Photosynthetic Reaction Center Models Research Articles

Computational Investigation of the Spin-Density Asymmetry in Photosynthetic Reaction Center Models from First Principles.

We present a computational analysis of the spin-density asymmetry in cation radical states of reaction center models from photosystem I, photosystem II, and bacteria from Synechococcus elongatus, Thermococcus vulcanus, and Rhodobacter sphaeroides, respectively. The recently developed frozen-density embedding (FDE)-diab methodology [J. Chem. Phys., 2018, 148, 214104] allowed us to effectively avoid the spin-density overdelocalization error characteristic for the standard Kohn-Sham density functional theory and to reliably calculate spin-density distributions and electronic couplings for a number of molecular systems ranging from inner pairs of (bacterio)chlorophyll a molecules in vacuum to large proteins including up to about 2000 atoms. The calculated spin densities show a good agreement with available experimental results and were used to validate reaction center models reported in the literature. Here, we demonstrate that the applied theoretical approach is very sensitive to changes in molecular structures and the relative orientation of molecules. This makes FDE-diab a valuable tool for electronic structure calculations of large photosynthetic models effectively complementing the existing experimental techniques.

(Nanocarbons Division Richard E. Smalley Research Award Address) Artificial Photosynthesis Utilizing Nanocarbons

Sustainable and clean energy resources using solar energy alternative to fossil fuels are urgently required in order to stop global warming and solve global energy and environmental issues. This award lecture focuses on our long-term developments in artificial photosynthesis for solar fuel production and its usage by utilizing nanocarbon materials. We have developed a number of photosynthetic reaction center models composed of metalloporphyrins and fullerenes with comparable charge-separated state lifetimes with that of the natural photosynthetic reaction center.[1-3] The light-harvesting unit and the charge-separation unit have been combined by utilizing coordination bonds, hydrogen bond, electrostatic interactions and p-p interactions.[4,5] The efficient photosynthetic reaction center models have been successfully combined with the catalytic water reduction unit to attain the most efficient photocatalytic hydrogen evolution system.[6-8] The activation of hydrogen has also been achieved by developing the first structural and functional model of hydrogenases,[9] which enables to replace precious Pt catalyst by much more earth-abundant metal catalyst for hydrogen evolution.[10] With regard to water oxidation, we have developed efficient and robust water oxidation catalysts using earth-abundant metal oxides nanoparticles.[11] We have also clarified the significant role of redox-inactive metal ions on enhancement of electron-transfer reactions of high-valent metal-oxo complexes, which is classified as metal ion-coupled electron transfer,[12,13] in relation with the pivotal role of Ca2+ in the oxygen evolving complex (OEC) in Photosystem II.[14,15] By combination of the catalytic four-electron water oxidation and the photocatalytic two-electron reduction of O2 using solar energy, we have succeeded in production of hydrogen peroxide (H2O2) as a solar fuel from water and O2 in the air.[16,17] We have also achieved efficient photocatalytic production of H2O2 from the most earth abundant seawater instead of precious pure water and O2 in the air using a two-compartment photoelectro-chemical cell with WO3 as a photocatalyst for water oxidation and a cobalt chlorin complex supported on a glassy-carbon substrate for the selective two-electron reduction of O2.[18] The highest solar energy conversion efficiency was determined to be 6.6% under simulated solar illumination adjusted to 0.05 sun after 1 h photocatalytic reaction (0.89% under 1sun illumination) when WO3 was replaced by the surface modified BiVO4 with iron(III) oxide(hydroxide) (FeO(OH)) as a water oxidation catalyst in the photoanode.[20] We have also developed one-compartment H2O2 fuel cells using H2O2 produced in seawater as a solar fuel and earth-abundant metal catalysts.[18,20,22] Thus, the combination of the photocatalytic H2O2 production from seawater and O2 using solar energy with one-compartment H2O2 fuel cells provides on-site production and usage of H2O2 as a more useful and promising solar fuel than H2.[18] References [1] Fukuzumi, S.; Ohkubo, K.; Suenobu, T. Acc. Chem. Res. 2014, 47, 1455-1464. [2] Fukuzumi, S. Phys. Chem. Chem. Phys. 2008, 10, 2283. [3] Fukuzumi, S.; Ohkubo, K. J. Mater. Chem. 2012, 22, 4575. [4] Fukuzumi, S.; Ohkubo, K.; D’Souza, F.; Sessler, J. L. Chem. Commun. 2012, 48, 9801-9815. [5] Yamada, M.; Ohkubo, K.; Shionoya, M.; Fukuzumi, S. J. Am. Chem. Soc. 2014, 136, 13240-13248. [6] Fukuzumi, S.; Yamada, Y.; Suenobu, T.; Ohkubo, K.; Kotani, H, Energy Environ. Sci. 2011, 4, 2754-2766. [7] Fukuzumi, S.; Yamada, Y. J. Mater. Chem. 2012, 22, 24284-24296. [8] Fukuzumi, S. Curr. Opinion Chem. Biol. 2015, 25, 18-26. [9] Ogo, S.; Kabe, R.; Uehara, K.; Kure, B.; Nishimura, T.; Menon, S. C.; Harada, R.; Fukuzumi, S.; Higuchi, Y.; Ohhara, T.; Tamada, T.; Kuroki, R. Science 2007, 316, 585-587. [10] Yamada, Y.; Miyahigashi, T.; Kotani, H.; Ohkubo, K.; Fukuzumi, S. Energy Environ. Sci. 2012, 5, 6111-6118. [11] Hong, D.; Yamada, Y.; Nagatomi, T.; Takai, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2012, 134, 19572-19575. [12] Fukuzumi, S. Prog. Inorg. Chem. 2009, 56, 49-153. [13] Fukuzumi, S.; Ohkubo, K.; Lee, Y.-M.; Nam, W. Chem.–Eur. J. 2015, 21, 17548-17559. [14] Fukuzumi, S.; Morimoto, Y.; Kotani, H.; Naumov, P.; Lee, Y.-M.; Nam, W. Nat. Chem. 2010, 2, 756-759. [15] Bang, S.; Lee, Y.-M.; Hong, S.; Nishida, Y.; Seo, M. S.; Sarangi, R.; Fukuzumi, S.; Nam, W. Nat. Chem. 2014, 6, 934-940. [16] Kato, S.; Jung, J.; Suenobu, T.; Fukuzumi, S. Energy Environ. Sci. 2013, 6, 3756-3764. [17] Isaka, Y.; Kato, S.; Hong, D.; Suenobu, T.; Yamada, Y.; Fukuzumi, S. J. Mater. Chem. A 2015, 3, 12404-12412. [18] Mase, K.; Yoneda, M.; Yamada, Y.; Fukuzumi, S. Nat. Commun. 2016, 7, 11470. [19] Mase, K.; Yoneda, M.; Yamada, Y.; Fukuzumi, S. ACS Energy Lett. 2016, 1, 913-919. [20] Yamada, Y.; Yoneda, M.; Fukuzumi, S. Energy Environ. Sci. 2015, 8, 1698-1701. [21] Fukuzumi, S.; Yamada, Y. ChemElectroChem 2016, 3, in press.

Effect of Spacer Connecting the Secondary Electron Donor Phenothiazine in Subphthalocyanine-Fullerene Conjugates in Promoting Electron Transfer Followed by Hole Shift Process.

Sequential electron/hole transfer between energetically well-positioned entities of photosynthetic reaction center models is one of the commonly employed mechanisms to generate long-lived charge-separated states. A wealth of information, applicable towards light energy harvesting and building optoelectronic devices, has been acquired from such studies. In the present study, we report on the effect of spacer (direct or via phenoxy linkage) connecting the hole shifting agent, phenothiazine (PTZ), on photoinduced charge stabilization in subphthalocyanine-fullerene donor-acceptor conjugates. In these conjugates, the subphthalocyanine (SubPc) and fullerene (C60 ) served as primary electron donor and acceptor, respectively, while the phenothiazine entities act as hole shifting agents. The newly synthesized compounds were characterized by optical absorption and emission, computational, and electrochemical methods. The redox potentials measured using differential pulse voltammetry were used to estimate free-energy changes for charge separation, hole migration, and charge recombination processes. Using femto- and nanosecond transient absorption techniques, evidence for charge separation, and kinetics of charge separation and recombination were obtained in polar benzonitrile and nonpolar toluene solvents. In the conjugate where the phenothiazine entities are directly linked to SubPc, evidence for sequential electron transfer followed by hole shift leading to long-lived charge separated state was weak, primarily due to the delocalization of HOMO on both SubPc and PTZ entities. However, in case of the conjugate where the PTZ and SubPc are linked via phenoxy spacers, sequential electron transfer/hole shift was observed leading to the formation of long-lived charge-separated states. The present study brings out the importance of the spacer group connecting the hole shifting agent in model donor-acceptor conjugates to generate long-lived charge-separated states.

A multi-pathway model for photosynthetic reaction center.

Charge separation occurs in a pair of tightly coupled chlorophylls at the heart of photosynthetic reaction centers of both plants and bacteria. Recently it has been shown that quantum coherence can, in principle, enhance the efficiency of a solar cell, working like a quantum heat engine. Here, we propose a biological quantum heat engine (BQHE) motivated by Photosystem II reaction center (PSII RC) to describe the charge separation. Our model mainly considers two charge-separation pathways which is more than that typically considered in the published literature. We explore how these cross-couplings increase the current and power of the charge separation and discuss the effects of multiple pathways in terms of current and power. The robustness of the BQHE against the charge recombination in natural PSII RC and dephasing induced by environments is also explored, and extension from two pathways to multiple pathways is made. These results suggest that noise-induced quantum coherence helps to suppress the influence of acceptor-to-donor charge recombination, and besides, nature-mimicking architectures with engineered multiple pathways for charge separations might be better for artificial solar energy devices considering the influence of environments.

Phenothiazine-BODIPY-fullerene triads as photosynthetic reaction center models: substitution and solvent polarity effects on photoinduced charge separation and recombination.

Novel photosynthetic reaction center model compounds of the type donor2 -donor1 -acceptor, composed of phenothiazine, BF2 -chelated dipyrromethene (BODIPY), and fullerene, respectively, have been newly synthesized using multistep synthetic methods. X-ray structures of three of the phenothiazine-BODIPY intermediate compounds have been solved to visualize the substitution effect caused by the phenothiazine on the BODIPY macrocycle. Optical absorption and emission, computational, and differential pulse voltammetry studies were systematically performed to establish the molecular integrity of the triads. The N-substituted phenothiazine was found to be easier to oxidize by 60 mV compared to the C-substituted analogue. The geometry and electronic structures were obtained by B3LYP/6-31G(dp) calculations (for H, B, N, and O) and B3LYP/6-31G(df) calculations (for S) in vacuum, followed by a single-point calculation in benzonitrile utilizing the polarizable continuum model (PCM). The HOMO-1, HOMO, and LUMO were, respectively, on the BODIPY, phenothiazine and fullerene entities, which agreed well with the site of electron transfer determined from electrochemical studies. The energy-level diagram deduced from these data helped in elucidating the mechanistic details of the photochemical events. Excitation of BODIPY resulted in ultrafast electron transfer to produce PTZ-BODIPY(.+) -C60 (.-) ; subsequent hole shift resulted in PTZ(.+) -BODIPY-C60 (.-) charge-separated species. The return of the charge-separated species was found to be solvent dependent. In nonpolar solvents the PTZ(.+) -BODIPY-C60 (.-) species populated the (3) C60 * prior to returning to the ground state, while in polar solvent no such process was observed due to relative positioning of the energy levels. The (1) BODIPY* generated radical ion-pair in these triads persisted for few nanoseconds due to electron transfer/hole-shift mechanism.

Synthesis and Photoinduced Electron Transfer Studies of a Tri(Phenothiazine)–Subphthalocyanine–Fullerene Pentad

A novel donor-acceptor pentad featuring subphthalocyanine and fullerene as the primary electron donor and acceptor, and three phenothiazine entities as secondary hole transferring agents, have been newly synthesized and characterized as an photosynthetic reaction center model compound. Occurrences of ultrafast photoinduced electron transfer (PET) and slower charge recombination are witnessed in the pentad from the femtosecond and nanosecond transient absorption studies.

Near‐IR Excitation Transfer and Electron Transfer in a BF2‐Chelated Dipyrromethane–Azadipyrromethane Dyad and Triad

A molecular dyad and triad, comprised of a known photosensitizer, BF(2)-chelated dipyrromethane (BDP), covalently linked to its structural analog and near-IR emitting sensitizer, BF(2)-chelated tetraarylazadipyrromethane (ADP), have been newly synthesized and the photoinduced energy and electron transfer were examined by femtosecond and nanosecond laser flash photolysis. The structural integrity of the newly synthesized compounds has been established by spectroscopic, electrochemical, and computational methods. The DFT calculations revealed a molecular-clip-type structure for the triad, in which the BDP and ADP entities are separated by about 14 Å with a dihedral angle between the fluorophores of around 70°. Differential pulse voltammetry studies have revealed the redox states, allowing estimation of the energies of the charge-separated states. Such calculations revealed a charge separation from the singlet excited BDP ((1)BDP*) to ADP (BDP(.+)-ADP(.-)) to be energetically favorable in nonpolar toluene and in polar benzonitrile. In addition, the excitation transfer from the singlet BDP to ADP is also envisioned due to good spectral overlap of the BDP emission and ADP absorption spectra. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of energy transfer from (1)BDP* to ADP (in benzonitrile and toluene) and electron transfer from BDP to (1)ADP* (in benzonitrile, but not in toluene). The kinetic study of energy transfer was measured by monitoring the rise of the ADP emission and revealed fast energy transfer (ca. 10(11) s(-1)) in these molecular systems. The kinetics of electron transfer via (1)ADP*, measured by monitoring the decay of the singlet ADP at λ=820 nm, revealed a relatively fast charge-separation process from BDP to (1)ADP*. These findings suggest the potential of the examined ADP-BDP molecules to be efficient photosynthetic antenna and reaction center models.

Co-facial magnesium porphyrin dimer complexed with fullerene: photosynthetic reaction center model of 'special pair' self-assembled to electron acceptor

Using a self-assembled supramolecular approach, a closer model for the photosynthetic reaction center 'special pair' assembled with an electron acceptor conjugate, is reported. As the 'special pair' donor, magnesium meso-(benzo-15-crown-5)porphyrin was self-assembled with K + to form a highly stable Mg porphyrin dimer with a co-facial geometry arrangement, which is a better electron donor since it is nearly 130 mV easier to oxidize compared to the earlier reported Zn porphyrin dimer analog. Further, pyridine and alkyl ammonium cation-functionalized fullerene was attached via axial coordination and crown ether-alkyl ammonium cation complexation to form the four-fold, donor-acceptor complex. The self-assembled supramolecular complex, constructed using this approach, possesses defined geometry and orientation. Efficient charge separation from the singlet excited state of the Mg porphyrin dimer to the fullerene entity and a relatively slow charge recombination, were revealed by time-resolved emission and nanosecond transient absorption spectral studies, supporting the impending exploitation of these novel biomimetic model systems for light energy harvesting applications.

Fullerene-porphyrin architectures; photosynthetic antenna and reaction center models.

Fullerenes and porphyrins are molecular architectures ideally suited for devising integrated, multicomponent model systems to transmit and process solar energy. Implementation of C60 as a 3-dimensional electron acceptor holds great expectations on account of their small reorganization energy in electron transfer reactions and has exerted a noteworthy impact on the improvement of light-induced charge-separation. This article describes how the specific compositions of porphyrin chromophores linked to C60--yielding artificial light harvesting antenna and reaction center mimics--have been elegantly utilized to tune the electronic couplings between donor and acceptor sites and the total reorganization energy. Specifically, the effects that these parameters have on the rate, yield and lifetime of the energetic charge-separated states are considered.

Development of exciplex chemistry: Some fundamental aspects

Abstract

Femtosecond‐Picosecond Laser Photolysis Studies on Photoinduced Electron Transfer Phenomena in Solutions

AbstractResults of our femtosecond‐picosecond laser photolysis studies on photoinduced electron transfer phenomena in solutions including exciplex dynamics and its solvent dependences, energy gap dependences of photoinduced charge separation and charge recombination of various geminate ion pairs, mechanisms of chemical reactions via exciplexes and ion pairs, dynamics of photoinduced election transfer in hydrogen bonding complexes, dynamics and mechanisms of photoinduced electron transfer in fixed distance donor acceptor dyads and photosynthetic reaction center models, and mechanisms of electron ejection from solute fluorescent state in polar solutions are summarized and discussed.

Chlorin-based supramolecular assemblies for artificial photosynthesis

Photosynthetic reaction center models consisting of zinc 9-desoxo-pyrochlorophyllide a primary electron donors, ZC, that are directly bonded at their 3-position to the 5-position of a 2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin, ZP, which is in turn bonded at its 15-position to 2-triptycenenaphthoquinone, 2-triptycenebenzoquinone, 1-triptycenebenzo-quinone, or N-(4-phenyl),N′-(n-octyl)-l,4,5,8-naphthalenediimide, 1, 2, 3, and 4, respectively, were prepared. Steric hindrance between adjacent substituents positions the π system of the chlorophyll perpendicular to that of the porphyrin. In turn, the π system of the chlorophyll is held about 60° to that of the O-O axis of the quinones in 1 and 2, parallel to the quinone O-O axis in 3, and parallel to the N-N axis in the diimide acceptor in 4. The resulting structures place the ZC donors in 1, 2, 3 and 4 at fixed center-to-center distances of 20, 18, 14, and 21 Å from the acceptors, respectively. Photoexcitation of 1–4 in 2-methyltetrahydrofuran glass at 77K results in a single observable electron transfer reaction: 1 ZC-ZP-X→ZC +-ZP-X −, where X = benzoquinone (BQ), naphthoquinone (NQ) or naphthalenediimide (NI), that occurs with τ = 4.5, 3.3, 2.0, and 2.0 ps for 1, 2, 3, and 4, respectively. The final ZC +-ZP-X − radical pairs live for 12.7, 8.4, 2.5, and 10 ms at 77K in 1, 2, 3, and 4, respectively, and exhibit spin-polarized EPR spectra characteristic of spin-correlated radical pairs. The EPR spectra of 1–4 can be simulated using the distances and orientations of the radicals relative to one another determined from the molecular structures of 1–4. These long-lived, spin-polarized radical ion pairs closely mimic the bacteriochlorophyll cation – quinone anion radical pair produced in photosynthetic reaction centers and provide a useful tool for studying the interaction of the surrounding medium with the charge separated radical ion pair.

Pioneering and Developing Exciplex Chemistry Part6. Investigations into Photo-Reaction Dynamics and Mechanisms of Photosynthetic Reaction Center Models and Proteins.

The multi-step and superexchange electron transfer mechanisms which are necessary for therapid formation of long-lived ion-pair state are discussed on the basis of our pioneering ultrafastlaser photolysis studies on multichromophoric photosynthetic reaction center models. Inaddition, results of our studies on time-resolved fluorescence decay of tryptophan in proteinare discussed from the viewpoint of protein dynamics which is an important basis for the elucidationof enzyme reactions.

Pioneering and Developing Exciplex Chemistry Part5. Elucidating the Most Fundamental Aspect of Electron Transfer Mehcanisms by Means of Ultrafast Laser Photolysis Studies on Distance-Fixed Porphyrin-Quinone Dyads.

General problems in relation to the investigations on the photosynthetic reaction centermodels including bichromophoric as well as multichromophoric systems are discussed. Moreover, the most fundamental aspects of electron transfer (ET) mechanisms are discussedon the basis of the results obtained by ultrafast laser photolysis studies on distance-fixedporphyrin-quinone dyads. The quite different nature of ET in the inverted region from that inthe normal region is demonstrated.

ChemInform Abstract: Synthesis of Quinone‐Linked Porphyrin Dimer, Trimer, and Tetramer as Models for Photosynthetic Reaction Center.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Synthesis of Quinone-linked Porphyrin Dimer, Trimer, and Tetramer As Models for Photosynthetic Reaction Center

Abstract As models for the photosynthetic reaction center, conformationally restricted quinone-linked porphyrin dimer, trimer, and tetramer were synthesized. The Soret bands of these zinc complexes are blue-shifted or split due to the exciton coupling depending on the geometries and compositions. The fluorescence intensities of these models are significantly reduced by the attached quinone, indicating the efficient electron transfer to the quinone.

Spin-Polarized Radical Ion Pair Formation Resulting from Two-step Electron Transfer from the Lowest Excited Singlet State of a Fixed-Distance Photosynthetic Model System at 5 K

Abstract A photosynthetic reaction center model consisting of a zinc porphyrin primary electron donor, ZP, positioned between a naphthoquinone electron acceptor, NQ, and an N,N,N,N–tetraalkyl–p–phenylenediamine secondary electron donor, TAPD, was synthesized. The resulting rigid structure places the TAPD donor at a fixed 23 A center–to–center distance from the NQ acceptor. Excitation of ZP with 540 nm light in butyronitrile glass at 5 K results in two–step sequential electron transfer: TAPD–1*ZP–NQ > TAPD–ZP+−NQ− −> TAPD+−ZP–NQ−. The final TAPD+−ZP–NQ− radical pair lives for 4 ms and exhibits a spin–polarized EPR spectrum characteristic of a spin–correlated radical pair. The EPR spectrum of this long–lived, spin–polarized radical ion pair closely mimics the bacteriochlorophyll cation – quinone anion radical pair produced in photosynthetic reaction centers.

ChemInform Abstract: High‐Quantum‐Yield Long‐Lived Charge Separation in a Photosynthetic Reaction Center Model.

Chemischer InformationsdienstVolume 17, Issue 1 Physical Organic Chemistry ChemInform Abstract: High-Quantum-Yield Long-Lived Charge Separation in a Photosynthetic Reaction Center Model. M. R. WASIELEWSKI, M. R. WASIELEWSKISearch for more papers by this authorM. P. NIEMCZYK, M. P. NIEMCZYKSearch for more papers by this authorW. A. SVEC, W. A. SVECSearch for more papers by this authorE. B. PEWITT, E. B. PEWITTSearch for more papers by this author M. R. WASIELEWSKI, M. R. WASIELEWSKISearch for more papers by this authorM. P. NIEMCZYK, M. P. NIEMCZYKSearch for more papers by this authorW. A. SVEC, W. A. SVECSearch for more papers by this authorE. B. PEWITT, E. B. PEWITTSearch for more papers by this author First published: January 7, 1986 https://doi.org/10.1002/chin.198601042Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume17, Issue1January 7, 1986 RelatedInformation

High-quantum-yield long-lived charge separation in a photosynthetic reaction center model

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTHigh-quantum-yield long-lived charge separation in a photosynthetic reaction center modelMichael R. Wasielewski, Mark P. Niemczyk, Walter A. Svec, and E. Bradley PewittCite this: J. Am. Chem. Soc. 1985, 107, 19, 5562–5563Publication Date (Print):September 1, 1985Publication History Published online1 May 2002Published inissue 1 September 1985https://doi.org/10.1021/ja00305a059RIGHTS & PERMISSIONSArticle Views437Altmetric-Citations156LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (278 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts