摘要:

AbstractQuasielastic light scattering is used to study the effect of ionic strength on the dynamic behaviour of DNA. In a first approach the spectrum of scattered light is analyzed in terms of a single relaxation process. The large difference between the observed behaviour and that expected according to a pure diffusional process reflects the contribution associated with internal modes, which increases with decreasing ionic strength. Such behaviour is better analyzed in terms of a double relaxation process by using two relaxation times, the reciprocals of which are equal toDK2andDK2+ τi−1(K), respectively, where τi(K) is an average value describing the set of modes observed at a givenKvalue. Relative intensity and relaxation times, which are the more accurate parameters, were used to interpret the results. The observed increase of the relative contribution of internal modes with decreasing ionic strength is actually a relative decrease of the diffusional contribution induced by a corresponding increase of the radius of gyrationRG. On the other hand, the reciprocal τi−1(K) of the relaxation time is a linear function ofK2in the analyzedKRGrange and is insensitive to ionic strength between 10−2Mand 1M. These results, when discussed according to Rouse's model, lead to define for each value of τi−1(K) a corresponding mean‐squared equilibrium length 〈μ i2〉 which is found to be a l

ISSN:0006-3525    

DOI:10.1002/bip.1977.360161002

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

摘要:

AbstractThe kinetics of complex formation between fluorescein mercuric acetate and heat‐denatured DNA were studied by measuring the fluorescence quenching of this reagent. This quenching process involved no immeasurably rapid phase and it was shown that this reaction follows simple second‐order kinetics. The rate constant at 25°C was estimated to be 2.9 × 104M−1sec−1for calf‐thymus DNA (42% G + C) and 1.1 × 104M−1sec−1forMicrococcus lysodeikticusDNA (72% G + C). Activation parameters for this reaction were calculated from the temperature dependence of the reaction rate, and the activation entropy was found to be highly negative (−27.5 cal/mol deg for calf‐thymus DNA and −25.5 cal/mol deg forM. lysodeikticusDNA). The binding of fluorescein mercuric acetate to native DNA, which requires the opening of the double‐helical structure, was also followed by measuring the absorbance change of this reagent. There was a lag phase in this binding process, and the enthalpy change for the opening step corresponded roughly to that for the opening of one base pair. These findings are discussed in relation to the results of a similar

ISSN:0006-3525    

DOI:10.1002/bip.1977.360161003

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

摘要:

AbstractThe formation of collagen fibrils from soluble monomers and aggregates by thermal gelation at neutral pH can be divided into two distinct stages: a nucleation phase and a growth phase. Turbidity studies of the kinetics of the precipitation reaction show that the lag‐phase time or nucleation reaction time,t′l, is markedly temperature dependent while the growth reaction time is temperature independent. The activation energy of the nucleation reaction is essentially constant over the temperature range studied. In monitoring the nucleation‐phase reaction by various physicochemical techniques, including viscosity, sedimentation equilibrium, and light scattering, no evidence for the formation of aggregates was observed. Enrichment of the initial collagen solution with aggregates accelerates nucleation, butde novonuclei formation is still required even in highly aggregated collagen preparations. Removal of pepsin and pronase susceptible peptides lengthens the nucleation reaction time and increases the sensitivity of the rate of nuclei formation to changes in ionic strength. Electron microscope studies show the fibrils formed from the protease‐treated collagen to be less well organized. With pepsin‐treated collagen, subfibrils and obliquely striated fibrils are seen, showing that while microfibrils are formed interactions between them are modulated by the enzyme susceptible peptides in the same way that these regions modulate nuclei assembly. It appears that pepsin and pronase susceptible peptide regions of collagen play a more prominent role in thein vitroassembly of collagen molecules to form D‐stagger nuclei and fibrils than do ionic interactions between helical molecular regions. A mechanism of nucleation of collagen fibrillogenesis i

ISSN:0006-3525    

DOI:10.1002/bip.1977.360161004

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

摘要:

AbstractHeat precipitation fibril formation in collagen solutions depends upon the prior thermal history of the solution. Collagen solutions were heat precipitated to various extents at 30°C, cooled, and then brought to a second precipitation. Kinetic analysis of the secondary precipitation demonstrated that only the nucleation phase of the precipitation was affected, not the fibril growth phase. Thermal history, or memory, is thus related to the formation of low‐temperature‐stable nuclei. A range of nuclei sizes is evident, supporting the concept of a homogeneous nucleation process. Schiffs base formation and establishment of cross‐linkages play no role in thein vitronucleation: thiosemicarbazide treated collagen behaves identically to untreated collagen in kinetics of assembly to fibrils. Low‐temperature‐stable nuclei formed at neutral pH are dissociated in the cold in acetic acid at pH 4. Pronase and pepsin susceptible molecular end regions are important in establishing the low‐temperature‐stable nuclei. Pronase treatment completely abolishes the acquisition of memory of prior thermal history in collagen solutions. We speculate that biological control mechanisms for fibril formationin vivorelate to specific interactions between non‐helical, enzyme susceptible regions on co

ISSN:0006-3525    

DOI:10.1002/bip.1977.360161005

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

摘要:

AbstractA statistical theory of the linear dichroism of DNA‐like chains is presented for two models which are discrete versions of the wormlike coil. In the final form the linear dichroism of the entire chain is related directly to the dichroic properties of a chain segment (base pair). Though the derivations are somewhat complicated, the result [Eq. (28)] is simple and the required statistical parameters can be easily calculated for either model from measured values of the persistence length. In fact, for molecules as stiff as double‐stranded DNA, the results can be reduced with good accuracy to the form\documentclass{article}\pagestyle{empty}\begin{document}$$ \frac{{\Delta \varepsilon (chain)}}{{\Delta \varepsilon (segment)}} = \frac{{P_{\infty} }}{l}\frac{r^{2}}{{\langle r^2 \rangle_0 }} $$\end{document}showing that the “optical persistence” given on the left is directly proportional to the structural persistence,P∞/l.As in previous theories the results are restricted to chains in their Gauss

ISSN:0006-3525    

DOI:10.1002/bip.1977.360161006

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

摘要:

AbstractThe flow birefringence and extinction angle over a velocity gradient range of approximately 5–100 sec−1, and the zero shear‐viscosity have been obtained from human umbilical cord hyaluronic acid at concentrations of 0.25, 0.125 and 0.0625%, and pHs 6.0, 6.5, 7.0, 7.5, 8.0, and 8.5 and constant ionic strength 0.1. The data indicate a large change in optical anisotropy as a function of pH, with most of the transition in the pH range 7.0–7.5, i.e., across the physiological range. The sign of the anisotropy changes between pH 8.0 and 8.5. These results, together with changes in the extinction angle and intrinsic viscosity as a function of pH, suggest a pH‐dependent structural change in the system. Due to the abruptness of the transition, as evidenced by the intrinsic viscosity and flow birefringence, it is probable that the structural transition is cooperative. If the data are interpreted in terms of the Rouse‐Zimm Gaussian subchain theory, a modification of the model in terms of the Haller‐Cerf concept of internal viscosity is required. Thus, the demonstrated properties of hyaluronate solutions indicate a system with memory of stress. Due to the presence of large concentration effects discernible in the extinction angle measurements, hyaluronic acid probably exists as a network in solution. The results are discussed with respect to the mechanoelectrical transducing properties of hyaluronates and stress‐dependent changes in ORD a

ISSN:0006-3525    

DOI:10.1002/bip.1977.360161007

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

摘要:

AbstractThe x‐ray crystal structures of 19 selected proteins are examined empirically for correlations between the amino acid sequence and long‐range,tertiaryconformation. There is clear evidence for preferential associations of certain types of amino acids, particularly among the hydrophobic aliphatic, aromatic, and cysteine residues. However, the likelihoods of forming these residue‐pair contacts are all less than 12%, so packing and geometric requirements must often take precedent over energetic considerations. The prediction of long‐range contacts is not substantially improved by taking into account the sequentially previous residues. The analysis of atom–atom contacts shows a similar lack of predictive ability, but the results show that a good approximation to the interresidue energy function must include different types of interactions at two or three different sites on some amino acids. Backbone–backbone long‐range interactions are relatively rare and nonspecific, whereas some “polar” side chains form hydrogen bonds from the polar groups while occasionally forming hydrophobic contacts with the remai

ISSN:0006-3525    

DOI:10.1002/bip.1977.360161008

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

摘要:

AbstractA histidine‐containing cyclic dipeptide, cyclo(D‐Leu‐L‐His), was almost 20 times as efficient a catalyst as imidazole in the hydrolysis ofp‐nitrophenyl laurate. The effect of dioxane on the hydrolysis showed that hydrophobic interaction between the cyclic dipeptide and the ester is very important. This reaction obeyed the Michaelis‐Menten kinetics, and the Michaelis constantKmwas as low as 9.98 × 10−5M. Since the linear dipeptide havingD‐Leu‐L‐His sequence was nearly inactive in the hydrolysis, the functional groups of cyclo(D‐Leu‐L‐His) in a specific arrangement held by the rigid backbone must have cooperated in the fast hydrolysis. Very weak catalysis by the diasteremeric cyclic dipeptide, cyclo(L‐Leu‐L‐His), in the hyd

ISSN:0006-3525    

DOI:10.1002/bip.1977.360161009

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

摘要:

AbstractIt has been reported previously that a cyclic dipeptide, cyclo(D‐Leu‐L‐His), showed a high hydrolytic activity toward a hydrophobic ester,p‐nitrophenyl laurate. In order to determine the reason for the high catalytic activity, the conformation of cyclo(D‐Leu‐L‐His) in aqueous solution was investigated by nuclear magnetic resonance and circular dichroism spectroscopy and compared with the conformation of cyclo(L‐Leu‐L‐His), which was nearly inactive in otherwise the same conditions for the hydrolysis. It was demonstrated that the spatial arrangement of the hydrophobic isobutyl group of theD‐leucyl residue and of the nucleophilic imidazolyl group of theL‐histidyl residue in cyclo(D‐Leu‐L‐His) matches very well with the long acyl chain and the active ester function ofp‐nitrophenyl laurate. On the other hand, in cyclo(L‐Leu‐L‐His) the hydrophobic and the nucleophilic pendant groups are too close with each other to cooperate intramolecularly for the hydrolysis. It was concluded that the different steric structures of the diastereomers can explain the large d

ISSN:0006-3525    

DOI:10.1002/bip.1977.360161010

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

摘要:

AbstractWhen pyrene butyric acid hydrazide or pyrene acetic acid hydrazide is attached to single‐strand RNA 3′ termini a red shift in absorbance and substantial hypochromicity are observed. A strong induced CD is seen and the fluorescence intensity is quenched by an order of magnitude. In double‐stranded samples, a further 10‐fold quenching of fluorescence is seen. Several lines of evidence suggest that the residual fluorescence of pyrene butyric acid hydrazide‐duplex conjugates arises from a minor species. The most likely possibility is dye reacted at a site other than the 3′ end. Some indication exists that 3′‐attached pyrene may perturb the relative stability of nearby duplex. Within the limits of this reservation, it appears that 3′‐pyrene conjugates may be rather useful for detecting the existence of duplex regions accessible to a dye at the 3

ISSN:0006-3525    

DOI:10.1002/bip.1977.360161011

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY