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PubMed2000

Effects of external trunk loads on lumbar spine stability.

Stability of the lumbar spine is an important factor in determining spinal response to sudden loading. Using two different methods, this study evaluated how various trunk load magnitudes and directions affect lumbar spine stability. The first method was a quick release procedure in which effective trunk stiffness and stability were calculated from trunk kinematic response to a resisted-force release. The second method combined trunk muscle EMG data with a biomechanical model to calculate lumbar spine stability. Twelve subjects were tested in trunk flexion, extension, and lateral bending under nine permutations of vertical and horizontal trunk loading. The vertical load values were set at 0, 20, and 40% of the subject's body weight (BW). The horizontal loads were 0, 10, and 20% of BW. Effective spine stability as obtained from quick release experimentation increased significantly (p<0.01) with increased vertical and horizontal loading. It ranged from 785 (S.D.=580) Nm/rad under no-load conditions to 2200 (S.D.=1015) Nm/rad when the maximum horizontal and vertical loads were applied to the trunk simultaneously. Stability of the lumbar spine achieved prior to force release and estimated from the biomechanical model explained approximately 50% of variance in the effective spine stability obtained from quick release trials in extension and lateral bending (0.53<R(2)<0.63). There was no such correlation in flexion trials. It was concluded that lumbar spine stability increased with increased trunk load magnitude to the extent that this load brought about an increase in trunk muscle activation. Indirectly, our data suggest that muscle reflex response to sudden loading can augment the lumbar spine stability level achieved immediately prior to the sudden loading event.

Adult↗
PubMed1999

Simulated movement termination for balance recovery: can movement strategies be sought to maintain stability in the presence of slipping or forced sliding?

Slipping during various kinds of movement often leads to potentially dangerous incidents of falling. The purpose of this study was to determine whether there was evidence to support the theory that movement strategies could be used by individuals to regain stability during an episode of slipping and whether forced sliding from a moving platform accurately simulated the effect of slipping on stability and balance. A single-link-plus-foot biomechanical model was used to mathematically simulate base of support (BOS) translation and body segment rotation during movement termination in sagittal plane. An optimization routine was used to determine region of stability [defined at given COM locations as the feasible range of horizontal velocities of the center of mass (COM) of human subject that can be reduced to zero with respect to the BOS while still allowing the COM to traverse within the BOS limits]. We found some 30% overlap in the region of stability for slipping and non-slipping conditions. This finding supports the theory that movement strategies can be sought for restoring stability and balance even if slipping unexpectedly occurs. We also found that forced sliding produces effects on stability that are similar to those of slipping, indicated by over 50% overlap in the regions of stability for the two conditions. In addition, forced sliding has distinctive effects on stability, including a "shift" of the region of stability extended beyond the BOS in the direction of sliding. These findings may provide quantifiable guidance for balance training aimed at reducing fall incidents under uncertain floor surface conditions.

Accidental Falls↗
PubMed1996

Experimental analysis of functional stability of sagittal split ramus osteotomies secured by miniplates and position screws.

PURPOSE: This study determined the relative functional stabilities of various miniplate systems and configurations used to stabilize sagittal split ramus osteotomies (SSROs) and compared them with conventional internal screw fixation. MATERIALS AND METHODS: The biomechanical model was a reproducible prototype of a mandible sagittal osteotomy with consistent material and geometric properties. After advancing the distal segment by 7 mm, each set of mandible analogs (1 set = 3 analogs) was fixed bilaterally by one of three miniplate systems applied in various configurations, and tested with and without a supplemental 2.4-mm bicortical screw applied in the retromolar region. Reduced analogs were placed in a straining frame, and simulated masticatory loads were applied alternatively to the mandibular first molars. Ensuing osteotomy site displacements were measured by transducers attached to a computer-based data acquisition program. A coordinate transformation procedure was used to convert the component displacements captured by the individual transducers into a common "instability factor" to reflect fixation stability for each construct and loading condition. Instability factors for the individual constructs were compared with each other and with those obtained from analogs reduced exclusively with 2.4-mm position screws. RESULTS: Osteotomies stabilized with a combination of miniplates and position screws were more stable than those stabilized exclusively with miniplates (P < .0001). Post-hoc comparisons of mean instability factors (Dunnet's method) showed the miniplate-position screw combinations to be more stable than the 2.4-mm position screw system used as standard (P < .05). Miniplate systems alone were the least stable of the test constructs, with differential rates of failure between the individual miniplate systems. CONCLUSIONS: Exclusive use of miniplate fixation may not provide the consistent stability necessary for early functional restoration after SSROs. The addition of a position screw in the retromolar region substantially enhances the fixation stability of miniplate systems. The use of miniplates with retromolar position screws offers both technical and stability advantages over conventional miniplate or internal screw fixation. The fixation stability of the miniplate-position screw combination is independent of the type of miniplate system used.

Analysis of Variance↗
PubMed1996

Functional stability of sagittal split ramus osteotomies: effects of positional screw size and placement configuration.

PURPOSE: This study examined the effects of positional screw size and placement configuration on osteotomy site stability in a sagittal split ramal osteotomy model. MATERIALS AND METHODS: A reproducible analog of a sagittal split ramus osteotomy was developed to facilitate equitable comparisons of fixation stability. The distal segment of each analog was advanced by 7 mm and fixed bilaterally with 2.0-mm or 2.4-mm cortical screws applied in either a linear or a triangular configuration. Reduced analogs were placed in a straining frame, and simulated masticatory loads wee applied alternatively to the mandibular first molars. Ensuring osteotomy site displacements were acquired and registered by displacement transducers attached to a computer-based data acquisition program. A coordinate transformation procedure was used to convert the component displacements captured by the individual transducers into a common "instability factor" to reflect fixation stability for each construct and loading condition. RESULTS: Between configurations, the magnitude of interfragmentary displacement was less when the screws were applied in a triangular than in a linear configuration (P < or = .0001). Within configurations, 2.4-mm cortical screws provided greater stability than 2.0-mm screws. The stability profiles of the 2.0-mm screws, applied in a linear configuration at the superior border, were very similar regardless of whether two or three screws were used. The loads at which the linear systems became unstable were significantly higher (P < or = .0001) than for the triangular systems. Despite the twofold or higher difference in the mean instability factor of the individual test constructs, the SDs for each construct were only one tenth to one fifth of the corresponding means. CONCLUSIONS: None of the internal screw systems were grossly unstable when used to stabilize ramal osteotomies. Other conditions, including quality and strength of investing bone, being equal, cortical screws placed in a triangular configuration provide greater stability than screws placed in a linear configuration. Osteotomy sites stabilized by 2.4-mm screws are more stable than those stabilized by 2.0-mm screws. It does not matter whether two or three screws are applied at the superior border in a linear configuration, provided that the outer screws are an inch apart. The model system has a very good measurement precision and has important implications in optimizing fixation device design and placement configuration.

Bite Force↗
PubMed2002

Validation of an in vitro method to determine infectivity of cryopreserved sporozoites in stabilates of Theileria spp.

The infectivity of 15 cryopreserved Theileria spp. sporozoite stabilates was assessed semi-quantitatively by titration using naive peripheral blood mononuclear cells (PBM) in vitro in multi-well plates. Using the method described, the effective dilution, which would result in 50% of replicate wells infected (ED50), was calculated. The ED50 for 11 Theileria annulata stabilates in bovine PBM ranged from 10(-2.6) to 10(-4.2) dilutions of 1 tick equivalent (t.e.) ml(-1), one stabilate of Theileria parva 10(-2.2)t.e.ml(-1); and three Theileria lestoquardi stabilates in ovine PBM, from 10(-1.5) to 10(-1.8)t.e.ml(-1). Two of the T. annulata stabilates had been used individually to infect groups of calves: stabilate 52 produced more severe disease responses than stabilate 67, as measured by prepatent period, parasitosis, parasitaemia and death or recovery. This corresponded with the sixfold difference found in vitro between the ED50's of these two stabilates. This method is useful not only to measure the infection potential of the sporozoite stabilates but also as an in vitro model for chemotherapeutic and immunological studies of the early stages of theileriosis.

Animals↗
PubMed2002

Use of the ion exchange method for the determination of stability constants of trivalent metal complexes with humic and fulvic acids II. Tb3+ Yb3+ and Gd3+ complexes in weakly alkaline conditions.

The conditional stability constants for tracer concentrations of Tb(III), Yb(II), and Gd(III) with three soil humic acids, three soil fulvic acids and a fulvic acid from weathered coal were determined at pH 9.0-9.1 (these values are similar to those in calcareous soils) in the presence of NaHCO3 by using the anion exchange method. It was found that 1:1 and 1:2 complexes were simultaneously formed in the weakly alkaline conditions. The conditional stability constants of these 1:1 and 1:2 complexes were calculated from the distribution coefficients of rare earth elements at various concentrations of humate or fulvate. The stability constants indicate the very high stability of trivalent Tb3+, Yb3+ and Gd3+ complexes with humic substances in weakly alkaline conditions. The key parameters necessary for the experimental determination of the conditional stability constants of metal ions with humic substances in the presence of NaHCO3 by using an anion exchange method were discussed. The conditional stability constants of these 1:1 and 1:2 complexes were compared in this paper. It was found that stabilities of Tb3+ 1:1 and 1:2 complexes with humic acid are greater than the corresponding ones with fulvic acid from the same soil. In addition, the effect of the presence of Ca2+ as a competitor on the stabilities of 1:1 and 1:2 complexes of Yb was examined and no pronounced change of stabilities of 1:1 complex was found, even though Ca2+ is in a 10(3) excess to Yb3+.

Journal Article↗
PubMed1995

Stabilization of triple-helical nucleic acids by basic oligopeptides.

Intermolecular triplex DNA is stabilized by metal cations and polyamines which reduce repulsion between the negatively charged phosphates of the three nucleic acid strands. We use a quantitative chemical-probing assay involving protection of duplex guanines in a homopyrimidine.homopurine (Py.Pu) sequence from dimethyl sulfate modification to study effects of basic oligopeptides on the stability of triplex DNA. An intermolecular protonated pyrimidine.purine.pyrimidine (Py.Pu*Py) triplex formed readily between a duplex DNA region and a 14-mer pyrimidine triplex-forming oligonucleotide (TFO) at pH 5. The triplex was stabilized at pH by the addition of magnesium ions. In the presence of spermine and lysine-rich peptides, the intermolecular triplex was stabilized up to pH 6.5-7.0. The effective peptide concentration required for stabilization was 10(-5)-10(-2) M. Of the basic peptides studied, pentalysine (Lys-Lys-Lys-Lys-Lys) was the most effective triplex stabilizer. It was effective at concentrations which are lower than those required for Lys-Gly-Lys-Gly-Lys and Lys-Ala-Lys-Ala-Lys and are similar to active concentrations of spermine. Basic peptides were more effective at stabilizing a Py.Pu*Py triplex than a pyrimidine.purine.purine (Py.Pu*Pu) triplex. At 1 mM, Lys-Lys-Lys-Lys-Lys stabilized the Py.Pu*Pu triplex at a level comparable to stabilization by Mn2+ and spermine, whereas Lys-Gly-Lys-Gly-Lys and Lys-Ala-Lys-Ala-Lys resulted in weaker TFO binding. The concentration of TFOs required to form triplex DNA were significantly reduced in the presence of peptides.(ABSTRACT TRUNCATED AT 250 WORDS)

Alkylating Agents↗
PubMed2000

Probing the lipid-free structure and stability of apolipoprotein A-I by mutation.

To probe the secondary structure of the C-terminus (residues 165-243) of lipid-free human apolipoprotein A-I (apoA-I) and its role in protein stability, recombinant wild-type and seven site-specific mutants have been produced in C127 cells, purified, and studied by circular dichroism and fluorescence spectroscopy. A double substitution (G185P, G186P) increases the protein stability without altering the secondary structure, suggesting that G185 and G186 are located in a loop/disordered region. A triple substitution (L222K, F225K, F229K) leads to a small increase in the alpha-helical content and stability, indicating that L222, F225, and F229 are not involved in stabilizing hydrophobic core contacts. The C-terminal truncation Delta(209-243) does not change the alpha-helical content but reduces the protein stability. Truncation of a larger segment, Delta(185-243), does not affect the secondary structure or stability. In contrast, an intermediate truncation, Delta(198-243), leads to a significant reduction in the alpha-helical content, stability, and unfolding cooperativity. The internal 11-mer deletion Delta(187-197) has no significant effect on the conformation or stability, whereas another internal 11-mer deletion, Delta(165-175), dramatically disrupts and destabilizes the protein conformation, suggesting that the presence of residues 165-175 is crucial for proper apoA-I folding. Overall, the findings suggest the presence of stable helical structure in the C-terminal region 165-243 of lipid-free apoA-I and the involvement of segment 209-243 in stabilizing interactions in the molecule. The effect of the substitution (G185P, G186P) on the exposure of tryptophans located in the N-terminal half suggests an apoA-I tertiary conformation with the C-terminus located close to the N-terminus.

Amino Acid Substitution↗
PubMed2001

How Ala-->Gly mutations in different helices affect the stability of the apomyoglobin molten globule.

The apomyoglobin molten globule has a complex, partly folded structure with a folded A[B]GH subdomain; the factors determining its stability are not yet known in detail. Ala-->Gly mutations, made at solvent-exposed positions, are used to probe the role of helix propensity of individual helices in stabilizing the molten globule. Molten globule stability is measured by reversible urea unfolding, monitored both by circular dichroism and by tryptophan fluorescence. Two-state unfolding is tested by superposition of these two unfolding curves, and stability data are reported only for variants which satisfy the superposition test. Results for sites Q8 in the A helix and E109 in the G helix confirm that the helix propensities of the A and G helices both strongly affect molten globule stability, in contrast to results for the G65A/G73A double mutant which show that changing the helix propensity of the E-helix sequence has no significant stabilizing effect. Changing the helix propensity of the B-helix sequence with the G23A/G25A double mutant affects molten globule stability to an intermediate extent, confirming an earlier report that this mutant has increased stability. These results are consistent with the bipartite structure for the molten globule in which the A, G, and H helices are stably folded, while the long E helix is unfolded and the B helix has intermediate stability. Some differences are found in the shapes of the unfolding curves of different mutants even though they satisfy the superposition test for two-state unfolding, and possible explanations are discussed.

Alanine↗
PubMed2002

Halide-stabilizing residues of haloalkane dehalogenases studied by quantum mechanic calculations and site-directed mutagenesis.

Haloalkane dehalogenases catalyze cleavage of the carbon-halogen bond in halogenated aliphatic compounds, resulting in the formation of an alcohol, a halide, and a proton as the reaction products. Three structural features of haloalkane dehalogenases are essential for their catalytic performance: (i) a catalytic triad, (ii) an oxyanion hole, and (iii) the halide-stabilizing residues. Halide-stabilizing residues are not structurally conserved among different haloalkane dehalogenases. The level of stabilization of the transition state structure of S(N)2 reaction and halide ion provided by each of the active site residues in the enzymes DhlA, LinB, and DhaA was quantified by quantum mechanic calculations. The residues that significantly stabilize the halide ion were assigned as the primary (essential) or the secondary (less important) halide-stabilizing residues. Site-directed mutagenesis was conducted with LinB enzyme to confirm location of its primary halide-stabilizing residues. Asn38Asp, Asn38Glu, Asn38Phe, Asn38Gln, Trp109Leu, Phe151Leu, Phe151Trp, Phe151Tyr, and Phe169Leu mutants of LinB were constructed, purified, and kinetically characterized. The following active site residues were classified as the primary halide-stabilizing residues: Trp125 and Trp175 of DhlA; Asn38 and Trp109 of LinB; and Asn41 and Trp107 of DhaA. All these residues make a hydrogen bond with the halide ion released from the substrate molecule, and their substitution results in enzymes with significantly modified catalytic properties. The following active site residues were classified as the secondary halide-stabilizing residues: Phe172, Pro223, and Val226 of DhlA; Trp207, Pro208, and Ile211 of LinB; and Phe205, Pro206, and Ile209 of DhaA. The differences in the halide stabilizing residues of three haloalkane dehalogenases are discussed in the light of molecular adaptation of these enzymes to their substrates.

Amino Acids↗
PubMed2004

Stability of KcsA tetramer depends on membrane lateral pressure.

The potassium channel KcsA forms an extremely stable tetramer. Despite this high stability, it has been shown that the membrane-mimicking solvent 2,2,2-trifluoroethanol (TFE) can induce tetramer dissociation [Valiyaveetil, F. I., et al. (2002) Biochemistry 41, 10771-7, and Demmers, J. A. A., et al. (2003) FEBS Lett. 541, 69-77]. Here we have studied the effect of TFE on the structure and oligomeric state of the KcsA tetramer, reconstituted in different lipid systems. It was found that TFE changes the secondary and tertiary structure of KcsA and that it can dissociate the KcsA tetramer in all systems used. The tetramer is stabilized by a lipid bilayer as compared to detergent micelles. The extent of stabilization was found to depend on the nature of the lipids: a strong stabilizing effect of the nonbilayer lipid phosphatidylethanolamine (PE) was observed, but no effect of the charged phoshosphatidylglycerol (PG) as compared to phosphatidylcholine (PC) was found. To understand how lipids stabilize KcsA against TFE-induced tetramer dissociation, we also studied the effect of TFE on the bilayer organization in the various lipid systems, using (31)P and (2)H NMR. The observed lipid dependency was similar as was found for tetramer stabilization: PE increased the bilayer stability as compared to PC, while PG behaved similar to PC. Furthermore, it was found that TFE has a large effect on the acyl chain ordering. The results indicate that TFE inserts primarily in the membrane interface. We suggest that the lipid bilayer stabilizes the KcsA tetramer by the lateral pressure in the acyl chain region and that this stabilizing effect increases when a nonbilayer lipid like PE is present.

Bacterial Proteins↗
PubMed2005

Electrostatic effects on the stability of discoidal high-density lipoproteins.

High-density lipoproteins (HDL) remove cholesterol from peripheral tissues and thereby help to prevent atherosclerosis. Nascent HDL are discoidal complexes composed of a phospholipid bilayer surrounded by protein alpha-helices that are thought to form extensive stabilizing interhelical salt bridges. Earlier we showed that HDL stability, which is necessary for HDL functions, is modulated by kinetic barriers. Here we test the role of electrostatic interactions in the kinetic stability by analyzing the effects of salt, pH, and point mutations on model discoidal HDL reconstituted from human apolipoprotein C-1 (apoC-1) and dimyristoyl phosphatidylcholine (DMPC). Circular dichroism, Trp fluorescence, and light scattering data show that molar concentrations of NaCl or Na(2)SO(4) increase the apparent melting temperature of apoC-1:DMPC complexes by up to 20 degrees C and decelerate protein unfolding. Arrhenius analysis shows that 1 M NaCl stabilizes the disks by deltaDeltaG* approximately equal 3.5 kcal/mol at 37 degrees C and increases the activation energy of their denaturation and fusion by deltaE(a) approximately equal deltaDeltaH* approximately equal 13 kcal/mol, indicating that the salt-induced stabilization is enthalpy-driven. Denaturation studies in various solvent conditions (pH 5.7-8.2, 0-40% sucrose, 0-2 M trimethylamine N-oxide) suggest that the salt-induced disk stabilization results from ionic screening of unfavorable short-range Coulombic interactions. Thus, the dominant electrostatic interactions in apoC-1:DMPC disks are destabilizing. Comparison of the salt effects on the protein:lipid complexes of various composition reveals an inverse correlation between the lipoprotein stability and the salt-induced stabilization and suggests that short-range electrostatic interactions significantly contribute to lipoprotein stability: the better-optimized these interactions are, the more stable the complex is.

Amino Acid Sequence↗
PubMed1996

Effects of protein stabilizing agents on thermal backbone motions: a disulfide trapping study.

Chemical stabilizers are widely used to enhance protein stability, both in nature and in the laboratory. Here, the molecular mechanism of chemical stabilizers is studied using a disulfide trapping assay to measure the effects of stabilizers on thermal backbone dynamics in the Escherichia coli galactose/ glucose binding protein. Two types of backbone fluctuations are examined: (a) relative movements of adjacent surface alpha-helices within the same domain and (b) interdomain twisting motions. Both types of fluctuations are significantly reduced by all six stabilizers tested (glycerol, sucrose, trehalose, L-glucose, D-glucose, and D-galactose), and in each case larger amplitude motions are inhibited more than smaller ones. Motional inhibition does not require a high-affinity stabilizer binding site, indicating that the effects of stabilizers are nonspecific. Overall, the results support the theory that effective stabilizing agents act by favoring the most compact structure of a protein, thereby reducing local backbone fluctuations away from the fully folded state. Such inhibition of protein backbone dynamics may be a general mechanism of protein stabilization in extreme thermal or chemical environments.

Calcium-Binding Proteins↗
PubMed2000

DNA sequence-dependent contributions of core histone tails to nucleosome stability: differential effects of acetylation and proteolytic tail removal.

Modulation of nucleosome stability in chromatin plays an important role in eukaryotic gene expression. The core histone N-terminal tail domains are believed to modulate the stability of wrapping nucleosomal DNA and the stability of the chromatin filament. We analyzed the contribution of the tail domains to the stability of nucleosomes containing selected DNA sequences that are intrinsically straight, curved, flexible, or inflexible. We find that the presence of the histone tail domains stabilizes nucleosomes containing DNA sequences that are intrinsically straight or curved. However, the tails do not significantly contribute to the free energy of nucleosome formation with flexible DNA. Interestingly, hyperacetylation of the core histone tail domains does not recapitulate the effect of tail removal by limited proteolysis with regard to nucleosome stability. We find that acetylation of the tails has the same minor effect on nucleosome stability for all the selected DNA sequences. A comparison of histone partitioning between long donor chromatin, acceptor DNA, and free histones in solution shows that the core histone tails mediate internucleosomal interactions within an H1-depleted chromatin fiber amounting to an average free energy of about 1 kcal/mol. Thus, such interactions would be significant with regard to the free energies of sequence-dependent nucleosome positioning. Last, we analyzed the contribution of the H2A/H2B dimers to nucleosome stability. We find that the intact nucleosome is stabilized by 900 cal/mol by the presence of the dimers regardless of sequence. The biological implications of these observations are discussed.

Acetylation↗
PubMed2005

Understanding effect of wall structure on the hydrothermal stability of mesostructured silica SBA-15.

Mesostructured silica SBA-15 materials with different structural parameters, such as pore size, pore volume, and wall thickness, etc., were prepared by varying the postsynthesis hydrothermal treatment temperature and adding inorganic salts. The hydrothermal stabilities of these materials in steam (100% water vapor) were systematically investigated using a variety of techniques including powder X-ray diffraction, transmission electron microscopy, nitrogen sorption, and (29)Si solid-state NMR. The effect of the pore size, microporosity or mesoporosity, and wall thickness on the stability was discussed. The results show that all of the SBA-15 materials have a good hydrothermal stability under steam of 600 degrees C for at least 24 h. N(2) sorption measurements show that the Brumauer-Emmett-Teller surface area of SBA-15 materials is decreased by about 62% after treatment under steam at 600 degrees C for 24 h. The materials with thicker walls and more micropores show relatively better hydrothermal stability in steam of 600 degrees C. Interestingly, we found that the microporosity of the mesostructured silica SBA-15 is a very important factor for the hydrothermal stability. To the materials with more micropores, the recombination of Si-O-Si bonds during the high-temperature steam treatment may not cause direct destruction to the wall structure. As a result, SBA-15 materials with more micropores show better stability in pure steam of 600 degrees C. Nevertheless, these materials are easily destroyed in steam of 800 degrees C for 6 h. Two methods to effectively improve the hydrothermal stability are introduced here: one is a high-temperature treatment, and another is a carbon-propping thermal treatment. Thermal treatment at 900 degrees C can enhance the polymerization degree of Si-O-Si bonds and effectively improve the hydrothermal stability of these SBA-15 materials in 800 degrees C steam for 12 h. But, this approach will cause very serious shrinkage of the mesopores, resulting in smaller pore diameter and low surface area. A carbon-propping thermal treating method was employed to enhance the polymerization of Si-O-Si bonds and minimize the serious shrinkage of mesopores at the same time. It was demonstrated to be an effective method that can greatly improve the hydrothermal stability of SBA-15 materials in 800 degrees C steam for 12 h. Furthermore, the SBA-15 materials obtained by using the carbon-propping method possess larger pores and higher surface area after the steam treatment at 800 degrees C compared to the materials from the direct thermal treatment method after the steam treatment.

Journal Article↗
PubMed2005

Stabilization energies of the hydrogen-bonded and stacked structures of nucleic acid base pairs in the crystal geometries of CG, AT, and AC DNA steps and in the NMR geometry of the 5'-d(GCGAAGC)-3' hairpin: Complete basis set calculations at the MP2 and CCSD(T) levels.

Stabilization energies of the H-bonded and stacked structures of a DNA base pair were studied in the crystal structures of adenine-thymine, cytosine-guanine, and adenine-cytosine steps as well as in the 5'-d(GCGAAGC)-3' hairpin (utilizing the NMR geometry). Stabilization energies were determined as the sum of the complete basis set (CBS) limit of MP2 stabilization energies and the Delta E(CCSD(T)) - Delta E(MP2) correction term evaluated with the 6-31G*(0.25) basis set. The CBS limit was determined by a two-point extrapolation using the aug-cc-pVXZ basis sets for X = D and T. While the H-bonding energies are comparable to those of base pairs in a crystal and a vacuum, the stacking energies are considerably smaller in a crystal. Despite this, the stacking is still important and accounts for a significant part of the overall stabilization. It contributes equally to the stability of DNA as does H-bonding for AT-rich DNAs, while in the case of GC-rich DNAs it forms about one-third of the total stabilization. Interstrand stacking reaches surprisingly large values, well comparable to the intrastrand ones, and thus contributes significantly to the overall stabilization. The hairpin structure is characterized by significant stacking, and both guanine...cytosine pairs possess stacking energies larger than 11.5 kcal/mol. A high portion of stabilization in the studied hairpin comes from stacking (similar to that found for AT-rich DNAs) despite the fact that it contains two GC Watson-Crick pairs having very large H-bonding stabilization. The DFT/B3LYP/6-31G** method yields satisfactory values of interaction energies for H-bonded structures, while it fails completely for stacking.

Adenine↗
PubMed2005

Bond dissociation energies and radical stabilization energies associated with model peptide-backbone radicals.

Bond dissociation energies (BDEs) and radical stabilization energies (RSEs) have been calculated for a series of models that represent a glycine-containing peptide-backbone. High-level methods that have been used include W1, CBS-QB3, U-CBS-QB3, and G3X(MP2)-RAD. Simpler methods used include MP2, B3-LYP, BMK, and MPWB1K in association with the 6-311+G(3df,2p) basis set. We find that the high-level methods produce BDEs and RSEs that are in good agreement with one another. Of the simpler methods, RBMK and RMPWB1K achieve good accuracy for BDEs and RSEs for all the species that were examined. For monosubstituted carbon-centered radicals, we find that the stabilizing effect (as measured by RSEs) of carbonyl substituents (CX=O) ranges from 24.7 to 36.9 kJ mol(-1), with the largest stabilization occurring for the CH=O group. Amino groups (NHY) also stabilize a monosubstituted alpha-carbon radical, with the calculated RSEs ranging from 44.5 to 49.5 kJ mol(-1), the largest stabilization occurring for the NH2 group. In combination, NHY and CX=O substituents on a disubstituted carbon-centered radical produce a large stabilizing effect ranging from 82.0 to 125.8 kJ mol(-1). This translates to a captodative (synergistic) stabilization of 12.8 to 39.4 kJ mol(-1). For monosubstituted nitrogen-centered radicals, we find that the stabilizing effect of methyl and related (CH2Z) substituents ranges from 25.9 to 31.7 kJ mol(-1), the largest stabilization occurring for the CH3 group. Carbonyl substituents (CX=O) destabilize a nitrogen-centered radical relative to the corresponding closed-shell molecule, with the calculated RSEs ranging from -30.8 to -22.3 kJ mol(-1), the largest destabilization occurring for the CH=O group. In combination, CH2Z and CX=O substituents at a nitrogen radical center produce a destabilizing effect ranging from -19.0 to -0.2 kJ mol(-1). This translates to an additional destabilization associated with disubstitution of -18.6 to -7.8 kJ mol(-1).

Carbon↗
PubMed2005

Effects of electrolyte concentration and pH on the coalescence stability of beta-lactoglobulin emulsions: experiment and interpretation.

Experimental results are presented about the effects of ionic strength and pH on the mean drop-size after emulsification and on the coalescence stability of emulsions, stabilized by a globular protein beta-lactoglobulin (BLG). The mean drop-size is determined by optical microscopy, whereas the coalescence stability is characterized by centrifugation. In parallel experiments, the zeta-potential and protein adsorption on drop surface are determined. The experiments are performed at two different BLG concentrations, 0.02 and 0.1 wt%. The electrolyte concentration in the aqueous phase, C(EL), is varied between 1.5 mM and 1 M, and pH is varied between 4.0 and 7.0. The experiments show that the mean drop-size after emulsification depends slightly on C(EL), at fixed protein concentration and natural pH = 6.2. When pH is varied, the mean drop-size passes through a maximum at fixed protein and electrolyte concentrations. A monolayer protein adsorption is registered in the studied ranges of C(EL) and pH at low BLG concentration of 0.02 wt%. In contrast, a protein multilayer is formed at higher BLG concentration, 0.1 wt%, above a certain electrolyte concentration (C(EL) > 100 mM, natural pH). The experimental results for the emulsion coalescence stability are analyzed by considering the surface forces acting between the emulsion drops. The electrostatic, van der Waals, and steric interactions are taken into account to calculate the barriers in the disjoining pressure isotherm at the various experimental conditions studied. The comparison of the theoretically calculated and the experimentally determined coalescence barriers shows that three qualitatively different cases can be distinguished. (1) Electrostatically stabilized emulsions, with monolayer protein adsorption, whose stability can be described by the DLVO theory. (2) Sterically stabilized emulsions, in which the drop-drop repulsion is created mainly by overlapping protein adsorption multilayers. A simple theoretical model is shown to describe emulsion stability in these systems. (3) Sterically stabilized emulsions with a monolayer adsorption on drop surface.

Adsorption↗