Role of Crystallization in Polymers and Proteins

Dynamically Crystalizing Liquid-Crystal Elastomers for an Expandable Endplate-Conforming Interbody Fusion Cage

Degenerative disc disease (DDD) is the leading cause of low back pain and radiating leg pain. DDD is commonly treated surgically using spinal fusion techniques, but in many cases failure occurs due to insufficient immobilization of the vertebrae during fusion. The fabrication and demonstration of a 3D-printed semi-crystalline liquid crystal elastomer (LCE) spinal fusion cage that addresses these challenges in particular subsidence are described. During implantation of the fusion cage, the LCE is rubbery and capable of deforming around and conforming to delicate anatomy. In the hours following implantation, the device crystallizes into a rigid, structural material with the modulus increasing tenfold from 8 to 80 MPa. In the crystalline regime, a 3D-printed prototype device is capable of enduring 1 million cycles of physiologic compressive loading with minimal creep-induced ratcheting. Effects of LCE molecular architecture on the rate and magnitude of modulus increase, material processability, and mechanical properties are explored. This fundamental characterization informs a proof-of-concept device-the first bulk 3D printed LCE demonstrated to date. Moreover, the novel deployment strategy represents an exciting new paradigm of spinal fusion cages, which addresses real clinical challenges in expandable interbody fusion cages.

The Mechanical Properties of Rat Tail Tendon

The load-strain and stress-relaxation behavior of wet rat tail tendon has been examined with respect to the parameters strain, rate of straining, and temperature. It is found that this mechanical behavior is reproducible after resting the tendon for a few minutes after each extension so long as the strain does not exceed about 4 per cent. If this strain is exceeded, the tendon becomes progressively easier to extend but its length still returns to the original value after each extension. Extensions of over 35 per cent can be reached in this way. Temperature has no effect upon the mechanical behavior over the range 0–37°C. Just above this temperature, important changes take place in the mechanical properties of the tendon which may have biological significance. The application of the techniques used here to studies of connective tissue disorders is suggested. Some of the mechanical properties of tendon have been interpreted with a simple model.

The structural principles of multidomain organization of the giant polypeptide chain of the muscle titin protein: SAXS/WAXS studies during the stretching of oriented titin fibres

Elasticity of titin is a key parameter that determines the mechanical properties of muscle. These include reversibility, i.e., the muscle's capacity to change its length many-fold and return to its original state, and the transduction of passive tension generated by the stretched muscle. The morphology and elastic properties of oriented fibres of titin molecules were studied using SAXS and WAXS (small- and wide-angle X-ray scattering, respectively) and mechanical techniques. We succeeded in obtaining oriented filaments of purified titin suitable for diffraction measurements. Our X-ray data suggest a model of titin as a nanoscale, morphological, and aperiodical array of rigid Ig- and Fn3-type domains covalently connected by conformationally variable short loops. The line group symmetry of the model can be defined as SM with axial translation tau(infinity). Both tension transduction and high elasticity of titin can be explained in terms of crystalline polymer physics. Titin stretching experiments show that each individual titin macromolecule can adopt a novel two-phase state within the fibre. Conversion between high elasticity and strength can be explained as a phase transition under external tension. In the terms of the concept of orientational melting the origin of the functional heterogeneity along the titin strand becomes interpretable.

The role of arthroscopic thermal capsulorrhaphy in the hip

Arthroscopic thermal modification of collagen in the hip capsular tissue appears to be a treatment option for patients with hip instability. Traumatic hip instability is associated with frank dislocation or a subluxation, and labral tears. Atraumatic hip instability is associated with evidence of generalized ligament laxity. It can be associated with bone-collagen type disorders, including Ehlers-Danlos syndrome, Down syndrome, arthrochalasis multiplex congenita, developmental dysplastic hip, and idiopathic type. As previously discussed by Bellabarba et al, capsular laxity may be the underlying cause of dynamic hip instability. The capsule is a fibrous, thick, and strong structure that encircles the proximal femur and the acetabulum. The capsule is thicker anteriorly than posteriorly, and consists of two sets of fibers, circular and longitudinal. The capsule ligaments play a very important role in hip stability. The hip joint capsule is reinforced by the iliofemoral, pubofemoral, and ischiofemoral ligaments. It remains sensitive to stretch and serves as a mechanism for muscular feedback and pain. The iliofemoral ligament limits hyperextension and lateral rotation of the hip joint and is taut in full extension. Full extension of the hip exposes the capsule and ligaments to a twisting and shortening effect that forces the head onto the acetabulum. We are currently studying the effect of iliofemoral ligament deficiency and its relationship to instability. Many of the properties of synovial lubrication depend on contact with articular surfaces, and incongruency due to instability may have some functional role in distribution of synovial fluid, leading to stresses from weightbearing and eventually to rapid deterioration of the articular surfaces. The high-level athletes in this series include two professional baseball players, three professional golfers (PGA), one professional football player (NFL), one figure skater (Olympic gold medalist), one gymnast (Olympic level, bilateral hips), and one ballet dancer; they returned to their pre-injury level of activity. The other patients returned to their pre-injury functional lifestyle. Hip instability appears to present consistently with stable gait abnormalities and painful sensation of instability. Recognizing the various patterns of hip instability is complicated, and therefore management and outcome of these disorders are quite variable. Bellabarba et al concluded that physical therapy alone had been unsuccessful and that temporary success of a posterior capsular "plication" in one patient showed promise. Arthroscopic thermal modification of collagen in the hip capsular tissue appears to be a treatment option for patients with hip instability. The hip joint capsule is predominantly type 1 collagen, and the mechanism of tissue shrinkage through type 1 collagen alteration is well documented in the literature. Short-term results appear promising, however, more studies are required to determine the long-term efficacy of this procedure in the treatment of this challenging disorder.

Intradiskal electrothermal therapy: a preliminary histologic study

OBJECTIVE

To characterize descriptively the histologic and temperature effects of intradiskal electrothermal annuloplasty on human cadaveric lumbar disks.

DESIGN

In vitro histologic study.

SETTING

Hospital-based soft-tissue research laboratory.

CADAVERS

Six human cadaveric lumbar disks, from 5 cadavers aged 39 to 79 who died from nonspine-related causes.

INTERVENTIONS

Intradiskal electrothermal therapy (IDET) by using a standard high-temperature heating protocol with the temperature of the probe gradually increased from 65 degrees C to 90 degrees C over 16.5 minutes. Disks were stained and examined by light microscopy and electron microscopy.

MAIN OUTCOME MEASURES

Temperatures in outer annulus, gross macroscopic changes, and histologic damage.

RESULTS

Gross inspection showed a small circumferential area of tissue alteration localized to the posterior annulus but not extending to the endplates. Light microscopy of the posterior aspect of the lumbar disks showed denaturation, shrinkage, and coalescence of annular collagen; the anterior portions, which served as internal controls, showed no evidence of damage. The endplates were structurally preserved and showed no evidence of damage. Electron microscopy showed extensive collagen disorganization, decreased quantity of collagen, collagen fibril shrinkage, and chondrocyte damage when compared with a control portion. The temperature curves showed parallel changes in temperature at the level of the probe and at the posterior portion of the disk.

CONCLUSIONS

IDET raises temperatures sufficiently to induce collagen denaturation and coalescence. These histologic changes may play a substantial role in the clinical efficacy of IDET.

Helix-coil transition in homopolypeptides under stretching

We consider the effect of an external applied force on the alpha-helix-coil transition of a single-stranded homopolypeptide chain. An annealed scenario is assumed, where the building amino acid monomers may interconvert between random-coiled and ordered alpha-helical configurations. By exact evaluation of the partition function of the freely jointed chain with helix-coil internal degrees of freedom in the thermodynamic limit, we obtain the result that the stress-strain characteristic has an asymmetrical sigmoid shape with a prominent pseudoplateau. Because of the one-dimensional nature of this system, fluctuations dominate over the mean-field approximation, which incorrectly predicts a second-order phase transition.

Glenohumeral translation after arthroscopic thermal capsuloplasty with a radiofrequency probe

The purpose of this study was to determine whether there are changes in anterior and posterior glenohumeral translation after arthroscopic thermal capsuloplasty with a radiofrequency probe. Anteriorly directed loads of 15 N and 20 N were sequentially applied to the humerus of each of 5 cadaveric glenohumeral joints, and anterior translation on the glenoid was measured through use of a customized translation apparatus and an electromagnetic tracking device. The tests were then repeated with posteriorly directed forces, and posterior translation was measured. During testing, the glenoid was rigidly fixed and the glenohumeral joint was positioned to simulate 90 degrees of shoulder abduction and 90 degrees of external rotation. By means of the radiofrequency probe, thermal energy was then applied to the anteroinferior capsuloligamentous structures; anterior and posterior translation measurements were repeated. The results showed a significant reduction in anterior and posterior translations after thermal capsuloplasty (P < .05). Anterior translation decreased from 6.8 to 4.0 mm (a 41% decrease) with the 15-N load and from 8.6 to 4.9 mm (a 42% decrease) with the 20-N load. Posterior translation decreased from 9.3 to 5.8 mm (a 36% decrease) with the 15-N load and from 10.4 to 6.5 mm (a 35% decrease) with the 20-N load. The results of this study indicate that the radiofrequency probe can be used to decrease both anterior and posterior glenohumeral translation in vitro. The biological effect on heat-treated tissues over time needs to be studied to prove that this is a satisfactory treatment for glenohumeral instability.

Extension of rod-coil multiblock copolymers and the effect of the helix-coil transition

The extension elasticity of rod-coil mutliblock copolymers is analyzed for two experimentally accessible situations. In the quenched case, when the architecture is fixed by the synthesis, the force law is distinguished by a sharp change in the slope. In the annealed case, where interconversion between rod and coil states is possible, the resulting force law is sigmoid with a pronounced plateau. This last case is realized, for example, when homopolypeptides capable of undergoing a helix-coil transition are extended from a coil state.

Thermal modification of collagen

Shoulder capsular shrinkage has recently been proposed as a therapeutic modality in a select group of patients with instability. Basic science research studying the mechanism of collagen shrinkage and the effect of shrinkage on the tissue's mechanical properties is essential to define the ideal process by which to achieve optimal tissue shrinkage. Tissue shrinkage is a function of both time and temperature. This relationship was studied, and a model was derived to describe the relationship mathematically. Tissue shrinkage rate was extremely sensitive to temperature changes. The purpose of this study, was to shrink collagenous tissue thermally and then to measure the mechanical property changes as a function of tissue shrinkage. Uniaxial tensile testing of normal and heat-shrunken bovine tendon was carried out, and a model was developed to express the relationship between shrinkage and mechanical properties. We found that the mechanical properties decreased with increasing shrinkage, and that the maximal allowable shrinkage before significant material property changes occurred was between 15% to 20%. Ultrastructural analysis with transmission electron microscopy showed denaturation of the collagen fibrillar structure and provided direct support for the observed material changes.

Tissue shrinkage with the holmium:yttrium aluminum garnet laser. A postoperative assessment of tissue length, stiffness, and structure

The effect of laser energy on the length, stiffness, and structure of connective tissue was examined in a rabbit patellar tendon model. A holmium:yttrium-aluminum-garnet laser was used to deliver a calculated dose of laser energy (300 J/cm2) to one randomly selected patellar tendon in each of 13 adult New Zealand White rabbits. The contralateral patellar tendon was used as a control. Radiopaque markers were placed in the patella and tibial tuberosity to allow for patellar tendon length measurements (via standard lateral radiographs) before and after laser application and at 4 and 8 weeks. Limbs were not immobilized during the postoperative period. The tendons were harvested at 0 weeks (N = 7) and 8 weeks (N = 6) and evaluated for tensile, stiffness, cross-sectional area, histologic changes, and electron microscopic appearance. The results demonstrated significant tendon shrinkage (6.6% +/- 1.4%) after application of the calculated laser energy dose. However, tendon length had increased significantly beyond the immediate postlaser length at 4 weeks and beyond its original length by 8 weeks. At 8 weeks, the lased tendons were significantly less stiff with significantly greater cross-sectional areas than contralateral controls. There was generalized fibroblastic response throughout the entire lased tendon characterized by a marked increase in cellularity. There was also a change from the normal bimodal pattern of large- and small-diameter collagen fibers to a unimodal pattern with predominantly small-diameter fibers in the lased tendons. The tissue alterations seen in this study suggest that the biologic response of connective tissue to laser energy causes a further compromise in tissue integrity, beyond that attributed to the initial physical effects of the laser. These alterations must be taken into consideration when determining postoperative rehabilitation of laser-modified tissues.

The thermal properties of bovine joint capsule. The basic science of laser- and radiofrequency-induced capsular shrinkage

Orthopaedic surgeons have recently adapted the holmium: yttrium-aluminum-garnet (YAG) laser for the shrinkage of capsular tissues for treatment of glenohumeral instability. The molecular mechanism of capsular shrinkage has not been documented to date. This study examined the effects of heating on bovine calf knee capsule and subsequent shrinkage of the capsule. Capsule specimens were placed in a saline bath at temperatures ranging from 55 degrees to 75 degrees C for 1, 3, 5, and 10 minutes. Shrinkage was quantified by digital imaging, and the tissue was examined by light and polarized light microscopy. Tissue contraction was not measurable at or below 57.5 degrees C. At 60 degrees C, tissue shrinkage occurred with corresponding basophilic staining and loss of birefringence in collagen fibers. For specimens heated at 60 degrees C and 62 degrees C, shrinkage directly correlated with duration of thermal exposure. Maximal shrinkage of approximately 50% in length occurred at and above 65 degrees C with thermal exposures of 1 minute or greater. This study demonstrates that thermal shrinkage of bovine knee capsule correlates with denaturation of collagen fibers and depends on both time and temperature. Capsular shrinkage treatments may be performed with any energy source that is capable of well-controlled heating of capsular tissue and does not depend on the special properties of laser light.

Chorda tympani responses under lingual voltage clamp: implications for NH4 salt taste transduction

Rat chorda tympani (CT) responses to NH4Cl, ammonium acetate (NH4Ac), and ammonium hippurate (NH4Hp) were obtained during simultaneous current and voltage clamping of the lingual field potential. Although functional and developmental similarities for gustation have been reported for NH4+ and K+ salts, we report here that significant differences are discernible in the CT responses to both salts. Unlike neural responses to KCl, those to NH4Cl are voltage sensitive, enhanced by submucosa negative and suppressed by positive voltage clamp. In this regard, NH4Cl responses are qualitatively similar to NaCl responses; however, the magnitude of NH4Cl voltage sensitivity is significantly less than that of NaCl. The concentration dependence of the CT response to NH4Cl manifests a biphasic nonlinear relationship not observed with KCl or NaCl. Below 0.3 M, the CT response increases as if to approach a saturation value. However, beyond 0.3 M an inflection appears in the CT-concentration curve because of an abrupt increase in CT responses. This kinetic profile is Cl-dependent and is correlated with an increase in transepithelial conductance that displays similar NH4Cl concentration dependence. The biphasic relation to salt concentration is not observed when acetate or hippurate is substituted for Cl-. As with Na+ and K+ salts, less mobile anions than Cl- (Ac- and Hp-) lower the CT responses. However, like Na+ salts, but in contrast to K+ salts, the onset kinetics of CT responses to NH4Ac or NH4Hp remained rapid, even under positive voltage-clamp conditions. Amiloride (100 microM) partially suppresses CT responses within the concentration range of 0.05-0.3 M (48-20% suppression). Amiloride also suppresses the voltage sensitivity of NH4Cl CT responses, but does not eliminate the sensitivity as it does for Na+ salts. In conclusion, the data suggest that taste transduction for NH4 salts is mediated over two NH+ conduction pathways in the taste bud. This is especially evident with NH4Cl, where the CT-concentration curves show two distinct kinetic regimes. Below 0.3 M the saturation with increasing concentration, clamp voltage response dependence, and amiloride sensitivity suggest an apical membrane transduction conductance. Above 0.3 M, the high anion dependence of the response and its amiloride insensitivity indicate participation of the paracellular pathway in transduction.

Pulsed n.m.r. studies of the crystallization kinetics of polyethylene from the melt and solution

Pulsed n.m.r. techniques have been used to determine the crystallization rates of polyethylene samples, both commercial polydispersed and monodispersed, in bulk and in approximately 5 % (by mass) solution in tetrachloroethylene. Where comparable, the results agree well with those obtained by Mandelkern (1956)using volume change measurements, but extend his temperature range towards the more rapid crystallization rates. The crystallization-time curves show the existence of an induction period, and a residual amorphous fraction which is lower in solution crystallization. The results agree well with the relation between bulk crystallization rates and degree of supercooling proposed by Mandelkern et al. (1954) provided appropriate melting temperatures,T m , are used. The technique has several advantages for the determination of polymer crystallization rates.

Phase transitions and the molecular mechanism of contraction

In this paper, the rotating cross-bridge mechanism for muscle contraction is discussed and much contradictory evidence is put forward. As an alternative, a model is given in which the motor of muscle contraction is placed in the myosin-rod hinge and/or in the actin filament. No definite choice for one of the proposed models can be made yet, although it is clear that some kind of phase transition plays an important role in the mechanism.

Thermal stress and Ca-independent contractile activation in mammalian skeletal muscle fibers at high temperatures

Temperature dependence of the isometric tension was examined in chemically skinned, glycerinated, rabbit Psoas, muscle fibers immersed in relaxing solution (pH approximately 7.1 at 20 degrees C, pCa approximately 8, ionic strength 200 mM); the average rate of heating/cooling was 0.5-1 degree C/s. The resting tension increased reversibly with temperature (5-42 degrees C); the tension increase was slight in warming to approximately 25 degrees C (a linear thermal contraction, -alpha, of approximately 0.1%/degree C) but became more pronounced above approximately 30 degrees C (similar behavior was seen in intact rat muscle fibers). The extra tension rise at the high temperatures was depressed in acidic pH and in the presence of 10 mM inorganic phosphate; it was absent in rigor fibers in which the tension decreased with heating (a linear thermal expansion, alpha, of approximately 4 x 10(-5)/degree C). Below approximately 20 degrees C, the tension response after a approximately 1% length increase (complete < 0.5 ms) consisted of a fast decay (approximately 150.s-1 at 20 degrees C) and a slow decay (approximately 10.s-1) of tension. The rate of fast decay increased with temperature (Q10 approximately 2.4); at 35-40 degrees C, it was approximately 800.s-1, and it was followed by a delayed tension rise (stretch-activation) at 30-40.s-1. The linear rise of passive tension in warming to approximately 25 degrees C may be due to increase of thermal stress in titin (connectin)-myosin composite filament, whereas the extra tension above approximately 30 degrees C may arise from cycling cross-bridges; based on previous findings from regulated actomyosin in solution (Fuchs, 1975), it is suggested that heating reversibly inactivates the troponin-tropomyosin control mechanism and leads to Ca-independent thin filament activation at high temperatures. Additionally, we propose that the heating-induced increase of endo-sarcomeric stress within titin-myosin composite filament makes the cross-bridge mechanism stretch-sensitive at high temperatures.

Elasticity of the human red cell membrane skeleton. Effects of temperature and denaturants

The molecular basis for the elasticity of the human erythrocyte membrane was explored. Skeletons were released from ghosts in Triton X-100 and their dimensions followed by dark-field microscopy and packed volume. The rest size of skeletons was assumed to reflect the balance point between expansion (deformation) driven by electrostatic repulsions among the excess of fixed negative charges on the proteins and contraction (recovery) driven by their elasticity. The size of skeletons decreased with increasing temperature. This finding suggests that entropy drives elasticity. The requisite entropy change could be associated with either the configurational freedom of flexible protein chains or with the solvation of side chains exposed during protein dissociation (hydrophobic effects). To distinguish between these two alternatives, we tested the impact of two weak denaturants, 10% ethanol and 20 nM lithium 3,5-diiodosalicylate. Both agents reversibly promoted the expansion of skeletons, presumably by reducing their elasticity. Since the conformation of random coils and globular proteins should not be significantly altered by these mild treatments, this finding strongly suggests a role for weak interdomain and/or interprotein associations. We conclude that the elasticity of the red cell membrane skeleton may not derive from the configurational entropy of flexible coils. Rather, the elastic energy may arise from reversible dissociations of weak but specific intramolecular and/or intermolecular contacts, presumably within deformed spectrin filaments.

Possible role of helix-coil transitions in the microscopic mechanism of muscle contraction

Local helix-coil transitions in the coiled coil portion of myosin have long been implicated as a possible origin of tension generation in muscle. From a statistical mechanical theory of conformational transitions in coiled coils, the free energy required to form a randomly coiled bubble in the hinge region of myosin of the type conjectured by Harrington (Harrington, W. F., 1979, Proc. Natl. Acad. Sci. USA, 76:5066-5070) is estimated to be approximately 25 kcal/mol. Unfortunately this is far more than the free energy available from ATP hydrolysis if the crossbridges operate independently. Thus, in solution such bubbles are predicted to be absent, and the theory requires that the rod portion of myosin be a hingeless, continuously deforming rod. While such bubble formation in vivo cannot be entirely ruled out, it appears to be unlikely. We further conjecture that in solution the swivel located between myosin subfragments 1 and 2 (S-2 and S-1) is due to a locally random conformation of the chains caused by the presence of a proline residue at the point that physically separates the coiled coil from the globular portion of myosin. On attachment of S-1 to actin in the strong binding state, the configurational entropy of the random coil in the swivel region is greatly reduced relative to the case where the ends are free. This produces a spontaneous coil-to-helix transition in the swivel region that causes rotation of S-1 and the translation of actin. Thus, the model predicts that the actin filaments are pushed rather than pulled past the thick filaments by the crossbridges. The specific mechanism of force generation is examined in detail, and a simple statistical mechanical realization of the model is proposed. We find that the model gives a substantial number of qualitative and at times quantitative predictions in accord with experiment, and is particularly appealing in that it provides a simple means of free energy transduction--the well known fact that topological constraints shift the equilibrium between helical and random coil states.

A proposed mechanism of contraction in which stepwise shortening is a basic feature

A simple model is put forth as an initial attempt to account for the observation that the contractile process is discrete and synchronized over a large region of space.

Organization of the neurofilamentous network

The neurofilamentous network of the normal rabbit brain (lateral vestibular nucleus) and of biopsies of human patients (cerebral cortex, sural nerve) was investigated electron microscopically. Thin sections of samples prepared by standard techniques and unfixed spreads of freshly isolated perikarya were utilized. The neurofilaments are assembled into a three-dimensional network associated with the axolemma, microtubules, mitochondria and polyribosomes. The elements of this network demonstrate helicity at several levels of organization. It is proposed that they are in a dynamic state of equilibrium between ordered lattice and open network paracrystalline states. Reversible phase transitions in the subunit proteins of the neurofilaments may lead to coiling and uncoiling of the filaments and induce alterations in the network structure of the neuroplasm. Giant axonal swellings in biopsies of the sural nerve are interpreted as accumulations of cytoskeletal elements in the absence of the orienting effect of microtubules. In cortical neurons of patients with Alzheimer's disease parts of the neurofilamentous network are in altered paracrystalline states; virus-like particles occur within this modified network. These concepts of cytoskeletal organization - network, helicity, phase transitions, and paracrystallinity - are useful for the interpretation of pathological alterations of the cytoskeleton and for an understanding of cytoskeletal organization in general.

Thermal transitions of myosin and its helical fragments. I. Shifts in proton equilibria accompanying unfolding

The thermal transitions of myosin and its helical fragments have been studied with pH as the observable. Heating unbuffered solutions of these proteins near their pI values causes an abrupt rise in pH at a characteristic temperature (the "melting temperature," Tm) which is due to structural changes within the protein. Since the pH shift turns out to be insensitive to the degree of protein aggregation, we have obtained acceptable melting curves even under conditions where the protein coagulates during melting. The melting profiles and Tm vlaues of myosin, myosin rod, and light meromyosin have been found to be remarkably similar (Tm equal to 40 plus or minus 1 degree, 0.5 M KCl, pH 5.9). Proton binding which occurs during melting coincides with the unfolding of a section of myosin rod. Taken in the context of other studies, the proton binding is thought to occur near the "hinge region."

Thermal transitions of myosin and its helical fragments. II. Solvent-induced variations in conformational stability

The melting behavior of myosin and myosin rod has been studied over the pH range 5.4-7.0, in 0.5 M KCl. Both proteins exhibit almost identical T-m values, which increase from about 37 to 43 degrees as the pH is elevated through the range of study, T-m follows a sigmoidal dependence upon pH, and the inflection point occurs near pH 6.5. The influence of salt concentration on T-m was studied, by the variation of KCl from 0.15 to 2.92 M. With an increasing KCl concentration, both proteins exhibit similar, sigmoidal declines in T-m from about 44 to 34 degrees. Under all conditions of pH and ionic strength studied, melting is accompanied by an absorption of H+ by the protein. The concentration of the protein, the age of the preparation, and the presence of divalent metal ions fail to exert a significant effect on the T-m values obtained by our methods. The effects of salt concentration and pH on the thermal stability of myosin and myosin rod are discussed in terms of the location of the melting process within the myosin molecule. Myosin is shown to possess several of the requisite features for energy transduction via a proton-coupling mechanism. These features provide a framework for investigating some aspects of the molecular basis of muscle contraction within the context of the sliding filament model.