Flexible Substrate of Cellulose Fiber/Structured Plasmonic Silver Nanoparticles Applied for Label-Free SERS Detection of Malathion.

Surface-enhanced Raman scattering (SERS) is considered an efficient technique providing high sensitivity and fingerprint specificity for the detection of pesticide residues. Recent developments in SERS-based detection aim to create flexible plasmonic substrates that meet the requirements for non-destructive analysis of contaminants on curved surfaces by simply wrapping or wiping. Herein, we reported a flexible SERS substrate based on cellulose fiber (CF) modified with silver nanostructures (AgNS). A silver film was fabricated on the membrane surface with an in situ silver mirror reaction leading to the formation of a AgNS-CF substrate. Then, the substrate was decorated through in situ synthesis of raspberry-like silver nanostructures (rAgNS). The SERS performance of the prepared substrate was tested using 4-mercaptobenzoic acid (4-MBA) as a Raman probe and compared with that of the CF-based plasmonic substrates. The sensitivity of the rAgNS/AgNS-CF substrate was evaluated by determining the detection limit of 4-MBA and an analytical enhancement factor, which were 10 nM and ~107, respectively. Further, the proposed flexible rAgNS/AgNS-CF substrate was applied for SERS detection of malathion. The detection limit for malathion reached 0.15 mg/L, which meets the requirements about its maximum residue level in food. Thus, the characteristics of the rAgNS/AgNS-CF substrate demonstrate the potential of its application as a label-free and ready-to-use sensing platform for the SERS detection of trace hazardous substances.

Comparative Absorption Dynamics of the Singlet Excited States of Chlorophylls a and d.

Transient absorption dynamics of chlorophylls a and d dissolved in tetrahydrofuran was measured by the broadband femtosecond laser pump-probe spectroscopy in a spectral range from 400 to 870 nm. The absorption spectra of the excited S1 singlet states of chlorophylls a and d were recorded, and the dynamics of the of the Qy band shift of the stimulated emission (Stokes shift of fluorescence) was determined in a time range from 60 fs to 4 ps. The kinetics of the intramolecular conversion Qx→Qy (electronic transition S2→S1) was measured; the characteristic relaxation time was 54 ± 3 and 45 ± 9 fs for chlorophylls a and d, respectively.

Current state of the primary charge separation mechanism in photosystem I of cyanobacteria.

This review analyzes new data on the mechanism of ultrafast reactions of primary charge separation in photosystem I (PS I) of cyanobacteria obtained in the last decade by methods of femtosecond absorption spectroscopy. Cyanobacterial PS I from many species harbours 96 chlorophyll a (Chl a) molecules, including six specialized Chls denoted Chl1A/Chl1B (dimer P700, or PAPB), Chl2A/Chl2B, and Chl3A/Chl3B arranged in two branches, which participate in electron transfer reactions. The current data indicate that the primary charge separation occurs in a symmetric exciplex, where the special pair P700 is electronically coupled to the symmetrically located monomers Chl2A and Chl2B, which can be considered together as a symmetric exciplex Chl2APAPBChl2B with the mixed excited (Chl2APAPBChl2B)* and two charge-transfer states P700 +Chl2A - and P700 +Chl2B -. The redistribution of electrons between the branches in favor of the A-branch occurs after reduction of the Chl2A and Chl2B monomers. The formation of charge-transfer states and the symmetry breaking mechanisms were clarified by measuring the electrochromic Stark shift of ß-carotene and the absorption dynamics of PS I complexes with the genetically altered Chl 2B or Chl 2A monomers. The review gives a brief description of the main methods for analyzing data obtained using femtosecond absorption spectroscopy. The energy levels of excited and charge-transfer intermediates arising in the cyanobacterial PS I are critically analyzed.

The Development of New Low-Molecular-Weight Factor Xa Inhibitors that are Potential Anticoagulants.

BACKGROUND: Despite the introduction of direct oral anticoagulants, the search for new oral anticoagulants remains an urgent task. OBJECTIVE: By using docking and scoring, based on physical methods, simple chemical rules, methods of synthesis, and activity measurement, develop new low-molecular-weight inhibitors of factor Xa, which are potential anticoagulants. METHODS: The development of leads was based on chemical synthesis and structure-based drug design methods. The basic idea was to combine the two approaches: one based on predictive modeling and the other based on the experimental data. RESULTS: In this study, we developed some nanomolar leads. Further chemical modification improved the inhibition constant by more than one order. DISCUSSION: The method proposed in this paper, as well as other methods, includes virtual screening, chemical synthesis, and activity measurement. However, the most time-consuming process in this method (chemical synthesis) was simplified, and the cost was reduced to the extent that was allowed; a very simple chemical reaction was chosen, i.e., the formation of an amide bond. CONCLUSION: In this work, we demonstrated how using simple chemical rules based on the structurebased drug design, substances with a nanomolar concentration of activity can be developed.

Primary charge separation within the structurally symmetric tetrameric Chl2APAPBChl2B chlorophyll exciplex in photosystem I.

In Photosystem I (PS I), the role of the accessory chlorophyll (Chl) molecules, Chl2A and Chl2B (also termed A-1A and A-1B), which are directly adjacent to the special pair P700 and fork into the A- and B-branches of electron carriers, is incompletely understood. In this work, the Chl2A and Chl2B transient absorption ΔA0(λ) at a time delay of 100 fs was identified by ultrafast pump-probe spectroscopy in three pairs of PS I complexes from Synechocystis sp. PCC 6803 with residues PsaA-N600 or PsaB-N582 (which ligate Chl2B or Chl2A through a H2O molecule) substituted by Met, His, and Leu. The ΔA0(λ) spectra were quantified using principal component analysis, the main component of which was interpreted as a mutation-induced shift of the equilibrium between the excited state of primary donor P700⁎ and the primary charge-separated state P700+Chl2-. This equilibrium is shifted to the charge-separated state in wild-type PS I and to the excited P700 in the PS I complexes with the substituted ligands to the Chl2A and Chl2B monomers. The results can be rationalized within the framework of an adiabatic model in which the P700 is electronically coupled with the symmetrically arranged monomers Chl2A and Chl2B; such a structure can be considered a symmetric tetrameric exciplex Chl2APAPBChl2B, in which the excited state (Chl2APAPBChl2B)* is mixed with two charge-transfer states P700+Chl2A- and P700+Chl2B-. The electron redistribution between the two branches in favor of the A-branch apparently takes place in the picosecond time scale after reduction of the Chl2A and Chl2B monomers.

Generation of ion-radical chlorophyll states in the light-harvesting antenna and the reaction center of cyanobacterial photosystem I.

The energy and charge-transfer processes in photosystem I (PS I) complexes isolated from cyanobacteria Thermosynechococcus elongatus and Synechocystis sp. PCC 6803 were investigated by pump-to-probe femtosecond spectroscopy. The formation of charge-transfer (CT) states in excitonically coupled chlorophyll a complexes (exciplexes) was monitored by measuring the electrochromic shift of ß-carotene in the spectral range 500-510 nm. The excitation of high-energy chlorophyll in light-harvesting antenna of both species was not accompanied by immediate appearance of an electrochromic shift. In PS I from T. elongatus, the excitation of long-wavelength chlorophyll (LWC) caused a pronounced electrochromic effect at 502 nm assigned to the appearance of CT states of chlorophyll exciplexes. The formation of ion-radical pair P700+A1- at 40 ps was limited by energy transfer from LWC to the primary donor P700 and accompanied by carotenoid bleach at 498 nm. In PS I from Synechocystis 6803, the excitation at 720 nm produced an immediate bidentate bleach at 690/704 nm and synchronous carotenoid response at 508 nm. The bidentate bleach was assigned to the formation of primary ion-radical state PB+Chl2B-, where negative charge is localized predominantly at the accessory chlorophyll molecule in the branch B, Chl2B. The following decrease of carotenoid signal at ~ 5 ps was ascribed to electron transfer to the more distant molecule Chl3B. The reduction of phylloquinone in the sites A1A and A1B was accompanied by a synchronous blue-shift of the carotenoid response to 498 nm, pointing to fast redistribution of unpaired electron between two branches in favor of the state PB+A1A-.

Evidence that chlorophyll f functions solely as an antenna pigment in far-red-light photosystem I from Fischerella thermalis PCC 7521.

The Photosystem I (PSI) reaction center in cyanobacteria is comprised of ~96 chlorophyll (Chl) molecules, including six specialized Chl molecules denoted Chl1A/Chl1B (P700), Chl2A/Chl2B, and Chl3A/Chl3B that are arranged in two branches and function in primary charge separation. It has recently been proposed that PSI from Chroococcidiopsis thermalis (Nürnberg et al. (2018) Science 360, 1210-1213) and Fischerella thermalis PCC 7521 (Hastings et al. (2019) Biochim. Biophys. Acta 1860, 452-460) contain Chl f in the positions Chl2A/Chl2B. We tested this proposal by exciting RCs from white-light grown (WL-PSI) and far-red light grown (FRL-PSI) F. thermalis PCC 7521 with femtosecond pulses and analyzing the optical dynamics. If Chl f were in the position Chl2A/Chl2B in FRL-PSI, excitation at 740 nm should have produced the charge-separated state P700+A0- followed by electron transfer to A1 with a τ of ≤25 ps. Instead, it takes ~230 ps for the charge-separated state to develop because the excitation migrates uphill from Chl f in the antenna to the trapping center. Further, we observe a strong electrochromic shift at 685 nm in the final P700+A1- spectrum that can only be explained if Chl a is in the positions Chl2A/Chl2B. Similar arguments rule out the presence of Chl f in the positions Chl3A/Chl3B; hence, Chl f is likely to function solely as an antenna pigment in FRL-PSI. We additionally report the presence of an excitonically coupled homo- or heterodimer of Chl f absorbing around 790 nm that is kinetically independent of the Chl f population that absorbs around 740 nm.

The Development of New Factor Xa Inhibitors Based on Amide Synthesis.

BACKGROUND: Factor Xa (FXa) is known to play a central role in blood coagulation cascade and considered to be one of the most attractive targets for oral anticoagulants of new generation. OBJECTIVE: Our approach for the development of directly acting oral anticoagulants (DOAC), FXa inhibitors was demonstrated in this work. METHOD: Chemical synthesis is the base of our approach for the development of potential inhibitors. In this work, the substances like R1-(CONH)-R2-(CONH)-R3 are being developed, using previously described docking and screening methods, where R1, R2 and R3 are some chemical groups and (CONH) are amide bonds connecting R1, R2 and R3. The direction of amide bond (CONH) could be arbitrary for R1, R2 and R2, R3. RESULTS: Chemical modifications were made in the frame of the results, taking into account the structure of FXa, chemical synthesis capabilities, as well as patentability of the target compounds. Subnanomolar potency of several developed compounds was achieved. Several analyzers and various testing-suites have been used to measure the concentration that doubled the prothrombin time (PTx2). Moreover, in human plasma the PTx2 concentration of the compound 217 (DD217) turned out to be 80±20 nM. The compound efficacy has proved by in vivo assays including oral administrations in rats, rabbits and monkeys. CONCLUSION: The pharmacodynamic profile of DD217 for oral administration in cynomolgus monkeys proves the efficacy of the compound, which makes it promising for the future preclinical trials.

Mechanism of adiabatic primary electron transfer in photosystem I: Femtosecond spectroscopy upon excitation of reaction center in the far-red edge of the QY band.

The ultrafast primary charge separation in Photosystem I (PS I) excited by femtosecond pulses centered at 720 and 760nm was studied by pump-to-probe laser spectroscopy. The absorbance in the red edge of PS I absorption spectrum has an unusual exponential dependence on wavelength. The cutoff of short wavelength components of 760nm pulse allows direct excitation of reaction center chlorophyll molecules without involvement of light-harvesting antenna. The transient spectrum manifests the features of the primary ion-radical pair P700+A0- at time delay <180fs, followed by formation of the secondary pair P700+A1- with a characteristic time of 26ps. The obtained data are rationalized in the framework of adiabatic three-state model that includes the chlorophyll dimer P700 and two symmetrically arranged nearest chlorophyll molecules of A0. The arrangement of chlorophylls results in strong electronic coupling between P700 and A0. Excitation in the maximum of P700 absorption generates electronic states with the highest contribution from P700*, whereas excitation in the far-red edge predominantly generates charge transfer state P700+A0- in both branches of redox-cofactors. The three-level model accounts for a flat-bottomed potential surface of the excited state and adiabatic character of electron transfer between P700 and A0, providing a microscopic explanation of the ultrafast formation of P700+A0- and exponential decline of PS I absorption.