Analyis of structure/property relationships in silkworm (Bombyx mori) and spider dragline (Nephila edulis) silks using Raman spectroscopy

The molecular deformation of both silkworm (Bombyx mori) and spider dragline (Nephila edulis) silks has been studied using a combination of mechanical deformation and Raman spectroscopy. The stress/strain curves for both kinds of silk showed elastic behavior followed by plastic deformation. It was found that both materials have well-defined Raman spectra and that some of the bands in the spectra shift to lower frequency under the action of tensile stress or strain. The band shift was linearly dependent upon stress for both types of silk fiber. This observation provides a unique insight into the effect of tensile deformation upon molecular structure and the relationship between structure and mechanical properties. Two similar bands in the Raman spectra of both types of silk in the region of 1000-1300 cm(-1) had significant identical rates of Raman band shift of about 7 cm(-1)/GPa and 14 cm(-1)/GPa demonstrating the similarity between the silk fibers from two different animals.     

Modeling crystal and molecular deformation in regenerated cellulose fibers

Experimental deformation micromechanics of regenerated cellulose fibers using Raman spectroscopy have been widely reported. Here we report on computer modeling simulations of Raman band shifts in modes close to the experimentally observed 1095 cm(-1) band, which has previously been shown to shift toward a lower wavenumber upon application of external fiber deformation. A molecular mechanics approach is employed using a previously published model structure of cellulose II. Changing the equilibrium c-spacing of this structure and then performing a minimization routine mimics tensile deformation. Normal-mode analysis is then performed on the minimized structure to predict the Raman-intensive vibrations. By using a dot-product analysis on the predicted eigenvectors it is shown that some Raman active modes close to the 1095 cm(-1) band interchange at certain strain levels. Nevertheless, when this is taken into account it is shown that it is possible to find reasonable agreement between theory and experiment. The effect of the experimentally observed broadening of the Raman bands is discussed in terms of crystalline and amorphous regions of cellulose, and this is compared to the lack of X-ray broadening to explain why discrepancies between theory and experiment are present. A hybrid model structure with a series-parallel arrangement of amorphous and misaligned amorphous-crystalline domains is proposed which is shown to agree with what is observed experimentally. Finally, the theoretical crystal modulus for cellulose II is reported as 98 GPa, which is shown to be in agreement with other studies and with an experimental measurement using synchrotron X-ray diffraction.     

Elastic modulus and stress-transfer properties of tunicate cellulose whiskers

Experimental deformation micromechanics of natural cellulose fibers using Raman spectroscopy and X-ray diffraction have been widely reported. However, little has been published on the direct measurements of the mechanical properties, and in particular the elastic modulus, of the highly crystalline material in the native state. Here we report on measurements of the elastic modulus of tunicate cellulose using a Raman spectroscopic technique. A dispersed sample of the material is deformed using a four-point bending test, and a shift in a characteristic Raman band (located at 1095 cm(-1)) is used as an indication of the stress in the material. Relatively little intensity change of the Raman band located at 1095 cm(-1) is shown to occur for samples oriented parallel and perpendicular to the polarization direction of the laser, as compared to a highly oriented flax sample. This indicates that the tunicate sample is a two-dimensional in-plane random network of fibers. By use of this result, the Raman shift, and calibrations with strain from other materials, it is shown that the modulus of the material is very high, at about 143 GPa, and a lack of Raman band broadening is thought to be due to the fact that there is pure crystalline deformation occurring without the effect of crystalline/amorphous fractions. A strain sensitivity of the shift in the 1095-cm(-1) Raman peak for this specimen is shown to be -2.4 +/- 0.2 cm(-1)/%. A molecular mechanics approach, using computer simulation and an empirical force field, was used to predict the modulus of a highly oriented chain of the material, and this is found to be 145 GPa, which is in agreement with the experimental data. However, by use of a normal-mode analysis, it is found that a number of modes have positions close to the central positions of the experimental Raman band. One in particular is found to shift at a rate of 2.5 cm(-1)/%, but due to the complex nature of the structure, it is not entirely conclusive that this band is representative of the experimental findings.     

FTIR analysis of cellulose treated with sodium hydroxide and carbon dioxide

Cellulose samples treated with sodium hydroxide (NaOH) and carbon dioxide in dimethylacetamide (DMAc) were analyzed by FTIR spectroscopy. Absorbance of hydrogen-bonded OH stretching was considerably decreased by the treatment of NaOH and carbon dioxide. The relative absorbance ratio (A(4000-2995)/A(993)) represented the decrease of absorbance as a criterion of hydrogen-bond intensity (HBI). The absorbance of the band at 1430cm(-1) due to a crystalline absorption was also decreased by NaOH treatment. The absorbance ratio of the bands at 1430 and 987-893cm(-1) (A(1430)/A(900)), adopted as crystallinity index (CI), was closely related to the portion of cellulose I structure. With the help of FTIR equipped with an on-line evacuation apparatus, broad OH bending due to bound water could be eliminated. FTIR spectra of the carbon dioxide-treated cellulose samples at 1700-1525cm(-1) were divided into some bands including 1663, 1635, 1616, and 1593cm(-1). The broad OH bending due to bound water at 1641-1645cm(-1) was resolved to two bands at 1663 and 1635cm(-1). As a trace of DMAc, the band at 1616cm(-1) is disappeared by washing for the cellulose treated with carbon dioxide (Cell 1-C and Cell 2/60-C). The decrease of HBI, the easy removal of DMAc, and the band at 1593cm(-1) supported the introduction of new chemical structure in cellulose. The bands shown at 1593 and 1470cm(-1) was assigned as hydrogen-bonded carbonyl stretching and O-C-O stretching of the carbonate ion.     

Effective Young's modulus of bacterial and microfibrillated cellulose fibrils in fibrous networks

The deformation micromechanics of bacterial cellulose (BC) and microfibrillated cellulose (MFC) networks have been investigated using Raman spectroscopy. The Raman spectra of both BC and MFC networks exhibit a band initially located at ≈ 1095 cm(-1). We have used the intensity of this band as a function of rotation angle of the specimens to study the cellulose fibril orientation in BC and MFC networks. We have also used the change in this peak's wavenumber position with applied tensile deformation to probe the stress-transfer behavior of these cellulosic materials. The intensity of this Raman band did not change significantly with rotation angle, indicating an in-plane 2D network of fibrils with uniform random orientation; conversely, a highly oriented flax fiber exhibited a marked change in intensity with rotation angle. Experimental data and theoretical analysis shows that the Raman band shift rate arising from deformation of networks under tension is dependent on the angles between the axis of fibrils, the strain axis, the incident laser polarization direction, and the back scattered polarization configurations. From this analysis, the effective moduli of single fibrils of BC and MFC in the networks were estimated to be in the ranges of 79-88 and 29-36 GPa, respectively. It is shown also that for the model to fit the data it is necessary to use a negative Poisson's ratio for MFC networks and BC networks. Discussion of this in-plane "auxetic" behavior is given.     

Stress transfer in cellulose nanowhisker composites--influence of whisker aspect ratio and surface charge

The mechanically induced molecular deformation of cellulose nanowhiskers embedded in subpercolation concentration in an epoxy resin matrix was monitored through Raman spectroscopy. Cellulose nanowhiskers isolated by sulfuric acid hydrolysis from tunicates and by sulfuric acid hydrolysis and hydrochloric acid hydrolysis from cotton were used to study how the aspect ratio (ca. 76 for tunicate and 19 for cotton) and surface charges (38 and 85 mmol SO(4)(-)/kg for sulfuric acid hydrolysis of cotton and tunicate, respectively; no detectable surface charges for hydrochloric acid hydrolysis) originating from the isolation process influence stress transfer in such systems. Atomic force microscopy confirmed that uncharged cellulose nanowhiskers produced by hydrochloric acid hydrolysis have a much higher tendency to aggregate than the charged cotton or tunicate nanowhiskers. Each of these nanowhisker types was incorporated in a concentration of 0.7 vol % in a thermosetting epoxy resin matrix. Mechanically induced shifts of the Raman peak initially located at 1095 cm(-1) were used to express the level of deformation imparted to the nanowhiskers embedded in the resin. Much larger shifts of the diagnostic Raman band were observed for nanocomposites with tunicate nanowhiskers than for the corresponding samples comprising cotton nanowhiskers. In the case of nanocomposites comprising nanowhiskers produced by hydrochloric acid hydrolysis, no significant Raman band shift was observed. These results are indicative of different modes of stress transfer, which in turn appear to originate from the different sample morphologies.     

Load distribution in native cellulose

The properties of cellulose materials are dependent on interactions between and within the cellulose chains. To investigate the deformation behavior of cellulose and its relation to molecular straining, sheets with fibers oriented preferably in one direction were studied by dynamic FT-IR spectroscopy. Celluloses with different origins (spruce pulp, Cladophora cellulose, cotton linters) were used. The sheets were stretched sinusoidally at low strains and small amplitudes while being irradiated with polarized infrared radiation. The cellulose fibers showed mainly an elastic response. The cellulose fibers showed mainly an elastic response. The glucose rings and the C-O-C bridges connecting adjacent rings, as well as the O(3)H.O(5) intramolecular hydrogen bonds are the components mainly deformed under stress, whereas the O(2)H.O(6) intramolecular hydrogen bonds play a minor role. The load distribution was also found to be different in the different allomorphic forms of cellulose I, namely, I(alpha) and I(beta).     

Effect of fiber orientation in dynamic FTIR study on native cellulose

The properties of cellulose materials are highly dependent on the interactions between and within the cellulose chains mainly related to inter- and intramolecular hydrogen bonds. To investigate the deformation behavior of cellulose and its relation to molecular straining, cellulose sheets with different fiber orientations were studied by dynamic FTIR spectroscopy. The sheets were stretched sinusoidally at low strains while being irradiated with polarized infrared light. It is shown that the polarization direction determines the dynamic IR response to a higher extent than the fiber direction in the sample sheets. Different polarization modes give different dynamic signals, allowing conclusions to be drawn on the structural orientation of submolecular groups in the cellulose molecules. The bands in the spectra mainly affected by the deformation of the sheets were derived from skeletal vibrations that include the C-O-C bridge connecting adjacent rings and from the hydrogen bonds. The conclusion that these groups are the ones that are mainly deformed under load has thereby experimentally demonstrated the theoretical calculations from Tashiro and Kobayashi [Tashiro, K.; Kobayashi, M. Polymer 1991, 32, 1516-1526].     

Structural characteristics and properties of Bombyx mori silk fiber obtained by different artificial forcibly silking speeds

To study the spinning condition of natural biopolymer silk, the silk fibers were directly acquired from Bombyx mori silkworm, N140 x C140 by a simple artificial forcibly silking method at the speed of 60, 120, 180 and 240 cm min(-1), respectively and its microstructure and physical properties were evaluated. The fine silk fibers (about 8 microm) were obtained at faster spinning speed, 240 cm min(-1). The tensile properties of silk fibers were remarkably increased with raising the forcibly spinning speeds. The beta-sheet structure contents of silk fibers obtained at higher speed were considerably increased. The fibers obtained by different spinning speeds exhibited a fairly similar X-ray crystallinity, while the degree of molecular orientation increased with decreasing the fiber diameter. The fine silk fibers obtained at higher speed (240 cm min(-1)) exhibited a slightly higher thermal stability, as shown by the upward shift of differential scanning calorimetry (DSC) decomposition temperature.     

Crystalline structure analysis of cellulose treated with sodium hydroxide and carbon dioxide by means of X-ray diffraction and FTIR spectroscopy

Crystalline structures of cellulose (named as Cell 1), NaOH-treated cellulose (Cell 2), and subsequent CO2-treated cellulose (Cell 2-C) were analyzed by wide-angle X-ray diffraction and FTIR spectroscopy. Transformation from cellulose I to cellulose II was observed by X-ray diffraction for Cell 2 treated with 15-20 wt% NaOH. Subsequent treatment with CO2 also transformed the Cell 2-C treated with 5-10 wt% NaOH. Many of the FTIR bands including 2901, 1431, 1282, 1236, 1202, 1165, 1032, and 897 cm(-1) were shifted to higher wave number (by 2-13 cm(-1)). However, the bands at 3352, 1373, and 983 cm(-1) were shifted to lower wave number (by 3-95 cm(-1)). In contrast to the bands at 1337, 1114, and 1058 cm(-1), the absorbances measured at 1263, 993, 897, and 668 cm(-1) were increased. The FTIR spectra of hydrogen-bonded OH stretching vibrations at around 3352 cm(-1) were resolved into three bands for cellulose I and four bands for cellulose II, assuming that all the vibration modes follow Gaussian distribution. The bands of 1 (3518 cm(-1)), 2 (3349 cm(-1)), and 3 (3195 cm(-1)) were related to the sum of valence vibration of an H-bonded OH group and an intramolecular hydrogen bond of 2-OH ...O-6, intramolecular hydrogen bond of 3-OH...O-5 and the intermolecular hydrogen bond of 6-O...HO-3', respectively. Compared with the bands of cellulose I, a new band of 4 (3115 cm(-1)) related to intermolecular hydrogen bond of 2-OH...O-2' and/or intermolecular hydrogen bond of 6-OH...O-2' in cellulose II appeared. The crystallinity index (CI) was obtained by X-ray diffraction [CI(XD)] and FTIR spectroscopy [CI(IR)]. Including absorbance ratios such as A1431,1419/A897,894 and A1263/A1202,1200, the CI(IR) was evaluated by the absorbance ratios using all the characteristic absorbances of cellulose. The CI(XD) was calculated by the method of Jayme and Knolle. In addition, X-ray diffraction curves, with and without amorphous halo correction, were resolved into portions of cellulose I and cellulose II lattice. From the ratio of the peak area, that is, peak area of cellulose I (or cellulose II)/total peak area, CI(XD) were divided into CI(XD-CI) for cellulose I and CI(XD-CII) for cellulose II. The correlation between CI(XD-CI) (or CI(XD-CII)) and CI(IR) was evaluated, and the bands at 2901 (2802), 1373 (1376), 897 (894), 1263, 668 cm(-1) were good for the internal standard (or denominator) of CI(IR), which increased the correlation coefficient. Both fraction of the absorbances showing peak shift were assigned as the alternate components of CI(IR). The crystallite size was decreased to constant value for Cell 2 treated at >or= 15 wt% NaOH. The crystallite size of Cell 2-C (cellulose II) was smaller than that of Cell 2 (cellulose I) treated at 5-10 wt% NaOH. But the crystallite size of Cell 2-C (cellulose II) was larger than that of Cell 2 (cellulose II) treated at 15-20 wt% NaOH.     

Intramolecular and intermolecular perturbation on electronic state of FAD free in solution and bound to flavoproteins: FTIR spectroscopic study by using the C = O stretching vibrations as probes

The intramolecular and intermolecular perturbation on the electronic state of FAD was investigated by FTIR spectroscopy by using the C=O stretching vibrations as probes in D(2)O solution. Natural and artificial FADs, i.e. 8-CN-, 8-Cl-, 8-H-, 8-OCH(3)-, and 8-NH(2)-FAD labelled by 2-(13)C, (18)O=C(2), or 4,10a-(13)C(2) were used for band assignments. The C(2)=O and C(4)=O stretching vibrations of oxidized FAD were shifted systematically by the substitution at the 8-position, i.e. the stronger the electron-donating ability (NH(2) > OCH(3) > CH(3) > H > Cl > CN) of the substituent, the lower the wavenumber region where both the C(2)=O and C(4)=O bands appear. In contrast, the C(4)=O band of anionic reduced FAD scarcely shifted. The 1,645-cm(-1) band containing C(2)=O stretching vibration shifted to 1,630 cm(-1) in the medium-chain acyl-CoA dehydrogenase (MCAD)-bound state, which can be explained by hydrogen bonds at C(2)=O of the flavin ring. The band was observed at 1,607 cm(-1) in the complex of MCAD with 3-thiaoctanoyl-CoA. The 23 cm(-1) shift was explained by the charge-transfer interaction between oxidized flavin and the anionic acyl-CoA. In the case of electron-transferring flavoprotein, two bands associated with the C(4)=O stretching vibration were obtained at 1,712 and 1,686 cm(-1), providing evidence for the multiple conformations of the protein.     

Analysis of cell-wall polymers during cotton fiber development

Although the fibers of cotton (Gossypium hirsutum L.) are single cells with a secondary wall composed primarily of cellulose, the cell-wall polymers of the fibers are technically difficult to characterize with respect to molecular weights. This limitation hinders understanding how the fiber wall composition changes during development, particularly with respect to genotypic variations, and how the molecular composition is related to physical properties. We analyzed cell-wall polymers from cotton fibers (cultivar, Texas Marker-1) at several developmental stages (8–60 days post-anthesis; DPA) by gel-permeation chromatography of components soluble in dimethyl acetamide and lithium chloride. This procedure solubilizes fiber cell-wall components directly without prior extraction or derivatization, processes that could lead to degradation of high-molecular-weight components. Cellwall polymers from fibers at primary cell-wall stages had lower molecular weights than the cellulose from fibers at the secondary wall stages; however, the high-molecularweight cellulose characteristic of mature cotton was detected as early as 8 DPA. High-molecular-weight material decreased during the period of 10–18 DPA with concomitant increase in lower-molecular-weight wall components, possibly indicating hydrolysis during the later stages of elongation.Abbreviations DMAC dimethyl acetamide - DP degree of polymerization - DPA days post anthesis - GPC gel-permeation chromatography - MW molecular weight - MWD molecular-weight distribution - TM-1 Texas Marker 1     

Structure of a histidine ligand in the photosynthetic oxygen-evolving complex as studied by light-induced fourier transform infrared difference spectroscopy.

Fourier transform infrared (FTIR) signals of a histidine side chain were identified in flash-induced S(2)/S(1) difference spectra of the oxygen-evolving complex (OEC) of photosystem II (PS II) using PS II membranes from globally (15)N-labeled spinach and PS II core complexes from Synechocystis cells in which both the imidazole nitrogens of histidine were selectively labeled with (15)N. A negative band at 1113-1114 cm(-1) was downshifted by 7 cm(-1) upon both global (15)N-labeling and selective [(15)N]His labeling, and assigned to the C-N stretching mode of the imidazole ring. This band was unaffected by H-D exchange in the PS II preparations. In addition, several peaks observed at 2500-2850 cm(-1) all downshifted upon global and selective (15)N-labeling. These were ascribed to Fermi resonance peaks on a hydrogen-bonding N-H stretching band of the histidine side chain. FTIR measurements of model compounds of the histidine side chain showed that the C-N stretching band around 1100 cm(-)(1) can be a useful IR marker of the protonation form of the imidazole ring. The band appeared with frequencies in the following order: Npi-protonated (>1100 cm(-1)) > imidazolate > imidazolium > Ntau-protonated (<1095 cm(-1)). The frequency shift upon N-deuteration was occurred in the following order: imidazolium (15-20 cm(-1)) > Ntau-protonated (5-10 cm(-1)) > Npi-protonated approximately imidazolate ( approximately 0 cm(-1)). On the basis of these findings together with the Fermi resonance peaks at >2500 cm(-1) as a marker of N-H hydrogen-bonding, we concluded that the histidine residue in the S(2)/S(1) spectrum is protonated at the Npi site and that this Npi-H is hydrogen bonded. This histidine side chain probably ligated the redox-active Mn ion at the Ntau site, and thus, oxidation of the Mn cluster upon S(2) formation perturbed the histidine vibrations, causing this histidine to appear in the S(2)/S(1) difference spectrum.     

Heterogeneous acylation of flax fibers. Reaction kinetics and surface properties

Flax fibers composed mainly of cellulose were subjected to heterogeneous valerylation reaction. The progress of the chemical modification was assessed by transmission FTIR. The heterogeneous esterification reaction followed first-order kinetics, and a plateau was reached already after 30 min. The intensity of the FTIR hydroxyl absorption band (nu = 3400 cm(-1)) did not appreciably decrease during the acylation reaction, showing that only a small fraction of the fiber hydroxyls was involved in the reaction. The degree of valerate substitution (DS) at the fiber surface (50 A thick layer) was evaluated by means of ESCA. Surface valerylation increased with reaction time and leveled off at DS around 1 after 30 min, in agreement with the FTIR data. The chemically modified fibers maintain the Cellulose I crystal structure and the original crystallinity degree up to the longest reaction time investigated (180 min). Dynamic contact angle measurements showed that surface hydrophobicity as indicated by advancing contact angle rapidly increased upon valerylation reaching a plateau after about 10 min. Chemical modification does not appreciably alter fiber thermal stability (by TGA) and morphology (by SEM).     

Spectroscopic markers of the T<-->R quaternary transition in human hemoglobin

In this work, we use a sol-gel protocol to trap and compare the R and T quaternary states of both the deoxygenated (deoxyHb) and carbonmonoxide (HbCO) derivatives of human hemoglobin. The near infrared optical absorption band III and the infrared CO stretching band are used to detect the effect of quaternary structure on the spectral properties of deoxyHb and HbCO; comparison with myoglobin allows for an assessment of tertiary and quaternary contributions to the measured band shifts. The R<-->T transition is shown to cause a blue shift of the band III by approximately 35 cm(-1) for deoxyHb and a red shift of the CO stretching band by only approximately 0.3 cm(-1) for HbCO. This clearly shows that quaternary structure changes are transmitted to the heme pocket and that effects on deoxyHb are much larger than on HbCO, at least as far as the band energies are concerned. Experiments performed in the ample temperature interval of 300-10K show that the above quaternary structure effects are "static" and do not influence the dynamic properties of the heme pocket, at least as probed by the temperature dependence of band III and of the CO stretching band. The availability of quaternary structure sensitive spectroscopic markers and the quantitative measurement of the quaternary structure contribution to band shifts will be of considerable help in the analysis of flash-photolysis experiments on hemoglobin. Moreover, it will enable one to characterize the dynamic properties of functionally relevant hemoglobin intermediates and to study the kinetics of both the T-->R and R-->T quaternary transitions through time-resolved spectroscopy.     

Flexural rigidity of marginal bands isolated from erythrocytes of the newt

The marginal band is a bundle of microtubules residing at the periphery of nucleated erythrocytes of nonmammalian vertebrates and some invertebrates. Marginal bands from erythrocytes of the newt (Notopthalmus viridescens) were isolated from the cells as intact structures by treatment with detergent and either mild protease or high salt. Isolated bands were subjected to mechanical testing by stretching the band between a glass microhook and a calibrated glass fiber. The deflection of the fiber provided a measure of the force on the band. The flexural rigidity of the band was determined from measurements of the band deformation as a function of applied force. Bands isolated with either of two proteases (pepsin or elastase) or with high salt exhibited elastic behavior with a flexural rigidity of approximately 9.0 X 10(-12) dyn.cm2. Treatment of bands with chymopapain caused an increase in band rigidity and inelastic behavior. Estimates of the contribution of the band to cellular rigidity are made based on the measurements of the structural properties of the isolated band. The band provides the cell with a large resistance to indentations at the rim and to large extensions, while maintaining a high degree of flexibility in small extensions or flexure.     

FTIR spectra of solid poly-l-lysine in the stretching NH mode range

Three bands at 3270 cm(-1), 3200 cm(-1) and 3030 cm(-1) are found in the IR stretching proton (nu(1)) mode spectral range in spectra of solid poly-l-lysine (PLL). Strong quantitative changes of these bands are observed in samples dried from water solutions with different pH. The narrow band at 3270 cm(-1), which is strong in the spectrum of PLL precipitated from pH=12 alkaline medium, is assigned to the nu(1) peptide proton mode of NH-CO (amide A) of the beta-sheet structure type. The band at 3200 cm(-1), which is intensified in PLL precipitated from pH=1 acidic medium, relates to the nu(1) peptide mode in the random coil structure. The band at 3030 cm(-1), whose peak intensity increases two-fold in going from alkaline to acidic medium, is assigned to the nu(1) modes of protonated NH(3)(+) side chain groups. The frequencies of all bands were used for estimating H-bond energy relying on an empirical correlation between this property and the red shift of the nu(1) band. The enthalpy of the secondary structure transition from beta-sheet to the random coil, which is observed in PLL at the change of pH from 11 to 1 amounts to 4.7 kJ mol(-1).     

Comparison of the effects of cyclic stretching and compression on endothelial cell morphological responses

Recent results demonstrate the exquisite sensitivity of cell orientation responses to the pattern of imposed deformation. Cells undergoing pure in-plane uniaxial stretching orient differently than cells that are simply elongated--likely because the latter stimulus produces simultaneous compression in the unstretched direction. It is not known, however, if cells respond differently to pure stretching than to pure compression. This study was performed to address this issue. Human aortic endothelial cells were seeded on deformable silicone membranes and subjected to various magnitudes and rates of pure stretching or compression. The cell orientation and cytoskeletal stress fiber organization responses were examined. Both stretching and compression resulted in magnitude-dependent but not rate-dependent orientation responses away from the deforming direction. Compression produced a slower temporal response than stretching. However, stress fiber reorganization responses-early disruption followed by reassembly into parallel arrays along the cells' long axes were similar between the two stimuli. Moreover, the cell orientation and stress fiber responses appeared to be uncoupled since disruption of stress fibers was not required for the cell orientation. Moreover, parallel actin stress fibers were observed at oblique angles to the deforming direction indicating that stress fibers can reassemble when undergoing deformation.     

Effect of hydrophobic surfactant proteins SP-B and SP-C on binary phospholipid monolayers: II. Infrared external reflectance-absorption spectroscopy

In situ external reflection infrared spectroscopy at the air-water interface was used to study the influence on phospholipid structure of an endogenous mixture of the two hydrophobic surfactant proteins, SP-B and SP-C, which are thought to play pivotal roles in the adsorption and function of pulmonary surfactant. Mixtures studied were 1:1, 2:1, and 7:1 (mol:mol) DPPC-d(62):DPPG, and 7:1 DPPC-d(62):DOPG, alone and in the presence of 0.5-10 wt % mixed SP-B/C purified chromatographically from calf lung surfactant extract. Perdeuteration of DPPC produced a shift in vibrational frequencies so that it could be differentiated spectroscopically from the phosphoglycerol component in the surface monolayer. CH(2) antisymmetric and symmetric stretching bands ( approximately 2920 and 2852 cm(-1)) along with the analogous CD(2) stretching bands ( approximately 2194 and 2089 cm(-1)) were analyzed, and band heights and peak wavenumber positions were assessed as a function of monolayer surface pressure. Small, near-physiological contents of 1-2 wt % SP-B/C typically produced the maximum observed spectroscopic effects, which were abolished at high protein contents of 10 wt %. Analysis of CH(2) and CD(2) stretching bands and C-H/C-D band height ratios indicated that SP-B/C affected PC and PG lipids differently within the surface monolayer. SP-B/C had preferential interactions with DPPG in 1:1, 2:1, and 7:1 DPPC-d(62):DPPG films that increased its acyl chain order. SP-B/C also interacted specifically with DOPG in 7:1 DPPC-d(62):DOPG monolayers, but in this case an increase in CH(2) band heights and peak wavenumber positions indicated a further disordering of the already fluid DOPG acyl chains. CD(2) band height and peak wavenumber analysis indicated that SP-B/C had no significant effect on the structure of DPPC-d(62) chains in 7:1 films with DPPG or DOPG, and had only a slight tendency to increase the acyl chain order in 1:1 films of DPPC-d(62):DPPG. SP-B/C had no significant effect on DPPC-d(62) structure in films with DOPG. Infrared results also indicated that interactions involving SP-B/C and lipids led to exclusion of PC and PG lipids from the compressed interfacial monolayer, in agreement with our previous report on the phase morphology of lipid monolayers containing 1 wt % SP-B/C.     

Polarized Raman spectroscopy of double-stranded RNA from bacteriophage phi6: local Raman tensors of base and backbone vibrations

Raman tensors for localized vibrations of base (A, U, G, and C), ribose and phosphate groups of double-stranded RNA have been determined from polarized Raman measurements on oriented fibers of the genomic RNA of bacteriophage phi6. Polarized Raman intensities for which electric vectors of both the incident and scattered light are polarized either perpendicular (I[bb]) or parallel (I[cc]) to the RNA fiber axis have been obtained by Raman microspectroscopy using 514.5-nm excitation. Similarly, the polarized Raman components, I(bc) and I(cb), for which incident and scattered vectors are mutually perpendicular, have been obtained. Spectra collected from fibers maintained at constant relative humidity in both H2O and D2O environments indicate the effects of hydrogen-isotopic shifts on the Raman polarizations and tensors. Novel findings are the following: 1) the intense Raman band at 813 cm(-1), which is assigned to phosphodiester (OPO) symmetrical stretching and represents the key marker of the A conformation of double-stranded RNA, is characterized by a moderately anisotropic Raman tensor; 2) the prominent RNA band at 1101 cm(-1), which is assigned to phosphodioxy (PO2-) symmetrical stretching, also exhibits a moderately anisotropic Raman tensor. Comparison with results obtained previously on A, B, and Z DNA suggests that tensors for localized vibrations of backbone phosphodiester and phosphodioxy groups are sensitive to helix secondary structure and local phosphate group environment; and 3) highly anisotropic Raman tensors have been found for prominent and well-resolved Raman markers of all four bases of the RNA duplex. These enable the use of polarized Raman spectroscopy for the determination of purine and pyrimidine base residue orientations in ribonucleoprotein assemblies. The present determination of Raman tensors for dsRNA is comprehensive and accurate. Unambiguous tensors have been deduced for virtually all local vibrational modes of the 300-1800 cm(-1) spectral interval. The results provide a reliable basis for future evaluations of the effects of base pairing, base stacking, and sequence context on the polarized Raman spectra of nucleic acids.