Low-temperature dynamic light scattering. I. Structural reorganization and physical gel formation in cellulose triacetate/methyl acetate dilute solution at -99 - 45 degrees C

Curious low-temperature solubility of cellulose triacetates (CTA; here we use nominally "CTA," but the sample still contains 7% of C-6 position hydroxyls) in an organic solvent, methyl acetate (MA), was studied by a newly designed low-temperature type of DLS apparatus, which enabled for the first time to investigate the structural change of CTA in solution from 45 degrees C down to -100 degrees C. A molecularly dissolved CTA was found to coexist with three types of self-assemblies over all the temperature ranges except for the three specific temperatures T* of 30, -10, and -75 degrees C. However, these multiple self-assemblies are not in real thermodynamic equilibrium but in a metastable state, which could be stabilized effectively by the intermolecular hydrogen bonding (HB) with the help of the dipole interaction at low temperatures. In more detail, with decreasing temperature, these assemblies performed the structural reorganization drastically at three T*'s and would finally be frozen in a physical gel structure at -99 degrees C; around the freezing temperature of MA, CTA molecules could be trapped homogeneously in the frozen MA. The crucial role in such structural reorganizations is played by the balance between the intermolecular HB and the dipole interaction worked in the highly electronegative solvent. Because these interactions, which are mediated by the solvent electronegativity, change drastically with temperature, they result in the control of not only the single CTA chain conformation (= the intramolecular HB) but also the binding ways of the intermolecular HBs between CTA molecules and they induce multitudinous metastable structures in solution. Here it is noted that HB could work mainly between the C-6 position hydroxyls in the anhydroglucose units of CTA and are essentially effective at low temperatures.

Graft polymerization from a silica surface initiated by adsorbed peroxide macroinitiators. I. Adsorption and structure of the adsorbed layer of peroxide macroinitiators on a silica surface

The adsorption features of two peroxide macroinitiators (PMIs) with various functionalities from their semi-dilute solutions on the silica surface were thoroughly investigated in the present work. These investigations include the study of the adsorption kinetics of PMI in diverse solvents and a detailed examination of the adsorbed layer structure with the aid of ellipsometry, scanning force microscopy (SFM), and contact angle measurements. Rearrangements of PMI macromolecules at the solid surface are supposed to be the main reason for the appearance of extremes on the kinetic curves and, besides, have a more pronounceable effect on adsorption rate than their diffusion rate to the surface even at the initial stage of the process. Both island-like and densely packed structures of absorbed layers were revealed by combining contact angle measurements and SFM. Surprisingly, even in the case when saturation of the adsorbed layer is reached, PMI does not completely occupy the substrate surface which is at least particularly reachable for the wetting liquids. PMIs adsorbed at the solid surface are intended for the formation of tethered polymer "brushes" via the initiation of "grafting from" polymerization.

Structure transition in myosin association with the change of concentration: solubility equilibrium under specified KCl and pH condition

We observed, for the first time, the elementary process for the ordered self-assembly formation of myosin in solution. It was realized exclusively under the specific condition of 200 mM KCl, 5 mM phosphate buffer, pH 7.08, at 15-20 degrees C, which is called the transition-generating condition (TGC). Described more in detail: pure myosin extracted from rabbit skeletal muscle exhibited the structural transition in its association form only when the myosin concentration c was changed under TGC. The myosin solubility was saturated in both edges of the total myosin concentration c > 10.0 mg/mL (solubility region II) and c < or = 0.25 mg/mL (solubility region I). In the intermediate region, the association structure of myosin changed stepwise with decreasing c. The steps were classified into four regions: region I (c < or = 0.25 mg/mL), II (0.25 < or = c < or = 0.50 mg/mL), III (0.50 < or = c < or = 5.0 mg/mL), and IV (c > 5.0 mg/mL). In each region except II, the plot of the relative soluble myosin concentration c(aq)/c against c(-1) gave a straight line of different slopes, certifying that myosin constructs self-assemblies by the closed association mechanism and that the self-assembly takes dual structures in each region. In region II, a drastic transition occurred in the self-assembled dual structures. Here, a highly associated (insoluble) giant assembly would break into soluble assemblies composed of several myosin molecules. The solubility region I originates a driving force for this structural transition. The basic binding unit of the self-assembly would be a parallel myosin-dimer constructed by the intermolecular axial staggers of 14.3 and 43 nm, as is observed by X-ray diffraction for the thick filament assembly or light meromyosin paracrystals. Myosin could take a single rod-like chain form only in an extremely low concentration region of c < or = c(aq,0) (= 0.053 mg/mL). The association behavior revealed in the present study suggests strongly that the complementary charge cluster and its electrostatic interaction between parallel myosin rods play a crucial role for the ordered self-assembly formation and that the specific electrostatic atmosphere of the solution under TGC is essential to the association mechanism in skeletal muscle myosin, or the thick filament formation of the mammals.

Regioselectively substituted 6-O- and 2,3-di-O-acetyl-6-O-triphenylmethylcellulose: its chain dynamics and hydrophobic association in polar solvents

Two kinds of regioselectively substituted cellulose derivatives, i.e., 6-O-triphenylmethylcellulose (6TC) and 2,3-di-O-acetyl-6-O-triphenylmethylcellulose (2,3Ac6TC), were prepared via cellulose. In these samples, C-6 position hydroxyls in the anhydroglucose units (AGU) along the cellulose chain were selectively substituted by the hydrophobic triphenylmethyl groups, but C-2 and -3 position hydroxyls remained in 6TC or were substituted completely by O-acetyls in 2,3Ac6TC. Their chain dynamics in polar solvents, dimethyl sulfoxide (DMSO) and N,N-dimethylacetamide (DMAc), in dilute solution were investigated by dynamic light scattering in the viewpoint of cluster formation. The results were compared with those of cellulose diacetates (CDA) in DMAc where three hydroxyls in the AGU were statistically substituted up to 2.44 by O-acetyls but hydroxyls at C-6 positions remained predominantly. It was found that 6TC and 2,3Ac6TC formed a dynamic structure about 10 times larger than single chains and that the structure would be a temporary and local association due to concentration fluctuations (dynamic structures) which were originated from the hydrophobic interactions between intermolecular triphenylmethyl groups. The dynamics and structures were in clear contrast to those of CDA where a solvent-mediated hydrogen bonding between intermolecular C-6 position hydroxyls was essential to cluster formation. The present structures were so weak as to dissipate easily under low shear field.

Substituent Distribution in Cellulose Acetates: Its Control and the Effect on Structure Formation in Solution

Architecture-regulated cellulose derivatives were prepared by regioselective substitution of C-2, -3, and -6 position hydroxyls in cellulose by O-acetyls or trityls. Their dynamic structures formed in polar solvents were compared with those by the commercial cellulose acetates where the distribution of hydroxyls and O-acetyls was nearly random in the chain. It was found that the difference in the chain architecture induces a large difference in the chain conformation, the solubility, and the clustering mechanism and structures; i.e., they changed with solvent quality, concentration, temperature, strength of external field, and so on. Copyright 2000 Academic Press.

Structural characterization of the molten globule and native states of ovalbumin: a 1H NMR study

Molecular characteristics of ovalbumin (OVA) in the acidic (pD 3.08, the E-form) and neutral [pD 7.29, the N-form (native form)] regions were studied by measuring effective radii, 1H NMR spectra, spin-echo 1H NMR spectra and cross-relaxation times (TIS) from irradiated to observed protein protons which are particularly sensitive for detection of the mobile segments and/or structural looseness in proteins. 1H NMR spectra did not show significant differences between the N- and E-forms except for the spectral lines in the CH3, epsilon CH2 and aromatic regions. Effective radii and TIS values for main- and side-chains showed 1.08 and 1.5- to 2.0-fold increases on going from the N- to E-forms, respectively. The elongation of TIS values might indicate the appearance of the fluctuating tertiary structure in the E-form. Molecular characteristics of the E-form, inferred from reported far ultraviolet-circular dichroism (UV-CD) spectra in the peptide region, near UV-CD spectra in the aromatic region [Koseki et al. (1988) J. Biochem. 103, 425-430], effective radii and especially elongation of TIS values might indicate that the E-form could be in the molten globule state. The onset of denaturation of OVA using TIS measurements was also studied.