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The kinetics of the beta-hydride elimination reaction in propyl (CH3CH2CH2-Cu) and trifluoropropyl (CF3CH2CH2-Cu) groups on the Cu(111) surface were analyzed. The adsorption of propyl iodide (1-iodopropane, CH3CH2CH2I) and trifluoropropyl iodide (1,1,1-trifluoro-3-iodopropane, CF3CH2CH2I) develop these species at low temperatures (T is less than 120 kelvin). Trifluoropropylene (CF3CH double bonded with CH2) and propylene are formed by the decomposition of the adsorbed propyl groups through beta-hydride elimination. The obstacle to beta-hydride elimination rises by the fluorination of the terminal methyl group.

We present a new analysis of small-angle neutron-scattering data from rabbit muscle actin in the course of the polymerization from G-actin to F-actin as a function of temperature. The data, from Ivkov et al. [J. Chem. Phys. 108, 5599 (1998)], were taken in D2O buffer with Ca2+ as the divalent cation on the G-actin in the presence of ATP and with KCl as the initiating salt. The new analysis of the data using modeling and the method of generalized indirect fourier transform (O. Glatter, GIFT, University of Graz, Austria, http://physchem.kfunigraz.ac.at/sm/) provide shapes and dimensions of the G-actin monomer and of the growing actin oligomer in solution as a function of temperature and salt concentration. This analysis indicates that the G-actin monomer, under the conditions given above, is a sphere 50–54 Å in diameter as opposed to the oblate ellipsoid seen by x-ray crystallography. The F-actin dimensions are consistent with x-ray crystal structure determinations. [ABSTRACT FROM AUTHOR]

The protein actin can polymerize from monomeric globular G-actin to polymeric filamentary F-actin, under the regulation of thermodynamic variables such as temperature, pressure, and compositions of G-actin and salts. We present here new measurements of the extent of polymerization ([lowercase_phi_synonym]) of actin under pressure (P), for rabbit skeletal muscle actin in H2O buffer in the presence of adenosine triposphate and calcium ions and at low (5–15 mM) KCl concentrations. We measured [lowercase_phi_synonym] using pyrene-labeled actin, as a function of time (t) and temperature (T), for samples of fixed concentrations of initial G-actin and KCl and at fixed pressure. The [lowercase_phi_synonym](T,P) measurements at equilibrium have the same form as reported previously at 1 atm: low levels of polymerization at low temperatures, representing dimerization of the actin; an increase in [lowercase_phi_synonym] at the polymerization temperature (Tp); a maximum in [lowercase_phi_synonym](T) above Tp with a decrease in [lowercase_phi_synonym](T) beyond the maximum, indicating a depolymerization at higher T. From [lowercase_phi_synonym](T,P) at temperatures below Tp, we estimate the change in volume for the dimerization of actin, ΔVdim, to be -307±10 ml/mol at 279 K. The change of Tp with pressure dTp/dP=(0.3015±0.0009) K/MPa=(30.15±0.09) mK/atm. The [lowercase_phi_synonym](T,P) data at higher T indicate the change in volume on propagation, ΔVprop, to be +401±48 ml/mol at 301 K. The [lowercase_phi_synonym](t) measurements yield initial relaxation times rp(T) that reflect the behavior of [lowercase_phi_synonym](T) and support the presence of a depolymerization temperature. We also measured the density of polymerizing actin with a vibrating tube density meter, the results of which confirm that the data from this instrument are affected by viscosity changes and can be erroneous. [ABSTRACT FROM AUTHOR]

Studies of the dependence of actin polymerization on thermodynamic parameters are important for understanding processes in living systems, where actin polymerization and depolymerization are crucial to cell structure and movement. We report measurements of the extent of polymerization, F, of rabbit muscle actin as a function of temperature [T=(0 - 35)°C], initial G-actin concentration [[G[SUB0]]=(1 - 3) mg/ml], and initiating salt concentration [[KCl]=(5-15) mmol/l with bound Ca[SUP2+]], in H[SUB2]O and D[SUB2]O buffers and in the presence of adenosine triphosphate (ATP). A preliminary account of the data and analysis for H[SUB2]O buffers has appeared previously [P. S. Niranjan, J. G. Forbes, S. C. Greer, J. Dudowicz, K. F. Freed, and J. F. Douglas, J. Chem. Phys. 114, 10573 (2001)]. We describe the details of the studies for H[SUB2]O buffers, together with new data and analysis for D[SUB2]O buffers. The measurements show a maximum in φ(T) for H[SUB2]O buffers and D[SUB2]O buffers. For H[SUB2]O buffers, T[SUBp] decreases as either [G[SUB0]] or [KCl] increases. For D[SUB2]O buffers, T[SUBp] decreases as [KCl] increases, but T[SUBp] is not monotonic in [G[SUB0]]. The measurements are interpreted in terms of a Flory-Huggins-type lattice model that includes the essential steps: monomer activation, dimerization of activated species, and propagation of trimers to higher order polymers. The competition between monomer activation and chain propagation leads to the observed nonmonotonic variation of φ(T). The actin polymerization in D[SUB2]O buffer differs considerably from that in the H[SUB2]O buffer and underscores the significant deuterium effect on hydrophobic interactions and hydrogen bonding in the polymerization process. [ABSTRACT FROM AUTHOR]

Investigates the polymerization of actin. Use of small angle neutron scattering in the study; Onset of the polymerization of G-actin to F-actin with temperature rise; Relationship between average polymer size and temperature.

Nebulin is an about 1µm long intrinsically disordered scaffold for the thin filaments of skeletal muscle sarcomere. It is a multifunctional elastic protein that wraps around actin filament, stabilizes thin filaments, and regulates Ca-dependent actomyosin interactions. This study investigates whether the disorder profile of nebulin might encode guidelines for thin and thick filament interactions in the sarcomere of the skeletal muscle. The question was addressed computationally by analyzing the predicted disorder profile of human nebulin (6669 residues, ~200 actin-binding repeats) by pondr and the periodicity of the A-band stripes (reflecting the locations of myosin-associated proteins) in the electron micrographs of the sarcomere. Using the detrended fluctuation analysis, a scale factor for the A-band stripe image data with respect to the nebulin disorder profile was determined to make the thin and thick filaments aligned to have maximum correlation. The empirical mode decomposition method was then applied to identify hidden periodicities in both the nebulin disorder profile and the rescaled A-band data. The decomposition reveals three characteristic length scales (45 nm, 100 nm, and 200 nm) that are relevant for correlational analysis. The dynamical cross-correlation analyses with moving windows at various sarcomere lengths depict a vernierlike design for both periodicities, thus enabling nebulin to sense position and fine tune sarcomere overlap. This shows that the disorder profile of scaffolding proteins may encode a guideline for cellular architecture. [ABSTRACT FROM AUTHOR]

The nanomechanical properties of the coiled-coils of myosin are fundamentally important in understanding muscle assembly and contraction. Force spectra of single molecules of double-headed myosin, single-headed myosin, and coiled-coil tail fragments were acquired with an atomic force microscope and displayed characteristic triphasic force-distance responses to stretch: a rise phase (R) and a plateau phase (P) and an exponential phase (E). The R and P phases arise mainly from the stretching of the coiled-coils, with the hinge region being the main contributor to the rise phase at low force. Only the E phase was analyzable by the worm-like chain model of polymer elasticity. Restrained molecular mechanics simulations on an existing x-ray structure of scallop S2 yielded force spectra with either two or three phases, depending on the mode of stretch. It revealed that coiled-coil chains separate completely near the end of the P phase and the stretching of the unfolded chains gives rise to the E phase. Extensive conformational searching yielded a P phase force near 40 pN that agreed well with the experimental value. We suggest that the flexible and elastic S2 region, particularly the hinge region, may undergo force-induced unfolding and extend reversibly during actomyosin powerstroke.