State Reaction Research Articles

Learning reaction coordinates via cross-entropy minimization: Application to alanine dipeptide

We propose a cross-entropy minimization method for finding the reaction coordinate from a large number of collective variables in complex molecular systems. This method is an extension of the likelihood maximization approach describing the committor function with a sigmoid. By design, the reaction coordinate as a function of various collective variables is optimized such that the distribution of the committor pB * values generated from molecular dynamics simulations can be described in a sigmoidal manner. We also introduce the L2-norm regularization used in the machine learning field to prevent overfitting when the number of considered collective variables is large. The current method is applied to study the isomerization of alanine dipeptide in vacuum, where 45 dihedral angles are used as candidate variables. The regularization parameter is determined by cross-validation using training and test datasets. It is demonstrated that the optimal reaction coordinate involves important dihedral angles, which are consistent with the previously reported results. Furthermore, the points with pB *∼0.5 clearly indicate a separatrix distinguishing reactant and product states on the potential of mean force using the extracted dihedral angles.

Nuclear quantum effects on the hydrogen bond donor-acceptor exchange in water-water and water-methanol dimers.

We present results from path integral molecular dynamics simulations that describe effects from the explicit incorporation of nuclear quantum fluctuations on the topology of the free energy associated with the geared exchange of hydrogen bonds in the water-water dimer. Compared to the classical treatment, our results reveal important reductions in the free energy barriers and changes at a qualitative level in the overall profile. Most notable are those manifested by a plateau behavior, ascribed to nuclear tunneling, which bridges reactant and product states, contrasting with the usual symmetric double-well profile. The characteristics of the proton localizations along the pathway are examined. An imaginary time analysis of the rotational degrees of freedom of the partners in the dimer at the vicinities of transition states shows a clear "anticorrelation" between intermolecular interactions coupling beads localized in connective and dangling basins of attractions. As such, the transfer is operated by gradual concerted inter-basin migrations in opposite directions, at practically no energy costs. Modifications operated by partial deuteration and by the asymmetries in the hydrogen bonding characteristics prevailing in water-methanol heterodimers are also examined.

The condition of neuro-endocrine systems in acute experimental myocardial infarction against the background of various reactivity of the organism

The adequacy of recovery processes in myocardial infarction (MI) significantly depends on the state of reactivity of the organism, which, in turn, is determined by the adequacy of neuro-endocrine regulation, in particular, hypothalamic-pituitary-adrenocortical (HPA) and hypothalamic-pituitary-thyroid (HPT) system system.Purpose of the study. To investigate the condition of HPA and HPT in acute experimental MI against the background of different reactivity of the organism.Material and methods. The study was performed on 20 outbred adult dogs weighing 8-12 kg, in which MI was simulated by ligation of the anterior interventricular artery. Animals were divided into three groups (n =5 in each group). The condition of hyperreactivity was caused by the introduction of pyrogenal (group 2), the state of hyporeactivity - the introduction of azothioprine (group 3), in group 1 drugs were not used (state of normoreactivity). In the control group used 5 falsely operated dogs with thoracopericardiotomy. In animals before surgery, on 1, 4, 7, 11 and 15 days in the blood was determined by the content of adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH) , cortisol (Cr), aldosterone (Ald), thyroxine and triiodothyronine. Statistical processing was performed using license packages Statistica 5.5 (Stat Soft Rus), Statistica Neural Networks (Statsoft Inc.).Results. Normoreactive course of myocardial infarction was characterized by primary activation of HPA, which was most pronounced for Cortisol, followed by restoration of ACTH and Aldosterone levels and duration of activation of the central link up to 4 days, peripheral - up to 7-11 days; reciprocal in relation to HPA inhibition of the functional activity of the HPT, most pronounced on the 4th day and lasting up to 15 days. Such changes caused the healing of myocardial infarction through the formation of a full-fledged post-infarction scar. The hyperreactive course of myocardial infarction was characterized by hypersecretion of ACTH with hypercorticism and hyperaldosteronism and less pronounced suppression of HPT with the restoration of its functional activity on the 7th day. The hyporeactive course of myocardial infarction was characterized by a minimally pronounced and delayed up to 4 days activation of HPA with Aldosterone hyposecretion on the 15th day; deep hypothyroidism of both central and peripheral genesis. Impaired reactivity was accompanied by an increase in the area of necrosis and a decrease in the thickness of the heart wall with the formation of postinfarction aneurysm.Conclusions. Desynchronization of necrotic and reparative processes with a change in reactivity leads to a disruption in the relationship between necrosis and repair with the formation of postinfarction aneurysm, which is accompanied by different activities of the HPA and HPT.

Measuring Reactive Sulfur Species and Thiol Oxidation States: Challenges and Cautions in Relation to Alkylation-Based Protocols.

Significance: Redox biology is gaining ground in research related to human physiology (metabolism, signaling), pathophysiology (cancer, cardiovascular disease, neurodegeneration), and toxicology (radiation- or xenobiotic-induced damage). A major hurdle in advancing redox medicine is the current lack of understanding the mechanisms underpinning the observed detrimental or beneficial in vivo effects. To gain deeper insights into the underlying molecular pathways of redox regulation, we need to appreciate the strengths and limitations of the currently available methods. Recent Advances: Reactive sulfur species (RSS), including cysteine derivatives of peptides and proteins along with small molecules such as hydrogen sulfide or inorganic polysulfides, are major players in redox biology. RSS-mediated regulation of protein functions is a widely studied mechanism in the field, and considerable efforts have been devoted to the development of selective detection methods. Critical Issues: A large number of available methods rely on an alkylation step to freeze the dynamism of consecutive oxidation and reduction events among RSS at a particular time point inside the cell. This process uses the assumption that alkylation blocks all redox events instantaneously. We argue that unfortunately this is often not the case, which could have serious impacts on detected sulfur species speciation and confound experimental results. Future Directions: Novel technologies and prudent optimization of existing methods to accurately characterize the dynamic redox status of the thiol proteome as well as detailed understanding of regulatory and signaling capacities of protein polysulfidation are crucial to open new routes toward therapeutic interventions.

Association of the Novel Inflammatory Marker GlycA and Incident Heart Failure and Its Subtypes of Preserved and Reduced Ejection Fraction: The Multi-Ethnic Study of Atherosclerosis.

GlycA, a nuclear magnetic resonance composite marker of systemic inflammation, reflects serum concentration and glycosylation state of main acute phase reactants. Prior studies have shown plasma GlycA levels were associated with cardiovascular disease even after adjusting for other inflammatory markers. However, little is known about the association of GlycA with the heart failure (HF) subtypes: heart failure with preserved ejection fraction (HFpEF) or heart failure with reduced ejection fraction. We examined the association of GlycA with incident HF and its subtypes in a multiethnic cohort. We studied 6507 Multi-Ethnic Study of Atherosclerosis participants aged 45 to 84 without baseline cardiovascular disease or HF who had data on GlycA and incident hospitalized HF. We used multivariable-adjusted Cox hazards models to evaluate the association of GlycA with incident total HF, HFpEF, and heart failure with reduced ejection fraction. Models were adjusted for sociodemographics, cardiovascular disease risk factors, and inflammatory biomarkers. The mean (SD) for age was 62 (10) years and for GlycA was 375 (82) μmol/L; 53% women. Over a median follow-up of 14.0 years, participants in the highest quartile of GlycA, compared with the lowest, experienced increased risk of developing any HF (hazard ratio, 1.48 [95% CI, 1.01-2.18]) in fully adjusted models. However, this increased risk was only seen for HFpEF (2.18 [1.15-4.13]) and not heart failure with reduced ejection fraction [1.06 (0.63-1.79)]. There was no significant interaction by sex, age, or race/ethnicity. GlycA was associated with an increased risk of any HF, and in particular, HFpEF. Future studies should examine mechanisms that might explain differential association of GlycA with HF subtypes, and whether therapeutic lowering of GlycA can prevent HFpEF development. Registration: URL: https://www.clinicaltrials.gov; Unique identifier: NCT00005487.

Excited State Vibrations of Isotopically Labeled FMN Free and Bound to a Light-Oxygen-Voltage (LOV) Protein.

Flavoproteins are important blue light sensors in photobiology and play a key role in optogenetics. The characterization of their excited state structure and dynamics is thus an important objective. Here, we present a detailed study of excited state vibrational spectra of flavin mononucleotide (FMN), in solution and bound to the LOV-2 (Light-Oxygen-Voltage) domain of Avena sativa phototropin. Vibrational frequencies are determined for the optically excited singlet state and the reactive triplet state, through resonant ultrafast femtosecond stimulated Raman spectroscopy (FSRS). To assign the observed spectra, vibrational frequencies of the excited states are calculated using density functional theory, and both measurement and theory are applied to four different isotopologues of FMN. Excited state mode assignments are refined in both states, and their sensitivity to deuteration and protein environment are investigated. We show that resonant FSRS provides a useful tool for characterizing photoactive flavoproteins and is able to highlight chromophore localized modes and to record hydrogen/deuterium exchange.

Machine Learning for Observables: Reactant to Product State Distributions for Atom-Diatom Collisions.

Machine learning based models to predict product state distributions from a distribution of reactant conditions for atom-diatom collisions are presented and quantitatively tested. The models are based on function-, kernel-, and grid-based representations of the reactant and product state distributions. All three methods predict final state distributions from explicit quasi-classical trajectory simulations with R2 > 0.998. Although a function-based approach is found to be more than two times better in computational performance, the grid-based approach is preferred in terms of prediction accuracy, practicability, and generality. For the function-based approach, the choice of parametrized functions is crucial and this aspect is explicitly probed for final vibrational state distributions. Applications of the grid-based approach to nonequilibrium, multitemperature initial state distributions are presented, a situation common to energy and state distributions in hypersonic flows. The role of such models in direct simulation Monte Carlo and computational fluid dynamics simulations is also discussed.

Central Nervous System Targets: Glial Cell Mechanisms in Chronic Pain.

Interactions between central glial cells and neurons in the pain circuitry are critical contributors to the pathogenesis of chronic pain. In the central nervous system (CNS), two major glial cell types predominate: astrocytes and microglia. Injuries or pathological conditions which evoke pain are concurrently associated with the presence of a reactive microglia or astrocyte state, which is characterized by a variety of changes in the morphological, molecular, and functional properties of these cells. In this review, we highlight the changes that reactive microglia and astrocytes undergo following painful injuries and insults and discuss the critical and interactive role these two cell types play in the initiation and maintenance of chronic pain. Additionally, we focus on several crucial mechanisms by which microglia and astrocytes contribute to chronic pain and provide commentary on the therapeutic promise of targeting these pathways. In particular, we discuss how the inflammasome in activated microglia drives maturation and release of key pro-inflammatory cytokines, which drive pain through neuronal- and glial regulations. Moreover, we highlight several potentially-druggable hemichannels and proteases produced by reactive microglia and astrocytes in pain states and discuss how these pathways regulate distinct phases during pain pathogenesis. We also review two emerging areas in chronic pain research: 1) sexually dimorphic glial cell signaling and 2) the role of oligodendrocytes. Finally, we highlight important considerations for potential pain therapeutics targeting glial cell mediators as well as questions that remain in our conceptual understanding of glial cell activation in pain states.

Co-Designed Exposure Protocol in the Study of Idiopathic Environmental Intolerance Attributed to Electromagnetic Fields.

The hypothesis of an electromagnetic origin of idiopathic environmental intolerance (IEI) attributed to electromagnetic fields (EMF) has been widely investigated by provocation studies, which consist of deliberately exposing people with IEI-EMF in laboratory settings to particular EMF to observe volunteers' reactions. In the majority of these studies, reactions have been found to be independent of exposure. However, most of these studies sufferfrom design and methodological limitations that might biastheir findings or reducetheir precision. As provocation studies are best suited for isolating the effects of EMF, innovative protocols should be applied. In the ExpoComm project (PNREST Anses, EST/2017/2 RF/19), several innovations have been introduced: the involvement of people with IEI-EMF in the development of the protocol, the attenuation of the anxiogenic nature of the tests, the individualization of the protocol, the validation of the neutral or normal reactivity state before the test, and the use of a cocktail of real, rather than artificially generated, sources. The objective of involving people with IEI-EMF was to increase the relevance and acceptability of the protocol, while respecting technical constraints and scientific quality requirements. This paper describes the protocol resulting from the collaborative process. Bioelectromagnetics. 2020;41:425-437. © 2020 Bioelectromagnetics Society.

Semiclassical instanton formulation of Marcus-Levich-Jortner theory.

Marcus-Levich-Jortner (MLJ) theory is one of the most commonly used methods for including nuclear quantum effects in the calculation of electron-transfer rates and for interpreting experimental data. It divides the molecular problem into a subsystem treated quantum-mechanically by Fermi's golden rule and a solvent bath treated by classical Marcus theory. As an extension of this idea, we here present a "reduced" semiclassical instanton theory, which is a multiscale method for simulating quantum tunneling of the subsystem in molecular detail in the presence of a harmonic bath. We demonstrate that instanton theory is typically significantly more accurate than the cumulant expansion or the semiclassical Franck-Condon sum, which can give orders-of-magnitude errors and, in general, do not obey detailed balance. As opposed to MLJ theory, which is based on wavefunctions, instanton theory is based on path integrals and thus does not require solutions of the Schrödinger equation nor even global knowledge of the ground- and excited-state potentials within the subsystem. It can thus be efficiently applied to complex, anharmonic multidimensional subsystems without making further approximations. In addition to predicting accurate rates, instanton theory gives a high level of insight into the reaction mechanism by locating the dominant tunneling pathway as well as providing similar information to MLJ theory on the bath activation energy and the vibrational excitation energies of the subsystem states involved in the reaction.

Theoretical Study of Diels-Alder Reaction of But-3-en-2-one with Hexa-1,2,4-triene: A Density Functional Theory Study

The Diels-Alder reaction between but-3-en-2-one with hexa-1,2,4-triene was studied using density functional theory method at B3LYP-D3/6-311++G(d,p) level of theory. The geometries of the transition states were determined. Moreover, calculations of the vibrational frequencies permitted computation of the activation enthalpies and entropies. The computational results show that the cycloadducts from trans conformer have the lower relative energies (−46.48 and −47.50 kcal/mol) as compared to the cis conformer of cycloadducts (−44.45 and −45.87 kcal/mol). The global reactivity indices were analyzed at the ground state of reactants to predict the reactivity of the studied organic molecules in the cycloaddition reactions. The electronic chemical potential of hexa-1,2,4-trien found to be than but-3-en-2-one, which indicates that the net charge transfer will be from hexa-1,2,4-trien toward the electron-deficient but-3-en-2-one reactant.

The Expansion of Prevent: On The Politics of Legibility, Opacity And Decolonial Critique

It is argued here that the liberal state has authoritarian aspects that are irreducible to the authoritarian aspects of neoliberalism. The argument draws on James Scott's work on modern state ruling through bureaucratic 'legibility', and the decolonial work of S. Sayyid on how a form of political Islam he calls 'Islamism' challenges the west's construction of modernity as an intrinsically western project. The state's need for legibility undermines democracy by seeking to shape political debate and political activity to fit its bureaucratic channels for engagement, and Islamophobia caused by the UK state's reaction to Islamism, shapes how the UK state seeks control via legibility. Prevent expanded in 2011 from focusing on 'violent extremism' to 'extremism', with extremism defined in terms of normative commitments the state takes to be in tension with its conception of 'British values'. The state defined the Muslim population as opaque because they were taken to not be socially integrated. This was used to justify a repressive ubiquitous surveillance based on what is termed here a 'legibility of symptoms'. This was presented, after 2015, as paternalistic 'safeguarding', when workers in public sector bureaucracies became legally obligated to carry out Prevent surveillance. Left-wing and environmental organisations engaged in extra-parliamentary protest are now as defined as potentially extremist. With the expansion of Prevent in 2011, the state created a 'pre-crime' space in civil society that is taken to justify repressive surveillance, presented as paternalistic safeguarding to save individuals 'at risk' of 'radicalisation' from going on to commit criminal acts.

CD49f Is a Novel Marker of Functional and Reactive Human iPSC-Derived Astrocytes

New methods for investigating human astrocytes are urgently needed, given their critical role in the central nervous system. Here we show that CD49f is a novel marker for human astrocytes, expressed in fetal and adult brains from healthy and diseased individuals. CD49f can be used to purify fetal astrocytes and human induced pluripotent stem cell (hiPSC)-derived astrocytes. We provide single-cell and bulk transcriptome analyses of CD49f+ hiPSC-astrocytes and demonstrate that they perform key astrocytic functions invitro, including trophic support of neurons, glutamate uptake, and phagocytosis. Notably, CD49f+ hiPSC-astrocytes respond to inflammatory stimuli, acquiring an A1-like reactive state, in which they display impaired phagocytosis and glutamate uptake and fail to support neuronal maturation. Most importantly, we show that conditioned medium from human reactive A1-like astrocytes is toxic to human and rodent neurons. CD49f+ hiPSC-astrocytes are thus a valuable resource for investigating human astrocyte function and dysfunction in health and disease.

Inhibition of STAT3 phosphorylation attenuates impairments in learning and memory in 5XFAD mice, an animal model of Alzheimer's disease

The pathophysiological roles of astrocytes in the reactive state are thought to have important significance in the pathogenesis of neurodegenerative diseases, including Alzheimer's disease (AD). However, the detailed mechanisms underlying the transition of astrocytes from the resting state to the reactive state during neurodegenerative disease largely remain to be defined. Here, we investigated the pathways involved in activating astrocytes from the resting state to the reactive state in primary cultured astrocytes treated with oligomeric Aβ and in the hippocampus of 5XFAD mice. Treatment with oligomeric Aβ induced an increase in reactive astrocytes, as assessed by the protein level of glial fibrillary acidic protein (GFAP) and this increase was caused by STAT3 phosphorylation in primary cultured astrocytes. The administration of Stattic, an inhibitor of STAT3, rescued the activation of astrocytes in primary cultured astrocytes and in the hippocampus of 6-month-old 5XFAD mice as well as impairments in learning and memory. Collectively, these results demonstrated that reactive astrocytes in the AD brain are induced via STAT3 and the impairments in learning and memory observed in 5XFAD mice are rescued by STAT3 inhibition, suggesting that the inhibition of STAT3 phosphorylation in astrocytes may be a novel therapeutic target for cognitive impairment in AD.

Generalizing the Marcus equation

The Marcus equation for the rate of an electron-transfer reaction can be generalized to cover larger electronic-interaction matrix elements, irregular free-energy surfaces, and coupling to multiple vibrational modes and to recognize the different effects of vibrational relaxations and pure dephasing. Almost all the information needed to calculate the rate constant can be obtained from a quantum-classical molecular dynamics simulation of the system in the reactant state. Because the final expression for the rate constant does not depend on the reorganization energy, it is insensitive to slow relaxations that follow the reaction.

Global targeting of functional (phospho)tyrosines using sulfur-triazole exchange chemistry

Abstract Chemoproteomic probes serve as valuable tools to study and perturb protein function on a global scale. Despite advances in methodologies for cysteine and lysine profiling, a large fraction of the human proteome remains inaccessible with current activity-based probes. Here, we describe development and application of sulfur-triazole exchange (SuTEx) chemistry for developing covalent probes with broad applications for chemical proteomics. We show modifications to the triazole leaving group can produce sulfonyl probes with high chemoselectivity for tyrosines over other nucleophilic amino acids to investigate thousands of tyrosine sites in lysates and live cells. We discover hyper-reactive tyrosines are enriched in enzymatic, protein–protein interaction and nucleotide recognition domains. We apply SuTEx as a chemical phosphoproteomics strategy to monitor activation of phosphotyrosine sites. Our findings illustrate the broad potential for deploying SuTEx to globally investigate tyrosine reactivity, function and post-translational modification state in complex biological systems.

Vimentin citrullination probed by a novel monoclonal antibody serves as a specific indicator for reactive astrocytes in neurodegeneration.

Vimentin citrullination, the calcium (Ca2+ )-dependent peptidylarginine deiminase (PAD)-mediated conversion of an arginine residue of vimentin to a citrulline residue, has emerged as a pathophysiological outcome of autoimmune diseases and neurodegeneration. However, the roles, functions, and expression of citrullinated vimentin have not yet been elucidated because available antibodies are limited. We developed mouse monoclonal IgG1 and IgM specific for vimentin citrullinated at position R440 or R450 and applied Western blotting, immunohistochemistry, and immunofluorescent staining to investigate the pathogenesis of prion diseases in animal models, in patients with prion diseases, and in vitro. Vimentin was found to be highly citrullinated at R440 and R450, and these citrullinated forms were mainly expressed in reactive astrocytes in the brain tissues of scrapie-infected mice. Full-length and cleaved forms of citrullinated vimentin were found in the cerebral cortices of sporadic Creutzfeldt-Jakob disease (sCJD) patients. The distribution of citrullinated vimentin was mainly confirmed in vimentin-, GFAP-, and YKL-40-positive reactive astrocytes. Biochemically, citrullination promoted resistance to the caspase-3- and caspase-9-mediated fragmentation of vimentin. Additionally, citrullination led to increased cytoplasmic and integral membrane/organelle vimentin enrichment, which indicated changes in the intrinsic solubility and distribution of vimentin. Our observations suggest that citrullinated vimentin acts as a specific indicator for the reactive state of astrocytes under abnormal neurological conditions. In addition, these novel antibodies will be helpful for studying the role of citrullinated vimentin in the pathogenesis of human disorders.

Controlling the nonadiabatic electron-transfer reaction rate through molecular-vibration polaritons in the ultrastrong coupling regime

Recent experiments showed that the chemical reaction rate is modified, either increased or decreased, by strongly coupling a nuclear vibration mode to the single mode of an optical cavity. Herein we investigate how the rate of an electron-transfer reaction depends on the molecule-cavity coupling in the ultrastrong coupling regime, where the coupling strength is comparable in magnitude with both the vibrational and the cavity frequencies. We found two main factors that determine the modification of the reaction rate: the relative shifts of the energy levels induced by the coupling and the mixing of the ground and excited states of molecular vibration in the ground state of the hybrid molecule-plus-cavity system through which the Franck-Condon factor between the initial and final states of the transition is altered. The former is the dominant factor if the molecule-cavity coupling strengths for the reactant and product states differ significantly from each other and gives rise to an increase in the reaction rate over a wide range of system’s parameters. The latter dominates if the coupling strengths and energy levels of the reactant and product states are close to each other and it leads to a decrease in the reaction rate. The effect of the mixing of molecular vibrational states on the reaction rate is, however, suppressed in a system containing a large number of molecules due to the collective nature of the resulting polariton, and thus should be observed in a system containing a small number of molecules. In contrast, the effect of the relative shifts of the energy levels should be essentially independent of the number of molecules coupled to the cavity.

Effect of Caloric Restriction on the in vivo Functional Properties of Aging Microglia.

Throughout the lifespan, microglia, the primary innate immune cells of the brain, fulfill a plethora of homeostatic as well as active immune defense functions, and their aging-induced dysfunctionality is now considered as a key trigger of aging-related brain disorders. Recent evidence suggests that both organism’s sex and age critically impact the functional state of microglia but in vivo determinants of such state(s) remain unclear. Therefore, we analyzed in vivo the sex-specific functional states of microglia in young adult, middle aged and old wild type mice by means of multicolor two-photon imaging, using the microglial Ca2 + signaling and directed process motility as main readouts. Our data revealed the sex-specific differences in microglial Ca2 + signaling at all ages tested, beginning with young adults. Furthermore, for both sexes it showed that during the lifespan the functional state of microglia changes at least twice. Already at middle age the cells are found in the reactive or immune alerted state, characterized by heightened Ca2 + signaling but normal process motility whereas old mice harbor senescent microglia with decreased Ca2 + signaling, and faster but disorganized directed movement of microglial processes. The 6–12 months long caloric restriction (70% of ad libitum food intake) counteracted these aging-induced changes shifting many but not all functional properties of microglia toward a younger phenotype. The improvement of Ca2 + signaling was more pronounced in males. Importantly, even short-term (6-week-long) caloric restriction beginning at old age strongly improved microglial process motility and induced a significant albeit weaker improvement of microglial Ca2 + signaling. Together, these data provide first sex-specific in vivo characterization of functional properties of microglia along the lifespan and identify caloric restriction as a potent, cost-effective, and clinically relevant tool for rejuvenation of microglia.

Excited-State Vibronic Dynamics of Bacteriorhodopsin from Two-Dimensional Electronic Photon Echo Spectroscopy and Multiconfigurational Quantum Chemistry.

Owing to the ultrafast time scale of the photoinduced reaction and high degree of spectral overlap among the reactant, product, and excited electronic states in bacteriorhodopsin (bR), it has been a challenge for traditional spectroscopies to resolve the interplay between vibrational dynamics and electronic processes occurring in the retinal chromophore of bR. Here, we employ ultrafast two-dimensional electronic photon echo spectroscopy to follow the early excited-state dynamics of bR preceding the isomerization. We detect an early periodic photoinduced absorptive signal that, employing a hybrid multiconfigurational quantum/molecular mechanical model of bR, we attribute to periodic mixing of the first and second electronic excited states (S1 and S2, respectively). This recurrent interaction between S1 and S2, induced by a bond length alternation of the retinal chromohore, supports the hypothesis that the ultrafast photoisomerization in bR is initiated by a process involving coupled nuclear and electronic motion on three different electronic states.