PET microstructures are slowly degraded only by endo‐type chain scission causing no detectable weight loss. Hence, efficient thermostable biocatalysts are required to overcome the competitive physical aging process for the complete degradation of postconsumer PET materials close to the glass transition temperature of PET.

We show that narrow hydrophobic pores have an intrinsic ion selectivity by using single-walled carbon nanotube membranes as a model. We examined pores of radius 3.4-6.1 A, and conducted molecular dynamics simulations to show that Na+, K+, and Cl- face different free energy barriers when entering hydrophobic pores. Most of the differences result from the different dehydration energies of the ions; however, changes in the solvation shell structure in the confined nanotube interior and van der Waals interactions in the small tubes can both play a role. Molecular dynamics simulations conducted under hydrostatic pressure show that carbon nanotube membranes can act as ion sieves, with the pore radius and pressure determining which ions will permeate through the membrane. This work suggests that the intrinsic ion selectivity of biological pores of differing radii might also play a role in determining their selectivity, in addition to the more common explanations based on electrostatic effects. In addition, "hydrophobic gating" can arise in continuous water-filled pores.

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