Approach run increases preactivation and eccentric phases muscle activity during drop jumps from different drop heights

The purpose of this study was to investigate the effects of a horizontal approach run and drop height on the activation of lower extremity muscles during drop jumps. Ten participants performed drop jumps from drop heights of 15, 30, 45 and 60 cm with zero (standing), one, two, and three approach run steps. The EMG activities of the Gluteus Maximus (GM), Rectus Femoris (RF), Biceps Femoris (BF), Vastus Lateralis (VL), Tibialis Anterior (TA), Gastrocnemius (GA) and Soleus (SO) were recorded, full-wave rectified, and averaged (aEMG) during the preactivation (50 ms before touchdown), downward, and push-off phases. Increasing drop height did not enhance the muscle activation level of any examined muscles except GA. During the preactivation phase, the aEMG of all muscles except TA increased with the number of approach run steps. The aEMG of RF, BF, VL, and SO also increased with the number of approach run steps during the downward phase, while no aEMG changes were observed during the push-off phase. These results suggest that a horizontal approach run preceding the drop jump is an effective strategy for increasing the muscle preactivation level, which contributes to a higher level of muscle activity during the eccentric contraction phase and could potentially contribute to the reported higher power output during the concentric contraction phase.     

Effect of electrode location on EMG signal envelope in leg muscles during gait.

The aim of the study was to assess the variability of EMG signal envelope with electrode location during gait. Surface EMG signals were recorded from 10 healthy subjects from the tibialis anterior (TA), peroneus longus (PL), gastrocnemius medialis (GM), gastrocnemius lateralis (GL), and soleus (SO) muscles. From TA, PL, GL and GM, signals were acquired using a two-dimensional grid of 4 x 3 electrodes (10 x 15 mm in size, as used in most gait laboratories) with 20-mm interelectrode distance in both directions. A similar grid of 3 x 3 electrodes was used for SO. EMG envelope was characterized by its peak value, area after normalization by the peak value, and time instant corresponding to the maximum. The maximum relative change in peak value with electrode location, expressed as a percentage of the peak value in the central location, was (mean+/-SD) 31+/-18% for TA, 29+/-13% for PL, 25+/-15% for GL, 14+/-8% for GM, and 26+/-14% for SO. The maximum relative change in area was 29+/-13% for TA, 73+/-40% for PL, 31+/-23% for GL, 35+/-20% for GM, 20+/-13% for SO, and in the position of maximum, computed as distance from the maximum position in the central channel, it was 5+/-10% of the gait cycle for TA, 26+/-16% for PL, 3+/-2% for GL, 3+/-1% for GM, 3+/-3% for SO. A crosstalk index, defined on the basis of the expected intervals of muscle activation for healthy subjects, indicated that estimated crosstalk was present between TA and PL, in an amount which depended on electrode location. It was concluded that the estimate of muscle activation intensity during gait from surface EMG is variable with location of the electrodes while timing of muscle activity is more robust to electrode displacement and can be reliably extracted in those cases in which crosstalk is limited. These results are valid for healthy subjects, where the level of muscular activity during gait is much lower than maximum.     

Biomechanical analysis of movement strategies in human forward trunk bending. I. Modeling

 Two behavioral goals are achieved simultaneously during forward trunk bending in humans: the bending movement per se and equilibrium maintenance. The objective of the present study was to understand how the two goals are achieved by using a biomechanical model of this task. Since keeping the center of pressure inside the support area is a crucial condition for equilibrium maintenance during the movement, we decided to model an extreme case, called “optimal bending”, in which the movement is performed without any center of pressure displacement at all, as if standing on an extremely narrow support. The “optimal bending” is used as a reference in the analysis of experimental data in a companion paper. The study is based on a three-joint (ankle, knee, and hip) model of the human body and is performed in terms of “eigenmovements”, i.e., the movements along eigenvectors of the motion equation. They are termed “ankle”, “hip”, and “knee” eigenmovements according to the dominant joint that provides the largest contribution to the corresponding eigenmovement. The advantage of the eigenmovement approach is the presentation of the coupled system of dynamic equations in the form of three independent motion equations. Each of these equations is equivalent to the motion equation for an inverted pendulum. Optimal bending is constructed as a superposition of two (hip and ankle) eigenmovements. The hip eigenmovement contributes the most to the movement kinematics, whereas the contributions of both eigenmovements into the movement dynamics are comparable. The ankle eigenmovement moves the center of gravity forward and compensates for the backward center of gravity shift that is provoked by trunk bending as a result of dynamic interactions between body segments. An important characteristic of the optimal bending is the timing of the onset of each eigenmovement: the ankle eigenmovement onset precedes that of the hip eigenmovement. Without an earlier onset of the ankle eigenmovement, forward bending on the extremely narrow support results in falling backward. This modeling approach suggests that during trunk bending, two motion units – the hip and ankle eigenmovements – are responsible for the movement and for equilibrium maintenance, respectively. Received: 1 July 1999 / Accepted in revised form: 23 October 2000     

The interaction of apurinic acid aldehyde groups with pararosaniline in the Feulgen-Schiff and related staining procedures.

The equilibrium reactions involved in the formation of the apurinic acid (APA)-Schiff chromophores in the staining phase of the Feulgen-Schiff reaction do not allow a quantitative conversion of APA to these chromophores. By modification of the sulfite and dye concentrations and the pH of the staining reagents, or by using better solvents for pararosaniline like acetic acid or dimethylsulfoxide (DMSO) a shift of these equilibria was attempted in order to obtain a higher amount of APA-bound dye. A 40% higher absorbance, when compared with the normal Schiff-staining, was obtained in model films by staining with a saturated solution of pararosaniline in a 1:1 v/v mixture of DMSO and SO2-water, followed by rinsing in SO2-water. A doubling of the absorbance resulted in the same objects when a saturated solution of pararosaniline in a 2 M acetic acid/acetate buffer of pH 4.45 was used for staining, followed by a short rinse in SO2-water. Amino groups (as found in histones) are shown to compete with the amino groups of pararosaniline for the APA aldehydes. This effect, although causing lower staining intensities, is shown not to be the explanation for the differences in stain content found between more and less compact forms of chromatin. Depending on the pH, and dye and sulfite concentrations of the staining reagents, the following components are considered as possible contributors to the mixture of chromophores (Duijndam et al., 1973 b) formed between APA and Schiff's reagent or its modifications: 1. An acid labile component with a wavelength of maximal absorbance (lambda max) near 510 nm; its structure is probably the azomethine--CH=N--; 2. A relatively acid stable component with a high value of molecular absorbance (epsilon), an lambda max near 570 nm and possibly having an enamine structure--CH=CH--NH--; 3. A component with intermediate acid stability, low epsilon, and lambda max near 540 nm, and which is probably an alkylsulfonic acid --CH(SO3H)--NH--compound. Small differences in the staining conditions in the histochemical application of the Feulgen-Schiff reaction may cause a shift in the ratio between especially components 2 and 3, resulting in variations in stain content and in lambda max.     

Voltage-sensing domain mode shift is coupled to the activation gate by the N-terminal tail of hERG channels

Human ether-a-go-go-related gene (hERG) potassium channels exhibit unique gating kinetics characterized by unusually slow activation and deactivation. The N terminus of the channel, which contains an amphipathic helix and an unstructured tail, has been shown to be involved in regulation of this slow deactivation. However, the mechanism of how this occurs and the connection between voltage-sensing domain (VSD) return and closing of the gate are unclear. To examine this relationship, we have used voltage-clamp fluorometry to simultaneously measure VSD motion and gate closure in N-terminally truncated constructs. We report that mode shifting of the hERG VSD results in a corresponding shift in the voltage-dependent equilibrium of channel closing and that at negative potentials, coupling of the mode-shifted VSD to the gate defines the rate of channel closure. Deletion of the first 25 aa from the N terminus of hERG does not alter mode shifting of the VSD but uncouples the shift from closure of the cytoplasmic gate. Based on these observations, we propose the N-terminal tail as an adaptor that couples voltage sensor return to gate closure to define slow deactivation gating in hERG channels. Furthermore, because the mode shift occurs on a time scale relevant to the cardiac action potential, we suggest a physiological role for this phenomenon in maximizing current flow through hERG channels during repolarization.     

Study of the scapular muscle latency and deactivation time in people with and without shoulder impingement

Changes in muscle activities are commonly associated with shoulder impingement and theoretically caused by changes in motor program strategies. The purpose of this study was to assess for differences in latencies and deactivation times of scapular muscles between subjects with and without shoulder impingement. Twenty-five healthy subjects and 24 subjects with impingement symptoms were recruited. Glenohumeral kinematic data and myoelectric activities using surface electrodes from upper trapezius (UT), lower trapezius (LT), serratus anterior (SA) and anterior fibers of deltoid were collected as subjects raised and lowered their arm in response to a visual cue. Data were collected during unloaded, loaded and after repetitive arm raising motion conditions. The variables were analyzed using 2 or 3 way mixed model ANOVAs. Subjects with impingement demonstrated significantly earlier contraction of UT while raising in the unloaded condition and an earlier deactivation of SA across all conditions during lowering of the arm. All subjects exhibited an earlier activation and delayed deactivation of LT and SA in conditions with a weight held in hand. The subjects with impingement showed some significant differences to indicate possible differences in motor control strategies. Rehabilitation measures should consider appropriate training measures to improve movement patterns and muscle control.     

The Effect of Urea on Acid Phosphatase Deactivation

Acid phosphatase thermal deactivation follows a complex path consisting of an initial decay of the native enzyme towards an equilibrium distribution of two intermediate structures, mutually at equilibrium. This initial transition is followed by a final decay towards a completely inactive enzyme configuration.

All the relevant parameters (one equilibrium and two kinetic constants) of the phenomenon are environment-sensitive. It is shown that urea affects the deactivation, by increasing the rate of both structural transitions as well as the thermodynamics of the equilibrium between intermediate forms. For every urea concentration up to 2.4M, an equivalent temperature can be calculated that yields exactly the same activity versus time profile. The result suggests that enzyme deactivation is controlled by a single parameter. Entirely different environments, so long as they result in the same value of the latter, are therefore bound to produce the same deactivation profile.

Marked deviations from thermal equivalence become apparent at higher urea concentrations. Therefore, extremely high urea concentrations seems to give rise to a change in the deactivation mechanism.     

Characteristics of the maintenance of the vertical posture during standing with an asymmetrical load on the legs

Movements of the common center of pressure (CP) and the CPs of the right and left legs separately were studied during the maintenance of the vertical posture by subjects standing with symmetrical load on their legs or with the shift of the load to the right or left leg. It was shown that standing with a symmetrical load on the legs was accompanied by the movement of the CP of an individual leg along the straight line with small deviations aside, whereas movement of the common CP represented the curve with frequent changes in direction and filling up some space. The shift of the load to one leg resulted in the movement of the CP of the loaded leg that was similar to that observed during a symmetrical load on the legs. The movement of the CP of the unloaded leg was chaotic. The shift of the load to one leg decreased the correlation between the movements of the CPs of the left and right legs compared to standing with a symmetrical load on the legs. The velocity of movement of the CP of the leg loaded increased in the sagittal direction but remained stable in the frontal direction. The velocity of movement of the CP of the unloaded leg remained stable in the sagittal direction but increased in the frontal direction. We suppose that during standing with an asymmetrical load on the legs the role of the single in the maintenance of the vertical posture depend on the load on the leg.     

Biomechanical study of the programming of anticipatory postural adjustments associated with voluntary movement

The present research concerns anticipatory postural adjustments (APA), with the purpose of determining whether they are preprogrammed and of specifying their biomechanical finality. The experimental situation allowed us to distinguish between the voluntary movement itself (an upper limb elevation) and the postural adjustments associated with it. To this aim, the upper limb kinematics, evaluated from an accelerometer fixed at wrist level, were compared to the whole body dynamics, recorded by means of a force platform. Movements, executed in series of five, were studied according to three conditions: bilateral flexions (BF) and unilateral flexions (UF), with (IUF) and without (OUF) an additional inertia, of the stretched upper limb(s). Six right handed adults were tested twice. Results showed that the ground reaction resultant forces as well as the ground reaction resultant moment about the vertical axis presented reproducible variations before and after the onset of upper limb acceleration. The biomechanical organization of APA corresponded, for the three experimental conditions, to an upward and forward acceleration of the body center of gravity, and also, for UF, to a resultant moment directed towards the contralateral side. The duration of APA varied with the characteristics of the forthcoming voluntary movement, increasing significantly from BF to OUF and from OUF to IUF. It is concluded that APA correspond to dynamic phenomena which are centrally preprogrammed. The inertia forces associated with APA may, when the time comes, balance the inertia forces due to the movement of the mobile limb therefore counteracting the disturbance to postural equilibrium.(ABSTRACT TRUNCATED AT 250 WORDS)     

The Circadian Rhythm of Leaf Movement of Coleus blumei x C. frederici, a Short Day Plant. II. The Effects of Light and Temperature Signals

The phase response curve for the circadian rhythm of leaf movement of Coleus blumei x C. frederici, a short day plant, is generally similar to those reported for other organisms. An increase in the duration of the light signal caused an increase in the extreme values of the phase response curve and shortened the time for transition from maximum delays to maximum advances. Experiments with 2 light signals showed that the overt rhythm of leaf movement represents the rhythm of the light sensitive oscillator even during the transient period that followed the first light signal. A temperature decrease of 7° for 8 hr caused only a transient phase shift in the following 2 cycles but not in the steady state. The combination of such a temperature decrease and a light signal showed that only the overt rhythm of leaf movement was disturbed by the temperature decrease whereas the light sensitive oscillator was free running. A temperature decrease of 11° for 10 hr caused a steady state phase shift and affected the light sensitive oscillator as well.     

A subsequent movement alters lower extremity muscle activity and kinetics in drop jumps vs. drop landings

Drop landings and drop jumps are common training exercises and injury research model tasks. Drop landings have a single landing, whereas drop jumps include a subsequent jump after initial landing. With the expected ground impact, instant and landing surface suggested to modulate landing neuromechanics, muscle activity, and kinetics should be the same in both tasks when landing from the same height onto the same surface. Although previous researchers have noted some differences between these tasks across separate studies, little research has compared these tasks in the same study. Thus, we examined whether a subsequent movement after initial landing alters muscle activity and kinetics between drop landings and jumps. Fifteen women performed 10 drop landings and drop jumps each from 45 cm. Muscle onsets and integrated muscle activation amplitudes 150 milliseconds before (preactivity) and after landing (postactivity) in the medial and lateral quadriceps, hamstrings, and lateral gastrocnemius and peak and time-to-peak vertical ground reaction forces were examined across tasks (p ≤ 0.05). When performing drop jumps, subjects demonstrated later (p = 0.02) gastrocnemius and lesser lateral gastrocnemius (p = 0.002) and medial quadriceps (p = 0.02) preactivity followed by increased postactivity in all muscles (p = 0.006), with higher peak vertical ground reaction forces (p = 0.04) but no differences in times to these peaks (p = 0.60) than drop landings. The later gastrocnemius activation, higher gastrocnemius and quadriceps postlanding amplitudes, and higher ground reaction forces in drop jumps may allow subjects to propel the body vertically after the initial landing vs. simply absorbing impact in drop landings. Our results indicate that in addition to landing surface and height, anticipation of a subsequent task changes landing neuromechanics. Generalizations of results from landing-only studies should not be made with landing followed-by-subsequent-activity studies. Landing exercises should be incorporated based on sport-specific demands.     

Movement behavior of high-heeled walking: how does the nervous system control the ankle joint during an unstable walking condition?

The human locomotor system is flexible and enables humans to move without falling even under less than optimal conditions. Walking with high-heeled shoes constitutes an unstable condition and here we ask how the nervous system controls the ankle joint in this situation? We investigated the movement behavior of high-heeled and barefooted walking in eleven female subjects. The movement variability was quantified by calculation of approximate entropy (ApEn) in the ankle joint angle and the standard deviation (SD) of the stride time intervals. Electromyography (EMG) of the soleus (SO) and tibialis anterior (TA) muscles and the soleus Hoffmann (H-) reflex were measured at 4.0 km/h on a motor driven treadmill to reveal the underlying motor strategies in each walking condition. The ApEn of the ankle joint angle was significantly higher (p<0.01) during high-heeled (0.38±0.08) than during barefooted walking (0.28±0.07). During high-heeled walking, coactivation between the SO and TA muscles increased towards heel strike and the H-reflex was significantly increased in terminal swing by 40% (p<0.01). These observations show that high-heeled walking is characterized by a more complex and less predictable pattern than barefooted walking. Increased coactivation about the ankle joint together with increased excitability of the SO H-reflex in terminal swing phase indicates that the motor strategy was changed during high-heeled walking. Although, the participants were young, healthy and accustomed to high-heeled walking the results demonstrate that that walking on high-heels needs to be controlled differently from barefooted walking. We suggest that the higher variability reflects an adjusted neural strategy of the nervous system to control the ankle joint during high-heeled walking.     

Mapping P300 waves onto inhibition: Go/NoGo discrimination

Subjects viewed letters presented at 2 sec intervals and prepared a fast button press whenever an “O” appeared. If the next letter was an “X” the button press was executed (Go signal), but if the letter was a non-X character (T, H, Z) suppression of the response was required (NoGo cue). NoGo signals elicited a P300-like wave that was larger at central and frontal scalp sites contralateral to the prepared movement, compared to P300s elicited by Go cues which were symmetric about the sagittal midline and dominant at parietal sites. Subtraction of preparatory CNVs from the NoGo P300 did not remove differences in scalp topography, or reduce the amplitude of the NoGo P300 to that seen following control letters that required perceptual identification but did not call for suppression of prepared motor responses. Principal components analysis identified a middle positive wave following X-alone control stimuli whose topography resembled the NoGo P300. These findings suggest that the source of augmented NoGo P300s is a generator involved with sensorimotor inhibition. We discuss the mechanism of P300 waves and evidence linking these waves with inhibition in other task arrangements.     

Ankle flexor muscles in the cat: Length-active tension and muscle unit properties as related to locomotion

The mechanical properties of the whole muscle and fast-twitch muscle units of the cat hindlimb pretibial flexors have been explored and related to normal locomotion. Tibialis anterior (TA) is parallel-fibered and functionally crosses a single joint, the ankle, whereas extensor digitorum longus (EDL) is pinnate and spans the ankle, knee, metatarsophalangeal and interphalangeal joints. The active tetanic tension of TA remains near its peak value over a range of muscle lengths associated with normal ankle movement. In contrast, the length-tension curve of EDL is sharply peaked. However, normal corollary action of the knee, ankle and metatarsophalangeal joints during stepping minimizes EDL's excursion and maintains it at or near a length optimal for peak tension development. EDL is capable of producing synchronous but sterotyped digit and ankle movements while TA provides for independent ankle flexion at all relevant joint angles. The mechanical properties of 84 TA and 98 EDL fast-twitch muscle units were studied by measuring twitch contraction time (≤45 msec), peak tetanic tension, response to repetitive stimulation, and contractile fatigue resistance during electrical stimulation of single alpha axons, functionally isolated from ventral root filaments. These mechanical properties were essentially similar for both muscles with the exception of mean peak tetanic tension which was 30% lower for TA units (14 gm-wt) than for EDL units (20 gm-wt). A high proportion of units in both muscles demonstrated fatigue resistance which is reflective of the repetitive, phasic demand upon these muscles during locomotion.     

Subpopulations of splenic T cells regulating an anti-hapten antibody response. II. Distinct functions of, and sequential requirement for, helper and amplifier cells.

In adoptive transfer experiments, two classes of peripheral T lymphocytes, carrier-primed helper (TH) and carrier-primed amplifier (TA) cells, synergized in the induction of a primary anti-hapten IgG response by virgin B cells. Purified TH and TA had separate functions and were required at different times during the antigen-driven development of the response. TH were required early, and provided an initiating signal to B cells in the presence of the specific hapten-carrier conjugate. The differentiative nature of this signal was inferred from the threshold dose-response relationship and the insensitivity of the TH-directed event to the antimitotic agent vinblastine. TA were required 4 days later and provided an amplifying signal to B cells in the presence of the same hapten-carrier conjugate. The proliferative nature of this second signal was inferred from the exponential dose-response relationship and the exquisite sensitivity of the TA-directed event to vinblastine. Virgin B cells became susceptible to the TA signal only after having received the TH signal. TH and TA did not synergize, however, in true secondary responses since hapten-primed B cells depended only on the TH signal to generate large numbers of IgG antibody-forming cells.     

Sodium Channel Activation Gating Is Affected by Substitutions of Voltage Sensor Positive Charges in All Four Domains

The role of the voltage sensor positive charges in the activation and deactivation gating of the rat brain IIA sodium channel was investigated by mutating the second and fourth conserved positive charges in the S4 segments of all four homologous domains. Both charge-neutralizing (by glutamine substitution) and -conserving mutations were constructed in a cDNA encoding the sodium channel α subunit that had fast inactivation removed by the incorporation of the IFMQ3 mutation in the III–IV linker (West, J.W., D.E. Patton, T. Scheuer, Y. Wang, A.L. Goldin, and W.A. Catterall. 1992. Proc. Natl. Acad. Sci. USA. 89:10910–10914.). A total of 16 single and 2 double mutants were constructed and analyzed with respect to voltage dependence and kinetics of activation and deactivation. The most significant effects were observed with substitutions of the fourth positive charge in each domain. Neutralization of the fourth positive charge in domain I or II produced the largest shifts in the voltage dependence of activation, both in the positive direction. This change was accompanied by positive shifts in the voltage dependence of activation and deactivation kinetics. Combining the two mutations resulted in an even larger positive shift in half-maximal activation and a significantly reduced gating valence, together with larger positive shifts in the voltage dependence of activation and deactivation kinetics. In contrast, neutralization of the fourth positive charge in domain III caused a negative shift in the voltage of half-maximal activation, while the charge-conserving mutation resulted in a positive shift. Neutralization of the fourth charge in domain IV did not shift the half-maximal voltage of activation, but the conservative substitution produced a positive shift. These data support the idea that both charge and structure are determinants of function in S4 voltage sensors. Overall, the data supports a working model in which all four S4 segments contribute to voltage-dependent activation of the sodium channel.     

Tonic-to-phasic shift of lumbo-pelvic muscle activity during 8 weeks of bed rest and 6-months follow up.

Prior motor control studies in unloading have shown a tonic-to-phasic shift in muscle activation, particularly in the short extensors. Tonic muscle activity is considered critical for normal musculoskeletal function. The shift from tonic-to-phasic muscle activity has not been systematically studied in humans in unloading nor at the lumbo-pelvic (LP) region. Ten healthy young male subjects underwent 8 wk of bed rest with 6-mo follow up as part of the "Berlin Bed-Rest Study." A repetitive knee movement model performed in the prone position is used to stimulate tonic holding LP muscle activity, as measured by superficial EMG. Tonic and phasic activation patterns were quantified by relative height of burst vs. baseline electromyographic linear-envelope signal components. Statistical analysis shows a shift toward greater phasic activity during bed rest and follow up (P < 0.001) with a significant interaction across muscles (P < 0.001) specifically affecting the short lumbar extensors. These changes appear unrelated to skill acquisition over time (P all > or = 0.196). This change of a shift from tonic LP muscle activation to phasic is in line with prior research on the effects of reduced weight bearing on motor control.     

Biomechanical reorganisation of stepping initiation during acute dorsiflexor fatigue

During voluntary step initiation (SI), propulsive forces are generated during anticipatory postural adjustments (APA) which displace the centre-of-gravity (CoG) in the desired direction. These propulsive forces are implemented by ankle synergy, bilateral soleus inhibition followed by activation of tibialis anterior (TA). The aim of this study was to investigate the effect of fatigue applied to ankle dorsiflexors on APA associated with SI and on related motor performance. Eight young healthy participants initiated stepping before and after a protocol designed to generate fatigue in ankle dorsiflexors. Fatigue was induced by series of high-level isometric contractions performed until exhaustion. Results showed that, with fatigue, the level of TA activation during APA, anticipatory postural dynamics (backward centre-of-pressure displacement and forward CoG velocity) and related motor performance (peak of CoG velocity) were attenuated, while APA duration and total SI duration increased. These changes were interpreted as reflecting a protective strategy aiming to preserve the integrity of the fatigued muscles, rather than an impairment associated with muscle weakness.     

The effect of sorbitol on acid phosphatase deactivation

Acid phosphatase thermal deactivation follows a complex path: an initial decay toward an equilibrium distribution of at least two intermediate structures, mutually at the equilibrium, followed by a final breakdown toward a completely inactive enzyme configuration. The results obtained in the presence of sorbitol have been compared to those produced in the course of purely thermal deactivation of the native enzyme. For any sobitol concentration, an equivalent temperature is calculated that results in exactly the same activity-versus-time profile. This suggests enzyme deactivation to be controlled by a single, unchanging step. Immobilized enzyme runs have been performed, as well, by entrapping acid phosphates within a polymeric network formed onto the upstream surface of an ultrafiltration membrane. The stabilizing effect of entrapment cumulates with that produced by sorbitol. In this case, however, an equivalent temperature cannot be determined, thus indicating that a different deactivation mechanism is followed.     

The action of indophenols and nitrophenols on the deactivation reactions in the water-splitting system of photosynthesis

The dependence of the relative average oxygen yield per flash for repetitive excitation with single flashes as a function of the dark time, t_d, between the flashes has been investigated.

The decrease of = f(t_d) for long dark times (t_d) depends on the deactivation processes in the water-splitting system by which the number of precursors for photosynthetic oxygen evolution is diminished.

It is shown that the rate of the deactivation reactions can be either accelerated or retarded by indophenols and nitrophenols. These effects can be clearly correlated to the acidity of the hydroxyl group of these substances, but other factors have also to be considered in order to interpret completely the mode of action of these agents in the deactivation process. Possible mechanisms are discussed.