Dynamic auditory contributions to error detection revealed in the discrimination of Same and Different syllable pairs.

During speech production auditory and motor regions within the sensorimotor dorsal stream operate in concert to facilitate online error detection. As the dorsal stream also is known to activate in speech perception, the purpose of the current study was to probe the role of auditory regions in error detection during auditory discrimination tasks as stimuli are encoded and maintained in working memory. A priori assumptions are that sensory mismatch (i.e., error) occurs during the discrimination of Different (mismatched) but not Same (matched) syllable pairs. Independent component analysis was applied to raw EEG data recorded from 42 participants to identify bilateral auditory alpha rhythms, which were decomposed across time and frequency to reveal robust patterns of event related synchronization (ERS; inhibition) and desynchronization (ERD; processing) over the time course of discrimination events. Results were characterized by bilateral peri-stimulus alpha ERD transitioning to alpha ERS in the late trial epoch, with ERD interpreted as evidence of working memory encoding via Analysis by Synthesis and ERS considered evidence of speech-induced-suppression arising during covert articulatory rehearsal to facilitate working memory maintenance. The transition from ERD to ERS occurred later in the left hemisphere for Different trials than for Same trials, with ERD and ERS temporally overlapping during the early post-stimulus window. Results were interpreted to suggest that the sensory mismatch (i.e., error) arising from the comparison of the first and second syllable elicits further processing in the left hemisphere to support working memory encoding and maintenance. Results are consistent with auditory contributions to error detection during both encoding and maintenance stages of working memory, with encoding stage error detection associated with stimulus concordance and maintenance stage error detection associated with task-specific retention demands.

Audiovisual incongruence differentially impacts left and right hemisphere sensorimotor oscillations: Potential applications to production.

Speech production gives rise to distinct auditory and somatosensory feedback signals which are dynamically integrated to enable online monitoring and error correction, though it remains unclear how the sensorimotor system supports the integration of these multimodal signals. Capitalizing on the parity of sensorimotor processes supporting perception and production, the current study employed the McGurk paradigm to induce multimodal sensory congruence/incongruence. EEG data from a cohort of 39 typical speakers were decomposed with independent component analysis to identify bilateral mu rhythms; indices of sensorimotor activity. Subsequent time-frequency analyses revealed bilateral patterns of event related desynchronization (ERD) across alpha and beta frequency ranges over the time course of perceptual events. Right mu activity was characterized by reduced ERD during all cases of audiovisual incongruence, while left mu activity was attenuated and protracted in McGurk trials eliciting sensory fusion. Results were interpreted to suggest distinct hemispheric contributions, with right hemisphere mu activity supporting a coarse incongruence detection process and left hemisphere mu activity reflecting a more granular level of analysis including phonological identification and incongruence resolution. Findings are also considered in regard to incongruence detection and resolution processes during production.

Sensorimotor contributions to working memory differ between the discrimination of Same and Different syllable pairs.

Sensorimotor activity during speech perception is both pervasive and highly variable, changing as a function of the cognitive demands imposed by the task. The purpose of the current study was to evaluate whether the discrimination of Same (matched) and Different (unmatched) syllable pairs elicit different patterns of sensorimotor activity as stimuli are processed in working memory. Raw EEG data recorded from 42 participants were decomposed with independent component analysis to identify bilateral sensorimotor mu rhythms from 36 subjects. Time frequency decomposition of mu rhythms revealed concurrent event related desynchronization (ERD) in alpha and beta frequency bands across the peri- and post-stimulus time periods, which were interpreted as evidence of sensorimotor contributions to working memory encoding and maintenance. Left hemisphere alpha/beta ERD was stronger in Different trials than Same trials during the post-stimulus period, while right hemisphere alpha/beta ERD was stronger in Same trials than Different trials. A between-hemispheres contrast revealed no differences during Same trials, while post-stimulus alpha/beta ERD was stronger in the left hemisphere than the right during Different trials. Results were interpreted to suggest that predictive coding mechanisms lead to repetition suppression effects in Same trials. Mismatches arising from predictive coding mechanisms in Different trials shift subsequent working memory processing to the speech-dominant left hemisphere. Findings clarify how sensorimotor activity differentially supports working memory encoding and maintenance stages during speech discrimination tasks and have potential to inform sensorimotor models of speech perception and working memory.

Influences of cognitive load on sensorimotor contributions to working memory: An EEG investigation of mu rhythm activity during speech discrimination.

Sensorimotor activity during speech perception is highly variable and is thought to be related to the underlying cognitive processes recruited to meet task demands. The purpose of the current study was to evaluate the impact of cognitive load on sensorimotor-based attention and working memory processes during speech perception. Manipulations of set size and signal clarity were employed to alter cognitive load. Raw EEG data recorded from 42 subjects during accurate discrimination of CV syllable pairs were decomposed by Independent component analysis; identifying sensorimotor mu components from 37 subjects. Time-frequency analyses revealed event related desynchronization (ERD) across alpha and beta frequency bands during and following stimulus presentation in all conditions, reflecting working memory maintenance through covert articulatory rehearsal. No early attentional activity was observed, suggesting adaptation to tasks. However, modulation of late working memory activity was observed between degraded and non-degraded conditions. Weak and delayed alpha and beta ERD in degraded conditions were interpreted as evidence of delayed implementation of covert rehearsal due to the prolonged time necessary to extract a phonological representation from the auditory signal. Findings are interpreted within Analysis by Synthesis to characterize the multi-faceted and temporally distinct contributions of anterior sensorimotor regions to working memory in support of speech discrimination.

Sex differences in early sensorimotor processing for speech discrimination.

Sensorimotor activity in speech perception tasks varies as a function of context, cognitive load, and cognitive ability. This study investigated listener sex as an additional variable. Raw EEG data were collected as 21 males and 21 females discriminated /ba/ and /da/ in quiet and noisy backgrounds. Independent component analyses of data from accurately discriminated trials identified sensorimotor mu components with characteristic alpha and beta peaks from 16 members of each sex. Time-frequency decompositions showed that in quiet discrimination, females displayed stronger early mu-alpha synchronization, whereas males showed stronger mu-beta desynchronization. Findings indicate that early attentional mechanisms for speech discrimination were characterized by sensorimotor inhibition in females and predictive sensorimotor activation in males. Both sexes showed stronger early sensorimotor inhibition in noisy discrimination conditions versus in quiet, suggesting sensory gating of the noise. However, the difference in neural activation between quiet and noisy conditions was greater in males than females. Though sex differences appear unrelated to behavioral accuracy, they suggest that males and females exhibit early sensorimotor processing for speech discrimination that is fundamentally different, yet similarly adaptable to adverse conditions. Findings have implications for understanding variability in neuroimaging data and the male prevalence in various neurodevelopmental disorders with inhibitory dysfunction.

The Application of EEG Mu Rhythm Measures to Neurophysiological Research in Stuttering.

Deficits in basal ganglia-based inhibitory and timing circuits along with sensorimotor internal modeling mechanisms are thought to underlie stuttering. However, much remains to be learned regarding the precise manner how these deficits contribute to disrupting both speech and cognitive functions in those who stutter. Herein, we examine the suitability of electroencephalographic (EEG) mu rhythms for addressing these deficits. We review some previous findings of mu rhythm activity differentiating stuttering from non-stuttering individuals and present some new preliminary findings capturing stuttering-related deficits in working memory. Mu rhythms are characterized by spectral peaks in alpha (8-13 Hz) and beta (14-25 Hz) frequency bands (mu-alpha and mu-beta). They emanate from premotor/motor regions and are influenced by basal ganglia and sensorimotor function. More specifically, alpha peaks (mu-alpha) are sensitive to basal ganglia-based inhibitory signals and sensory-to-motor feedback. Beta peaks (mu-beta) are sensitive to changes in timing and capture motor-to-sensory (i.e., forward model) projections. Observing simultaneous changes in mu-alpha and mu-beta across the time-course of specific events provides a rich window for observing neurophysiological deficits associated with stuttering in both speech and cognitive tasks and can provide a better understanding of the functional relationship between these stuttering symptoms. We review how independent component analysis (ICA) can extract mu rhythms from raw EEG signals in speech production tasks, such that changes in alpha and beta power are mapped to myogenic activity from articulators. We review findings from speech production and auditory discrimination tasks demonstrating that mu-alpha and mu-beta are highly sensitive to capturing sensorimotor and basal ganglia deficits associated with stuttering with high temporal precision. Novel findings from a non-word repetition (working memory) task are also included. They show reduced mu-alpha suppression in a stuttering group compared to a typically fluent group. Finally, we review current limitations and directions for future research.

Power and phase coherence in sensorimotor mu and temporal lobe alpha components during covert and overt syllable production.

The sensorimotor dorsal stream is known to activate in both overt and covert speech production. However, overt production produces sensory consequences that are absent during covert production. Thus, the purpose of the current study is to investigate differences in dorsal stream activity between these two production conditions across the time course of utterances. Electroencephalography (EEG) was recorded from 68 channels while 23 participants overtly (Op) and covertly (Cp) produced orthographically cued bisyllabic targets. Sensorimotor mu and auditory alpha components (from anterior and posterior aspects of the dorsal stream) were identified using independent component analysis (ICA). Event-related spectral perturbation (ERSP) analyses identified changes in mu and alpha oscillatory power over time, while intercomponent phase coherence (IPC) measured anterior-posterior connectivity in the two conditions. Results showed greater beta (15-25 Hz) suppression during speech planning across left and right hemisphere sensorimotor and temporal ICs for Op relative to Cp. By contrast, greater intrahemispheric beta coherence was observed for Cp compared to Op during speech planning. During execution, greater beta suppression was observed along with greater low frequency (< 10 Hz) power enhancement and intrahemispheric phase coherence in Op compared to Cp. The findings implicate low frequency sensorimotor and posterior temporal phase coherence in the integration of somatosensory and acoustic feedback in overt relative to covert execution. Findings are consistent with early frontal-temporal forward models involved in planning and execution with modulations depending on whether the task goal is internal or overt syllable production.

Evaluating the efficiency of a photoelectrochemical electrode constructed with photosystem II-enriched thylakoid membrane fragments.

The photoelectrochemical electrode has been intensively studied in recent years as a means of generating electricity from light through the use of intact thylakoid membranes or highly purified photosystem II. PSII-enriched thylakoid membrane fragments (PSII-BBY), also have the potential to construct the photoelectrochemical anode. In this study, we examined the feasibility of utilizing PSII-BBY preparations to construct a relatively inexpensive photoelectrochemical anode with a comparable current density and a reasonable stability. Intact thylakoid membrane based photoelectrochemical electrode was also constructed to compare with the PSII-BBY based photoelectrochemical electrode with respect to the protein activity and current density. In addition, the practicability of replacing the popular gold nanoparticle modified gold slide with multi-walled carbon nanotube modified indium tin oxide coated slides was tested. In order to understand the surface change during slide surface modification, an atomic force microscope (AFM) was used to image the topography of the slide. Above all, we observed a current density of 20.44 ±â€¯1.58 µA/cm2 when PSII-BBY was used to construct the photoelectrochemical anode. Moreover, the PSII-BBY based photoelectrochemical anode showed high stability over time with the current decreasing at a rate of 0.78%/h.

Trait related sensorimotor deficits in people who stutter: An EEG investigation of µ rhythm dynamics during spontaneous fluency.

Stuttering is associated with compromised sensorimotor control (i.e., internal modeling) across the dorsal stream and oscillations of EEG mu (µ) rhythms have been proposed as reliable indices of anterior dorsal stream processing. The purpose of this study was to compare µ rhythm oscillatory activity between (PWS) and matched typically fluent speakers (TFS) during spontaneously fluent overt and covert speech production tasks. Independent component analysis identified bilateral µ components from 24/27 PWS and matched TFS that localized over premotor cortex. Time-frequency analysis of the left hemisphere µ clusters demonstrated significantly reduced µ-α and µ-ß ERD (pCLUSTER < 0.05) in PWS across the time course of overt and covert speech production, while no group differences were found in the right hemisphere in any condition. Results were interpreted through the framework of State Feedback Control. They suggest that weak forward modeling and evaluation of sensory feedback across the time course of speech production characterizes the trait related sensorimotor impairment in PWS. This weakness is proposed to represent an underlying sensorimotor instability that may predispose the speech of PWS to breakdown.

The Effects of Fluency Enhancing Conditions on Sensorimotor Control of Speech in Typically Fluent Speakers: An EEG Mu Rhythm Study.

Objective: To determine whether changes in sensorimotor control resulting from speaking conditions that induce fluency in people who stutter (PWS) can be measured using electroencephalographic (EEG) mu rhythms in neurotypical speakers. Methods: Non-stuttering (NS) adults spoke in one control condition (solo speaking) and four experimental conditions (choral speech, delayed auditory feedback (DAF), prolonged speech and pseudostuttering). Independent component analysis (ICA) was used to identify sensorimotor µ components from EEG recordings. Time-frequency analyses measured µ-alpha (8-13 Hz) and µ-beta (15-25 Hz) event-related synchronization (ERS) and desynchronization (ERD) during each speech condition. Results: 19/24 participants contributed µ components. Relative to the control condition, the choral and DAF conditions elicited increases in µ-alpha ERD in the right hemisphere. In the pseudostuttering condition, increases in µ-beta ERD were observed in the left hemisphere. No differences were present between the prolonged speech and control conditions. Conclusions: Differences observed in the experimental conditions are thought to reflect sensorimotor control changes. Increases in right hemisphere µ-alpha ERD likely reflect increased reliance on auditory information, including auditory feedback, during the choral and DAF conditions. In the left hemisphere, increases in µ-beta ERD during pseudostuttering may have resulted from the different movement characteristics of this task compared with the solo speaking task. Relationships to findings in stuttering are discussed. Significance: Changes in sensorimotor control related feedforward and feedback control in fluency-enhancing speech manipulations can be measured using time-frequency decompositions of EEG µ rhythms in neurotypical speakers. This quiet, non-invasive, and temporally sensitive technique may be applied to learn more about normal sensorimotor control and fluency enhancement in PWS.

Sensorimotor activity measured via oscillations of EEG mu rhythms in speech and non-speech discrimination tasks with and without segmentation demands.

Better understanding of the role of sensorimotor processing in speech and non-speech segmentation can be achieved with more temporally precise measures. Twenty adults made same/different discriminations of speech and non-speech stimuli pairs, with and without segmentation demands. Independent component analysis of 64-channel EEG data revealed clear sensorimotor mu components, with characteristic alpha and beta peaks, localized to premotor regions in 70% of participants.Time-frequency analyses of mu components from accurate trials showed that (1) segmentation tasks elicited greater event-related synchronization immediately following offset of the first stimulus, suggestive of inhibitory activity; (2) strong late event-related desynchronization in all conditions, suggesting that working memory/covert replay contributed substantially to sensorimotor activity in all conditions; (3) stronger beta desynchronization in speech versus non-speech stimuli during stimulus presentation, suggesting stronger auditory-motor transforms for speech versus non-speech stimuli. Findings support the continued use of oscillatory approaches for helping understand segmentation and other cognitive tasks.

EEG Mu (µ) rhythm spectra and oscillatory activity differentiate stuttering from non-stuttering adults.

Stuttering is linked to sensorimotor deficits related to internal modeling mechanisms. This study compared spectral power and oscillatory activity of EEG mu (µ) rhythms between persons who stutter (PWS) and controls in listening and auditory discrimination tasks. EEG data were analyzed from passive listening in noise and accurate (same/different) discrimination of tones or syllables in quiet and noisy backgrounds. Independent component analysis identified left and/or right µ rhythms with characteristic alpha (α) and beta (ß) peaks localized to premotor/motor regions in 23 of 27 people who stutter (PWS) and 24 of 27 controls. PWS produced µ spectra with reduced ß amplitudes across conditions, suggesting reduced forward modeling capacity. Group time-frequency differences were associated with noisy conditions only. PWS showed increased µ-ß desynchronization when listening to noise and early in discrimination events, suggesting evidence of heightened motor activity that might be related to forward modeling deficits. PWS also showed reduced µ-α synchronization in discrimination conditions, indicating reduced sensory gating. Together these findings indicate spectral and oscillatory analyses of µ rhythms are sensitive to stuttering. More specifically, they can reveal stuttering-related sensorimotor processing differences in listening and auditory discrimination that also may be influenced by basal ganglia deficits.

Auditory cortical deactivation during speech production and following speech perception: an EEG investigation of the temporal dynamics of the auditory alpha rhythm.

Sensorimotor integration (SMI) across the dorsal stream enables online monitoring of speech. Jenson et al. (2014) used independent component analysis (ICA) and event related spectral perturbation (ERSP) analysis of electroencephalography (EEG) data to describe anterior sensorimotor (e.g., premotor cortex, PMC) activity during speech perception and production. The purpose of the current study was to identify and temporally map neural activity from posterior (i.e., auditory) regions of the dorsal stream in the same tasks. Perception tasks required "active" discrimination of syllable pairs (/ba/ and /da/) in quiet and noisy conditions. Production conditions required overt production of syllable pairs and nouns. ICA performed on concatenated raw 68 channel EEG data from all tasks identified bilateral "auditory" alpha (α) components in 15 of 29 participants localized to pSTG (left) and pMTG (right). ERSP analyses were performed to reveal fluctuations in the spectral power of the α rhythm clusters across time. Production conditions were characterized by significant α event related synchronization (ERS; pFDR < 0.05) concurrent with EMG activity from speech production, consistent with speech-induced auditory inhibition. Discrimination conditions were also characterized by α ERS following stimulus offset. Auditory α ERS in all conditions temporally aligned with PMC activity reported in Jenson et al. (2014). These findings are indicative of speech-induced suppression of auditory regions, possibly via efference copy. The presence of the same pattern following stimulus offset in discrimination conditions suggests that sensorimotor contributions following speech perception reflect covert replay, and that covert replay provides one source of the motor activity previously observed in some speech perception tasks. To our knowledge, this is the first time that inhibition of auditory regions by speech has been observed in real-time with the ICA/ERSP technique.

Temporal dynamics of sensorimotor integration in speech perception and production: independent component analysis of EEG data.

Activity in anterior sensorimotor regions is found in speech production and some perception tasks. Yet, how sensorimotor integration supports these functions is unclear due to a lack of data examining the timing of activity from these regions. Beta (~20 Hz) and alpha (~10 Hz) spectral power within the EEG µ rhythm are considered indices of motor and somatosensory activity, respectively. In the current study, perception conditions required discrimination (same/different) of syllables pairs (/ba/ and /da/) in quiet and noisy conditions. Production conditions required covert and overt syllable productions and overt word production. Independent component analysis was performed on EEG data obtained during these conditions to (1) identify clusters of µ components common to all conditions and (2) examine real-time event-related spectral perturbations (ERSP) within alpha and beta bands. 17 and 15 out of 20 participants produced left and right µ-components, respectively, localized to precentral gyri. Discrimination conditions were characterized by significant (pFDR < 0.05) early alpha event-related synchronization (ERS) prior to and during stimulus presentation and later alpha event-related desynchronization (ERD) following stimulus offset. Beta ERD began early and gained strength across time. Differences were found between quiet and noisy discrimination conditions. Both overt syllable and word productions yielded similar alpha/beta ERD that began prior to production and was strongest during muscle activity. Findings during covert production were weaker than during overt production. One explanation for these findings is that µ-beta ERD indexes early predictive coding (e.g., internal modeling) and/or overt and covert attentional/motor processes. µ-alpha ERS may index inhibitory input to the premotor cortex from sensory regions prior to and during discrimination, while µ-alpha ERD may index sensory feedback during speech rehearsal and production.

Redox control and hydrogen bonding networks: proton-coupled electron transfer reactions and tyrosine Z in the photosynthetic oxygen-evolving complex.

In photosynthetic oxygen evolution, redox active tyrosine Z (YZ) plays an essential role in proton-coupled electron transfer (PCET) reactions. Four sequential photooxidation reactions are necessary to produce oxygen at a Mn(4)CaO(5) cluster. The sequentially oxidized states of this oxygen-evolving cluster (OEC) are called the S(n) states, where n refers to the number of oxidizing equivalents stored. The neutral radical, YZ•, is generated and then acts as an electron transfer intermediate during each S state transition. In the X-ray structure, YZ, Tyr161 of the D1 subunit, is involved in an extensive hydrogen bonding network, which includes calcium-bound water. In electron paramagnetic resonance experiments, we measured the YZ• recombination rate, in the presence of an intact Mn(4)CaO(5) cluster. We compared the S(0) and S(2) states, which differ in Mn oxidation state, and found a significant difference in the YZ• decay rate (t(1/2) = 3.3 ± 0.3 s in S(0); t(1/2) = 2.1 ± 0.3 s in S(2)) and in the solvent isotope effect (SIE) on the reaction (1.3 ± 0.3 in S(0); 2.1 ± 0.3 in S(2)). Although the YZ site is known to be solvent accessible, the recombination rate and SIE were pH independent in both S states. To define the origin of these effects, we measured the YZ• recombination rate in the presence of ammonia, which inhibits oxygen evolution and disrupts the hydrogen bond network. We report that ammonia dramatically slowed the YZ• recombination rate in the S(2) state but had a smaller effect in the S(0) state. In contrast, ammonia had no significant effect on YD•, the stable tyrosyl radical. Therefore, the alterations in YZ• decay, observed with S state advancement, are attributed to alterations in OEC hydrogen bonding and consequent differences in the YZ midpoint potential/pK(a). These changes may be caused by activation of metal-bound water molecules, which hydrogen bond to YZ. These observations document the importance of redox control in proton-coupled electron transfer reactions.

Proton Coupled Electron Transfer and Redox Active Tyrosines: Structure and Function of the Tyrosyl Radicals in Ribonucleotide Reductase and Photosystem II.

Proton coupled electron transfer (PCET) reactions are important in many biological processes. Tyrosine oxidation/reduction can play a critical role in facilitating these reactions. Two examples are photosystem II (PSII) and ribonucleotide reductase (RNR). RNR is essential in DNA synthesis in all organisms. In E. coli RNR, a tyrosyl radical, Y122(•), is required as a radical initiator. Photosystem II (PSII) generates molecular oxygen from water. In PSII, an essential tyrosyl radical, YZ(•), oxidizes the oxygen evolving center. However, the mechanisms, by which the extraordinary oxidizing power of the tyrosyl radical is controlled, are not well understood. This is due to the difficulty in acquiring high-resolution structural information about the radical state. Spectroscopic approaches, such as EPR and UV resonance Raman (UVRR), can give new information. Here, we discuss EPR studies of PCET and the PSII YZ radical. We also present UVRR results, which support the conclusion that Y122 undergoes an alteration in ring and backbone dihedral angle when it is oxidized. This conformational change results in a loss of hydrogen bonding to the phenolic oxygen. Our analysis suggests that access of water is an important factor in determining tyrosyl radical lifetime and function. TOC graphic.

Proton coupled electron transfer and redox-active tyrosine Z in the photosynthetic oxygen-evolving complex.

Proton coupled electron transfer (PCET) reactions play an essential role in many enzymatic processes. In PCET, redox-active tyrosines may be involved as intermediates when the oxidized phenolic side chain deprotonates. Photosystem II (PSII) is an excellent framework for studying PCET reactions, because it contains two redox-active tyrosines, YD and YZ, with different roles in catalysis. One of the redox-active tyrosines, YZ, is essential for oxygen evolution and is rapidly reduced by the manganese-catalytic site. In this report, we investigate the mechanism of YZ PCET in oxygen-evolving PSII. To isolate YZ(•) reactions, but retain the manganese-calcium cluster, low temperatures were used to block the oxidation of the metal cluster, high microwave powers were used to saturate the YD(•) EPR signal, and YZ(•) decay kinetics were measured with EPR spectroscopy. Analysis of the pH and solvent isotope dependence was performed. The rate of YZ(•) decay exhibited a significant solvent isotope effect, and the rate of recombination and the solvent isotope effect were pH independent from pH 5.0 to 7.5. These results are consistent with a rate-limiting, coupled proton electron transfer (CPET) reaction and are contrasted to results obtained for YD(•) decay kinetics at low pH. This effect may be mediated by an extensive hydrogen-bond network around YZ. These experiments imply that PCET reactions distinguish the two PSII redox-active tyrosines.

Proton-coupled electron transfer in photosystem II: proton inventory of a redox active tyrosine.

Photosystem II (PSII) catalyzes the light driven oxidation of water and the reduction of plastoquinone. PSII is a multisubunit membrane protein; the D1 and D2 polypeptides form the heterodimeric core of the PSII complex. Water oxidation occurs at a manganese-containing oxygen evolving complex (OEC). PSII contains two redox active tyrosines, Y(Z) and Y(D), which form the neutral tyrosyl radicals, Y(z)(*) and Y(D)(*). Y(D) has been assigned as tyrosine 160 in the D2 polypeptide through isotopic labeling and site-directed mutagenesis. Whereas Y(D) is not directly involved in the oxidation of water, it has been implicated in the formation and stabilization of the OEC. PSII structures have shown Y(D) to be within hydrogen-bonding distance of histidine 189 in the D2 polypeptide. Spectroscopic studies have suggested that a proton is transferred between Y(D) and histidine 189 when Y(D) is oxidized and reduced. In our previous work, we used (2)H(2)O solvent exchange to demonstrate that the mechanism of Y(D) proton-coupled electron transfer (PCET) differs at high and low pH. In this article, we utilize the proton inventory technique to obtain more information concerning PCET mechanism at high pH. The hypercurvature of the proton inventory data provides evidence for the existence of multiple, proton-donation pathways to Y(D)(*). In addition, at least one of these pathways must involve the transfer of more than one proton.

Proton-coupled electron transfer and tyrosine D of photosystem II.

Photosystem II (PSII) is a photosynthetic reaction center that oxidizes water and reduces bound plastoquinone. PSII electron transfer is mediated by two redox-active tyrosine residues. One of these residues, tyrosine D (YD), has been assigned as Tyr160 of the D2 polypeptide by site-directed mutagenesis and isotopic labeling. Previous spectroscopic evidence has established that His189 in the D2 subunit forms a hydrogen bond with YD* and donates a proton to YD* when the radical is reduced. However, the mechanism of this reaction has not been elucidated. In this report, EPR spectroscopy and 2H2O solvent exchange were used to investigate the pL dependence of the YD* reduction rate. The kinetic isotope effect (KIE), induced by solvent exchange, was also measured as a function of pL. Under the conditions employed, the reduction of YD* is attributed to recombination with the QA- plastoquinone acceptor of PSII. The kinetic data were fit with a biexponential function. The majority, slow phase exhibited a pL-dependent rate constant, with a minimum at pL 7.5. Solvent exchange gave significant KIE at values between pL 5.5 and 8.0. In particular, at high pL (> or =7.5), the values of the KIE were determined to be 2.1 +/- 0.6 and 2.4 +/- 0.5. These values are consistent with a coupled electron and proton reaction, which occurs with a single kinetic step at pL values > or =7.5. The lower KIE values and the rate acceleration observed at low pL may be consistent with a change of mechanism in which the protonation of YD* occurs first, followed by rate-limiting electron transfer. The more modest acceleration in rate at high pL values is attributed to a small, pL-induced change in the distance between YD* and QA-.