Correspondence between food consistency and suprahyoid muscle activity, tongue pressure, and bolus transit times during the oropharyngeal phase of swallowing.

This study aimed to evaluate the effects of food texture and viscosity on the swallowing function by measuring tongue pressure and performing a videofluorographic (VF) examination. Eleven normal adults were recruited for this study. Test foods with different consistencies and liquid contents, i.e., a half-solid nutrient made of 0.8 and 1.5% agar powder, syrup, and a liquid containing 40 wt/vol% barium sulfate, were swallowed, and the anterior (AT) and posterior tongue pressures (PT) and electromyographic (EMG) activity of the suprahyoid muscles were recorded, together with VF images. The timing of each event obtained from EMG, tongue pressure, and VF recordings was measured and then compared. We found that the AT and PT activity patterns were similar and showed a single peak. The peak, area, and time duration of all of the variables for AT and PT and EMG burst increased with increasing hardness of the bolus. The onset of the EMG burst always preceded those of the AT and PT activities, while there were no significant differences in peak and offset times among EMG burst, AT, and PT. Total swallowing time and oral ejection time were significantly longer during the swallowing of 1.5% agar than any other boluses, while pharyngeal transit time and clearance time were significantly longer during the swallowing of syrup, which was as hard as the liquid, but showed a higher viscosity than the liquid. The results suggested that the major effects of food hardness were to delay oral ejection time, which strongly delays total swallowing time. In addition, pharyngeal bolus transit is not dependent on the hardness of food but on its viscosity.     

Absorption studies of Photosystem I photochemistry in the absence of vitamin K-1

Flash-induced absorption changes were studied on different timescales (nanosecond to millisecond) and at different temperatures (10 to 278 K) in highly enriched spinach PS I particles lacking vitamin K-1 and in which the electron transfer from the primary acceptor to the secondary acceptors was blocked. At all temperatures, the initial absorption change at 820 nm was followed by a fast decay (t_1/2 ≈ 47 ns at 278 K and ≈ 82 ns at 10 K) which is attributed to the decay of the primary radical pair (P-700⁺-A⁻₀). A slower phase of absorption decay is attributed to the P-700 triplet state, which was formed as a result of the biradical recombination, with a yield of about 30% at 278 K and about 75% at 10 K. Under air, the ³P-700 state decayed with a t_1/2 of about 50 μs at 278 K, whereas in the absence of oxygen it decayed with t_1/2 ≈ 560 μs. At 278 K, this yield was shown to depend on the presence of a magnetic field, with a maximum around 60 G. The ³P-700 decay halftime was nearly independent of temperature in the absence of oxygen (t_1/2 ≈ 1 ms at 10 K). The implications for the mechanisms involved in this decay are discussed. Addition of vitamin K-1 to these particles resulted in a decrease in the amplitude of the fast submicrosecond decay and a concomitant increase in the amplitude of a slow phase, indicating an efficient transfer from A⁻₀ to vitamin K-1. However, most functional properties of the acceptor A₁ were not reconstituted under these conditions.     

Adaptation of swallowing hyo-laryngeal kinematics is distinct in oral vs. pharyngeal sensory processing

Before a bolus is pushed into the pharynx, oral sensory processing is critical for planning movements of the subsequent pharyngeal swallow, including hyoid bone and laryngeal (hyo-laryngeal) kinematics. However, oral and pharyngeal sensory processing for hyo-laryngeal kinematics is not fully understood. In 11 healthy adults, we examined changes in kinematics with sensory adaptation, sensitivity shifting, with oropharyngeal swallows vs. pharyngeal swallows (no oral processing), and with various bolus volumes and tastes. Only pharyngeal swallows showed sensory adaptation (gradual changes in kinematics with repeated exposure to the same bolus). Conversely, only oropharyngeal swallows distinguished volume differences, whereas pharyngeal swallows did not. No taste effects were observed for either swallow type. The hyo-laryngeal kinematics were very similar between oropharyngeal swallows and pharyngeal swallows with a comparable bolus. Sensitivity shifting (changing sensory threshold for a small bolus when it immediately follows several very large boluses) was not observed in pharyngeal or oropharyngeal swallowing. These findings indicate that once oral sensory processing has set a motor program for a specific kind of bolus (i.e., 5 ml water), hyo-laryngeal movements are already highly standardized and optimized, showing no shifting or adaptation regardless of repeated exposure (sensory adaptation) or previous sensory experiences (sensitivity shifting). Also, the oral cavity is highly specialized for differentiating certain properties of a bolus (volume) that might require a specific motor plan to ensure swallowing safety, whereas the pharyngeal cavity does not make the same distinctions. Pharyngeal sensory processing might not be able to adjust motor plans created by the oral cavity once the swallow has already been triggered.     

An optimization model for mastication and swallowing in mammals.

Mammalian mastication is a process combining simultaneous food comminution and lubrication. The initiation of swallowing, which is voluntary, has been thought to depend on separate thresholds for food particle size and for particle lubrication. Instead of this duality, we suggest that swallowing is initiated when it is sensed that a batch of food particles is binding together under viscous forces so as to form a bolus. Bolus formation ensures that when the food mass is swallowed, it will pass the pharyngeal region safely without risk of inhaling small particles into the lower respiratory tract. Crucial for bolus formation is food particle size reduction by mastication. This allows the tongue to pack particles together tightly by pressure against the hard palate. A major function of salivation is to fill the gradually reducing spaces between particles, so increasing viscous cohesion and promoting bolus formation. If swallowing is delayed, excessive saliva floods the bolus, separating particles and reducing cohesion. Swallowing then becomes more precarious. Our model suggests that there is an optimum moment for a mammal to swallow, defined in terms of a peak cohesive force between food particles. The model is tested on human mastication with two foods, brazil nut and raw carrot, which have very different particle size breakdown rates. The peak cohesive force is much greater with brazil nuts but both foods are predicted to be swallowed after similar numbers of chews despite the very different food particle size reductions achieved at that stage. The predicted number of chews to swallow is in broad agreement with published data.     

Laser-induced reactions of P700 and cytochrome f in a blue-green alga, Plectonema boryanum

Kinetics of the absorption change of P700 (blue band) and cytochrome f in whole cells of a blue-green alga, Plectonema boryanum, have been studied by Q-switched ruby-laser flash excitation (694 nm; approx. 20 nsec) to elucidate the sequential relationship of these two components in photosynthetic electron transport. “P700” was photooxidized within 2 μsec and recovered in two phases t_1/2 10 μsec and 200 μsec). Under the same conditions cytochrome f was oxidized with a half time of 15 μsec. The magnitude of the fast phase of “P700” recovery, however, diminished at lower laser intensity while the cytochrome f change remained unaffected. The result suggests that cytochrome f and P700 may not be on the same electron-transport chain.     

Physiology of feeding in miniature pigs

The anatomy and physiology of feeding in miniature swine were studied, using gross dissection, electromyography, cinematography, and cineradiography. Small particles are preferred for ingestion, and large items are usually broken down outside of the oral cavity. The particles are initially picked up with the lower lip and then retrieved by the tongue; the tongue very rarely leaves the oral cavity. Geniohyoid, mylohyoid and digastric are the most active muscles during food collection. Mastication is fairly rapid (3 cycles/sec) and involves a transverse component which may be either medially or laterally directed. The direction of motion is generally reversed with every chew. Electromyography indicates that the transverse rotation is caused by a force couple consisting of protrusors (masseter, medial and lateral pterygoids) on one side and retrusors (zygomaticomandibularis and temporalis) on the other. The direction of the rotation is not necessarily related to the side containing the bolus. Mandibular depression is electromyographically biphasic. Concurrent tongue and hyoid movements complicate the interpretation of activity patterns in the oral floor. Both masticated food and liquid are stored between the tongue and the soft palate and epiglottis before being swallowed. Swallowing involves very strong activity in all of the hyoid and tongue muscles and very low activity in the adductors. Liquid is taken in by suction rather than lapping. Comparisons with other mammals are presented.     

Role of physical bolus properties as sensory inputs in the trigger of swallowing

Background

Swallowing is triggered when a food bolus being prepared by mastication has reached a defined state. However, although this view is consensual and well supported, the physical properties of the swallowable bolus have been under-researched. We tested the hypothesis that measuring bolus physical changes during the masticatory sequence to deglutition would reveal the bolus properties potentially involved in swallowing initiation.

Methods

Twenty normo-dentate young adults were instructed to chew portions of cereal and spit out the boluses at different times in the masticatory sequence. The mechanical properties of the collected boluses were measured by a texture profile analysis test currently used in food science. The median particle size of the boluses was evaluated by sieving. In a simultaneous sensory study, twenty-five other subjects expressed their perception of bolus texture dominating at any mastication time.

Findings

Several physical changes appeared in the food bolus as it was formed during mastication: (1) in rheological terms, bolus hardness rapidly decreased as the masticatory sequence progressed, (2) by contrast, adhesiveness, springiness and cohesiveness regularly increased until the time of swallowing, (3) median particle size, indicating the bolus particle size distribution, decreased mostly during the first third of the masticatory sequence, (4) except for hardness, the rheological changes still appeared in the boluses collected just before swallowing, and (5) physical changes occurred, with sensory stickiness being described by the subjects as a dominant perception of the bolus at the end of mastication.

Conclusions

Although these physical and sensory changes progressed in the course of mastication, those observed just before swallowing seem to be involved in swallowing initiation. They can be considered as strong candidates for sensory inputs from the bolus that are probably crucially involved in the triggering of swallowing, since they appeared in boluses prepared in various mastication strategies by different subjects.     

Photosystem I photochemistry at low temperature. Heterogeneity in pathways for electron transfer to the secondary acceptors and for recombination processes

Electron transport has been studied by flash absorption and EPR spectroscopies at 10–30 K in Photosystem I particles prepared with digitonin under different redox conditions. In the presence of ascorbate, an irreversible charge separation is progressively induced at 10 K between P-700 and iron-sulfur center A by successive laser flashes, up to a maximum which corresponds to about two-thirds of the reaction centers. In these centers, heterogeneity of the rate for center A reduction is also shown. In the other third of reaction centers, the charge separation is reversible and relaxes with a t_1/2 ≈ 120 μs. When the iron-sulfur centers A and B are prereduced, the 120 μs relaxation becomes the dominant process (70–80% of the reaction centers), while a slow component (t_1/2 = 50–400 ms) reflecting the recombination between P-700⁺ and center X⁻ occurs in a minority of reaction centers (10–15%). Flash absorption and EPR experiments show that the partner of P-700⁺ in the 120 μs recombination is neither X nor a chlorophyll but more probably the acceptor A⁻₁ as defined by Bonnerjea and Evans (Bonnerjea, J. and Evans, M.C.W. (1982) FEBS Lett. 148, 313–316). The role of center X in low-temperature electron flow is also discussed.     

Assessment of intraluminal impedance for the detection of pharyngeal bolus flow during swallowing in healthy adults

Intraluminal impedance, a nonradiological method for assessing bolus flow within the gut, may be suitable for investigating pharyngeal disorders. This study evaluated an impedance technique for the detection of pharyngeal bolus flow during swallowing. Patterns of pharyngoesophageal pressure and impedance were simultaneously recorded with videofluoroscopy in 10 healthy volunteers during swallowing of liquid, semisolid, and solid boluses. The timing of bolus head and tail passage recorded by fluoroscopy was correlated with the timing of impedance drop and recovery at each recording site. Bolus swallowing produced a drop in impedance from baseline followed by a recovery to at least 50% of baseline. The timing of the pharyngeal and esophageal impedance drop correlated with the timing of the arrival of the bolus head. In the pharynx, the timing of impedance recovery was delayed relative to the timing of clearance of the bolus tail. In contrast, in the upper esophageal sphincter (UES) and proximal esophagus, the timing of impedance recovery correlated well with the timing of clearance of the bolus tail. Impedance-based estimates of pharyngoesophageal bolus clearance time correlated with true pharyngoesophageal bolus clearance time. Patterns of intraluminal impedance recorded in the pharynx during bolus swallowing are therefore more complex than those in the esophagus. During swallowing, mucosal contact between the tongue base and posterior pharyngeal wall prolongs the duration of pharyngeal impedance drop, leading to overestimation of bolus tail timing. Therefore, we conclude that intraluminal impedance measurement does not accurately reflect the bolus transit in the pharynx but does accurately reflect bolus transit across the UES and below.     

Mechanical basis for lingual deformation during the propulsive phase of swallowing as determined by phase-contrast magnetic resonance imaging.

The tongue is an intricately configured muscular organ that undergoes a series of rapid shape changes intended to first configure and then transport the bolus from the oral cavity to the pharynx during swallowing. To assess the complex array of mechanical events occurring during the propulsive phase of swallowing, we employed tongue pressure-gated phase-contrast MRI to represent the tissue's local strain rate vectors. Validation of the capacity of phase-contrast MRI to represent local compressive and expansive strain rate was obtained by assessing deformation patterns induced by a synchronized mechanical plunger apparatus in a gelatinous material phantom. Physiological strain rate data were acquired in the sagittal and coronal orientations at 0, 200, 400, and 600 ms relative to the gating pulse during 2.5-ml water bolus swallows. This method demonstrated that the propulsive phase of swallowing is associated with a precisely organized series of compressive and expansive strain rate events. At the initiation of propulsion, bolus position resulted from obliquely aligned compressive and expansive strain, vertically aligned compressive strain and orthogonal expansion, and compressive strain aligned obliquely to the styloid process. Bolus reconfiguration and translocation resulted from a combination of compressive strain occurring in the middle and posterior tongue aligned obliquely between the anterior-inferior and the posterior-superior regions with commensurate orthogonal expansion, along with bidirectional contraction in the distribution of the transversus and verticalis muscle fibers. These data support the concept that propulsive lingual deformation is due to complex muscular interactions involving both extrinsic and intrinsic muscles.     

An experimental model to investigate the biomechanical determinants of pharyngeal mucosa coating during swallowing

The development of innovative experimental approaches is necessary to gain insights in the complex biomechanics of swallowing. In particular, unraveling the mechanisms of formation of the thin film of bolus coating the pharyngeal mucosa after the ingestion of liquid or semi-liquid food products is an important challenge, with implication in dysphagia treatment and sensory perceptions.The aim here is to propose an original experimental model of swallowing (i) to simulate the peristaltic motions driving the bolus from the oral cavity to the esophagus, (ii) to mimic and vary complex physiological variables of the pharyngeal mucosa (lubrication, deformability and velocity) and (iii) to measure the thickness and the composition of the coatings resulting from bolus flow. Three Newtonian glucose solutions were considered as model food boli, through sets of experiments covering different ranges of each physiological parameter mimicked.The properties of the coatings (thickness and dilution in saliva film) were shown to depend significantly on the physical properties of food products considered (viscosity and density), but also on physiological variables such as lubrication by saliva, velocity of the peristaltic wave, and to a lesser extent, the deformability of the pharyngeal mucosa.The biomechanical peristalsis simulator developed here can contribute to unravel the determinants of bolus adhesion on pharyngeal mucosa, necessary both for the design of alternative food products for people affected by swallowing disorders, and for a better understanding of the dynamic mechanisms of aroma perception.     

Influence of bite force and tongue pressure on oro-pharyngeal residue in the elderly

Objective: To investigate the influence of maximal bite force, maximal tongue pressure, number of mastications and swallowing on the oro‐pharyngeal residue in the elderly. Background: Oro‐pharyngeal residue in the elderly is an indication of dysphagia. Pharyngeal residue is especially critical as it may cause aspiration pneumonia, which is one of the major causes of death in elderly. Materials and methods: Videofluorographic recordings were performed on 14 elderly volunteers (six males, eight females, age range 65–93 years) without any history or symptoms of dysphagia. The subjects were instructed to consume 9 g of barium containing bread in two manners; free mastication and swallow (FMS: masticate and swallow freely), and limited mastication and swallow (LMS: swallow once after 30 chewing actions). The amount of oral and pharyngeal residue was evaluated using a 4‐point rating scale. Maximal occlusal force was measured by a pressure sensitive sheet, and maximal tongue pressure using a handy probe. Multiple regression analysis was performed to examine the influence of these items on the amount of oral and pharyngeal residue in FMS and LMS. Results: In FMS, age was found to be a factor which increased oral residue (p = 0.053), and the number of swallowing (p = 0.017) and the state of the prosthesis (p = 0.030) reduced the pharyngeal residue. In LMS, tongue pressure was a factor which reduced oral residue (p = 0.015) and increased pharyngeal residue (p = 0.008). Conclusion: It is suggested that in the elderly tongue pressure contributed to propulsion of the food bolus from oral cavity into the pharynx, and multiple swallowing contributed to the reduction in the amount of pharyngeal residue.     

Lung volume effects on pharyngeal swallowing physiology.

The experiment was a prospective, repeated-measures design intended to determine how the variation of lung volume affects specific measures of swallowing physiology. Swallows were recorded in 28 healthy subjects, who ranged in age from 21 to 40 yr (mean age of 29 yr), by using simultaneous videofluoroscopy, bipolar intramuscular electromyography, and respiratory inductance plethysmography. Each subject swallowed three standardized pudding-like consistency boluses at three randomized lung volumes: total lung capacity, functional residual capacity, and residual volume. The results showed that pharyngeal activity duration of deglutition for swallows produced at residual volume was significantly longer than those occurring at total lung capacity or at functional residual capacity. No significant differences were found for bolus transit time or intramuscular electromyography of the superior constrictor. The results of this experiment lend support to the hypothesis that the respiratory system may have a regulatory function related to swallowing and that positive subglottic air pressure may be important for swallowing integrity. Eventually, new treatment paradigms for oropharyngeal dysphagia that are based on respiratory physiology may be developed.     

Stress relaxation function of bone and bone collagen

Relaxation Young's and shear moduli of bovine bone and bone collagen were investigated. It was found that each relaxation process observed had two stages, which were referred to as process I and process II in order of time. Process II was described by a simple exponential decay while process I was not. The Kohlrausch-Williams-Watts (KWW) function, ψ(t) = exp[t/τ₁)^B] (0 < B < 1), was found to be suitable to describe process I. The normalized relaxation modulus, M_r(t), was expressed by the combination of the simple exponential type relaxation function and the KWW function

M_r=A₁exp[−(t/τ₁)^B]+A₂exp[(t/τ₁)](0<B1)

On the basis of this equation, the relaxation mechanism in bone and bone collagen was identified. According to the model proposed for the KWW relaxation function, the stress relaxation process in bone was considered to be governed by viscoelastic properties of matrix collagen fiber. The model for the KWW relaxation function requires the disordered glassy structure of collagen fiber, which is consistent with the results of the structural investigations.     

Starch-based spherical aggregates: stability of a model flavouring compound, vanillin entrapped therein

Studies on the stability of the vanillin entrapped within the spherical aggregates obtained from amaranth (Amaranthus paniculatus L.), quinoa (Chenopodium quinoa L.), rice (Oryza sativa L.) and colocasia (Colocasia esculenta L.) in the presence of gum Arabic, carboxymethyl cellulose (CMC) and carrageenan at 0.1–1.0% as bonding agents, were obtained by spray drying a 20% (w/w) starch dispersion at 120 °C. Vanillin was used at 5% based on starch (bos). The loss of vanillin over a 6-week storage period followed a first order kinetics. The stability was evaluated in terms of t_1/2 (weeks) from a semi-log plot of percentage retention of vanillin vs. storage time in weeks. The t_1/2 for the total vanillin and entrapped vanillin within the spherical aggregates prepared from different starches decreased in the order, amaranth>colocasia>chenopodium>rice. The t_1/2 decreased with an increase in the amylose content of the starches, although it was not linear. With respect to the bonding agent the stability decreased in the order, gum Arabic>CMC>carrageenan. While CMC and carrageenan gave an increasing value of t_1/2 with an increase in concentration from 0.5 to 1.0%, gum Arabic surprisingly gave a higher t_1/2 value at 0.5% as compared to 1.0%.     

Kinetics of in Vitro Bread Bolus Digestion with Varying Oral and Gastric Digestion Parameters

The rate of bolus disintegration in the stomach plays an important role in gastric emptying, which has been shown to be directly correlated with post-food consumption blood glucose levels and satiety. This study examined the effect of various oral and gastric factors on bread bolus disintegration, including flour type, hydrodynamic forces, saliva level, and presence of α-amylase. The kinetics of bolus disintegration were determined by measuring the mass retention of boluses and fitting their disintegration profiles to a linear-exponential model. Artificially masticated bread was mixed with simulated saliva to form a bolus, which was soaked in simulated gastric juice for 120 min. The mass retention kinetics during static and agitated soaking followed three distinct profiles: exponential, sigmoidal, and delayed sigmoidal. The differences in profiles were attributed to varying levels of water absorption, caused by variations in bread structure and moisture content. Increasing the bolus saliva level decreased cohesive forces and increased bolus disintegration rate. These changes could have been caused by the increase of water inside the food matrix, or by the softening effect of α-amylase, which was shown to have a significant effect on bolus texture and also caused an increase in bolus disintegration. This work studies the driving factors in the breakdown of different types of bread during simulated gastric digestion. Our results demonstrate that bread structure and moisture content are key features controlling the rate of bread breakdown during gastric digestion.     

Contribution of upper airway geometry to convective mixing

We investigated the axial dispersive effect of the upper airway structure (comprising mouth cavity, oropharynx, and trachea) on a traversing aerosol bolus. This was done by means of aerosol bolus experiments on a hollow cast of a realistic upper airway model (UAM) and three-dimensional computational fluid dynamics (CFD) simulations in the same UAM geometry. The experiments showed that 50-ml boluses injected into the UAM dispersed to boluses with a half-width ranging from 80 to 90 ml at the UAM exit, across both flow rates (250, 500 ml/s) and both flow directions (inspiration, expiration). These experimental results imply that the net half-width induced by the UAM typically was 69 ml. Comparison of experimental bolus traces with a one-dimensional Gaussian-derived analytical solution resulted in an axial dispersion coefficient of 200-250 cm(2)/s, depending on whether the bolus peak and its half-width or the bolus tail needed to be fully accounted for. CFD simulations agreed well with experimental results for inspiratory boluses and were compatible with an axial dispersion of 200 cm(2)/s. However, for expiratory boluses the CFD simulations showed a very tight bolus peak followed by an elongated tail, in sharp contrast to the expiratory bolus experiments. This indicates that CFD methods that are widely used to predict the fate of aerosols in the human upper airway, where flow is transitional, need to be critically assessed, possibly via aerosol bolus simulations. We conclude that, with all its geometric complexity, the upper airway introduces a relatively mild dispersion on a traversing aerosol bolus for normal breathing flow rates in inspiratory and expiratory flow directions.     

Mechanism of intraoral transport in macaques

All mammals have the same divisions of cyclic movement of tongue and hyoid during mastication: a protraction or forward phase that begins at minimum gape, and a retraction or return phase. Nonanthropoid mammals transport food from the oral cavity to the oropharynx during the return phase; food on the dorsal surface of the tongue moves distally while the tongue is retracted. Macaques, however, transport food during the protraction phase of tongue/hyoid movement. Food is squeezed posteriorly by contact between the tongue surface and the palate anterior to the food. This mechanism of transport is occasionally seen in nonanthropoid mammals when they are transporting liquids from the oral cavity to the oropharynx. It has, however, not been seen when these mammals transport solid food. One morphological basis for this difference is the reduction in height of the rugae of the palate of macaques. In most mammals these rugae are pronounced ridges that are able to hold food in place during protraction as the tongue slides forward beneath the food. Anthropoids and other mammals differ in the way they store food prior to swallowing. When macaques transport food to the oropharynx, usually they swallow in the next cycle, but always in the next 2 or 3 cycles. Most mammals transport and store food in the oropharynx for several cycles before a swallow clears that region of food. This behavior is correlated with differences in morphology of the oropharynx; anthropoids have reduced valleculae, the area in which other mammals store food prior to swallowing.     

Comparative thermodynamic elucidation of the structural stability of thermophilic proteins

Differential scanning calorimetry, circular dichroism, and visible absorption spectrophotometry were employed to elucidate the structural stability of thermophilic phycocyanin derived from Cyanidium caldarium, a eucaryotic organism which contains a nucleus, grown in acidic conditions (pH 3.4) at 54°C. The obtained results were compared with those previously reported for thermophilic phycocyanin derived from Synechococcus lividus, a procaryote containing no organized nucleus, grown in alkaline conditions (pH 8.5) at 52°C. The temperature of thermal unfolding (t_d) was found to be comparable between C. caldarium (73°C) and S. lividus (74°C) phycocyanins. The apparent free energy of unfolding (ΔG_[urea]=0) at zero denaturant (urea) concentration was also comparable: 9.1 and 8.7 kcal/mole for unfolding the chromophore part of the protein, and 5.0 and 4.3 kcal/mole for unfolding the apoprotein part of the protein, respectively. These values of t_d and ΔG_[urea]=0 were significantly higher than those previously reported for mesophilic Phormidium luridum phycocyanin (grown at 25°C). These findings revealed that relatively higher values of t_d and ΔG_[urea]=0 were characteristics of thermophilic proteins. In contrast, the enthalpies of completed unfolding (ΔH_d) and the half-completed unfolding (ΔH_d)1/2 for C. caldarium phycocyanin were much lower than those for S. lividus protein (89 versus 180 kcal/mole and 62 versus 115 kcal/mole, respectively). Factors contributing to a lower ΔH_d in C. caldarium protein and the role of charged groups in enhancing the stability of thermophilic proteins were discusse.     

Comparison in halide binding ability between the unsaturated clusters [Pd3(μ-dppm)3(CO)] and [PdPtCo(μ-dppm)2(CO)3(CNBu)] (dppm = Ph2PCH2PPh2)

The trinuclear clusters [Pd₃(μ-dppm)₃(CO)]²⁺ and [PtPdCo(μ-dppm)₂(CO)₃(CN^tBu)]⁺ exhibit a large and a small cavity, respectively, formed by the phenyl rings of the bridging diphosphine ligands. Their binding constants (K₁₁) with halide ions (X⁻) were obtained by UV-Vis spectroscopy. The binding ability varies as I⁻ > Br⁻ > Cl⁻, and [Pd₃(μ-dppm)₃(CO)]²⁺ > [ptPdCo(μ-dppm)₂-(CO)₃(CN^tBu)]⁺. The MO diagram for the related cluster [Pd₂Co(μ-dppm)₂(CO)₄]⁺ has been addressed theoretically in order to predict the nature of the lowest energy electronic bands. For this class of compounds, the lowest energy bands are assigned to charge transfers from the Co center to the Pd₂ centers.