摘要:

AbstractReduced viscosity and infrared spectra of low‐molecular‐weight poly‐γ‐benzyl‐L‐glutamate (which was prepared by polymerization of theN‐carboxyanhydride withn‐hexylamine initiation at [A]/[I]3, 4, and 8) have been measured in various organic solvents. Infrared spectra indicate that the polypeptide molecules consist of a series of residues of two forms, the solvated σ‐form and the hydrogen‐bonded β‐form, and relative abundance of the two forms depends on solvent species and polypeptide concentration. An approximate method is developed for estimating the content of β‐structure from a single spectrum of dissolved polypeptide. The reduced viscosity of some solutions is scarcely dependent in polypeptide concentration, in which a single conformation is predominantly kept over the concentration range. In the other solutions the reduced viscosity displays a strong concentration dependence or some anomalous behavior. The observed viscosity behavior has been attributed to the changes in size and shape of aggregates, which are determined by the number of hydrogen bonds in the aggregate. This unusual behavior is exhibited by solutions of the polypeptides which have a moderate content of β‐structure at a finite concentration. Both the content of β‐structure and the extent of association increase in the following solvents, ranked in order of effectiveness: dimethylformamide, trifluoroethanol

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120602

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

年代:1973

数据来源: WILEY

摘要:

AbstractThe interactions between chondroitin‐6‐sulfate and poly‐L‐arginine in aqueous salt solution have been investigated by circular dichroism techniques. In the presence of chondroitin‐6‐sulfate, at neutral pH, poly‐L‐arginine adopts the α‐helical conformation rather than “charged coil” form observed in the absence of mucopolysaccharide. This interaction is at a maximum when the ratio of arginine to disaccharide residues is 2:1. Elevation of the temperature leads to a sharp melting transition at 76.0 ± 1.0°C. This behavior is in marked contrast to that for poly‐L‐lysine‐chondroitin‐6‐sulfate interactions, which are at a maximum at a 1:1 residue ratio and have a melting transition at 47.0 ± 1.0°C. These results indicate a stronger interaction for poly‐L‐arginine than for poly‐L‐lysine. The positive arginine side chains appear to interact with both the negative sulfate and carboxyl residues, while those of the lysines are involved only with the sulfates. Poly‐L‐ornithine at neutral pH shows no conformation directing interaction with chondroitin‐6‐sulfate, although a small proportion of α‐helix is formed on dilution of the mixture with methanol. The extent of the interaction of cationic polypeptides with chondroitin‐6‐sulfate increases in the order poly‐L‐ornithine, poly‐L‐lysine,

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120603

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

年代:1973

数据来源: WILEY

摘要:

AbstractWe present absorption and circular dichroism (CD) spectra for the synthetic polymers poly d(AAT):d(AAT) and poly r(AAU):r(AAU), in both native and heat‐denatured forms. As a means of evaluating the first‐neighbor hypothesis, the CD spectra are compared with approximations derived from spectra of other synthetic polymers containing the same first‐neighbor sequences. This is the first instance where such a comparison has been possible using spectra of double‐stranded RNA sequences, and the agreement between the measured and approximated spectra for poly r(AAU):r(AUU) is surprisingly good. We have also subjected the CD spectrum of poly d(AAT):d(AAT) to a previously published analytical procedure for obtaining estimates of first‐neighbor frequencies. In this first independent test of the procedure, we find that the analysis does infer the existence of a majority (86%) of AA, TT, AT, and TA first neighbors but does not precisely indicate their relative pr

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120604

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

年代:1973

数据来源: WILEY

摘要:

AbstractMelting curves of tRNAfMetand two fragments derived from this molecule by limited ribonuclease T1 digestion (i.e., the anticodon arm and loop [K fragment] and the larger fragment representing three‐fourths of the tRNA chain from the 3′ terminus including two potential limbs of the cloverleaf structure [L fragment]) are presented. The profiles observed are consistent with the presence of base paired structures in all those molecules. At low salt concentration (0.02MNa+) the stabilities of these molecules measured by the apparent midpoints of the denaturation profiles are in the order K>L>tRNA. The relative stabilities approach each other at 0.2MNa+(the tRNA profile being biphasic), while at high salt (2M) the L fragment seems to be more stable than either K or t‐RNAfMet. Estimation of the enthalpy of denaturing the K structure in 0.02MNa+gives a value of 40 ± 3 kcal/mole corresponding to an enthalpy per effective G.C. base pair disrupted of 10 ± 1 kc

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120605

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

年代:1973

数据来源: WILEY

摘要:

AbstractDielectric relaxation and viscosity measurements were performed on delipidated and relipidated samples of bovine serum albumin (BSA) at urea concentrations between O and 6M. By the combined interpretation of these two hydrodynamic methods the characterization of conformational changes of the molecule during urea denaturation is possible.The denaturation of delipidated BSA results from two mechanisms. The first one is a slow, time‐dependent elongation of the molecule; the second one is a rapid swelling which becomes most pronounced at urea concentrations higher than 4M. For relipidated albumin, the slow elongation mechanism occurs but the presence of fatty acids protects the protein aganist molecular swelling. In both cases these conformational changes are accompanied by an increased disymmetry of charge repartition and a concomitant increase of the dipole moment.From these results it follows that lipidated albumin (as occurs under physiological conditions) is less sensitive to denaturation than delipidated albumi

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120606

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

年代:1973

数据来源: WILEY

摘要:

AbstractOn the basis of a set of nonbonded interatomic potential functions derived earlier from heats of sublimation and experimental crystal structures, we derive a second, less repulsive, set which is to be used in the absence of the expansion caused by thermal motion and in particular in macromolecular systems where thermal motion is much reduced compared with crystals of small molecules. Working with a pair of octane molecules, we calculate the intermolecular potential (U) in the presence of thermal motion from potentialsU° (in the absence of thermal motion), by letting a system of pairs of molecules assume a Boltzmann distribution over the intermolecular distance, in the presence of a force of varied magnitude applied to obtain different equilibrium distances. The potentialU° is adjusted until the calculated and the empirical potentialsUagree. Finally, best interatomic Lennard‐Jones potentials which reproduce the functionU° are calculated. The resulting functions are tested by calculating the crystal structure of benzene and comparing it with experimental data at low temperature, by energy minimization of the crystal structure of polyethylene and of the β‐structure of poly‐L‐alanine, and by comparing the energy of the α‐helix and the β‐structure of poly‐L‐alanine. In all cases, the corrected functions give more satisfactory results tha

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120607

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

年代:1973

数据来源: WILEY

摘要:

AbstractEarlier theoretical work on aggregation in imperfect gases and in solutions of protein molecules is reformulated here in such a way as to be applicable to a number of aggregation systems in solution. The theory is most easily developed for an “osmotic” solution. It is exact, in principle, and in particular does not treat the aggregates as an ideal mixture as is usually done. If osmotic virial coefficients are diffcult to measure in a particular case, it is then necessary to relate these coefficients to properties of the more conventional “constant pressure” solution. An important special case is used as an example in the last three sections: The stacking of bases, nucleosides, etc., is represented by a statistical mecchanical model in which the aggregates resemble vertical stacks of c

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120608

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

年代:1973

数据来源: WILEY

摘要:

AbstractAbsorbance‐temperature profiles have been determined for the following self‐complementary oligonucleotides or equimolar paris of complementary oligonucleotides containing GC base pairs: A2GCU2, A3GCU3, A4GCU4, A6CG + CGU6, A8CG + CGU8, A4G2+ C2U4, A5G2+ C2U5, A4G3+ C3U4, and A5G3+ C3U5. In all cases cooperative melting transitions indicate double‐helix formation. As was found previously, the stability of GC containing oligomer helices is much higher than that of AU helices of corresponding length. Moreover, helices with the same length and base composition but different sequences also have quite different stabilites. The melting curves were andlyzed using a zipper model and the thermodynamic parameters for the AU pairs determined previously. The effect of single‐strand stacking was considered separately. According to this model, the formation of a GC pair from unstacked single strands is associated with an ethalpy change of −15 kcal/mole. Due to the high degree of single‐strand stacking at room temperature the enthalpy change for the formation of GC pairs from unstacked single strands is only −5 to −6 kcal/mole. (The corresponding parameters for AU pairs are −10.7 kcal/mole and −5 to −6 kcal/mole.) The sequence dependence of helix stability seems to be primarily entropic since no differences in ΔHwere seen among the sequence isomers.The kinetics of helix formation was investigated for the same molecules using the temperature jump technique. Recombination of strands is second order with rate constants in the range of 105to 107M−1sec−1depending on the chain length and the nucleotide sequence. Within a series of oligomers of a given type, the rates of recombination decrease with increasing chain length. Oligomers with the sequence AnGCUnrecombine six to eight times slower than the other oligomers of corresponding chain length. The experimental enthalpies of activation of 6 to 9 kcal/mole suggest a nucleation length of one or two GC base pairs. The helix dissociation process has rate constants between 0.5 and 500 sec−1and enthalpies of activation of 25 to 50 kcal/mole. An increase of chain length within a given nucleotide series leads to decreased rates of dissociation and increased enthalpies of activation. An investigation of the effect of ionic strength on AnGCUnhelix formation showed that the rates of recombination increase considerably w

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120609

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

年代:1973

数据来源: WILEY

摘要:

AbstractThe intensities and other spectral parameters of the main components of infrared amide bands for the random and anti‐parallel pleated sheet forms have been determined for poly‐S‐carbobenzoxymethylcysteine, poly‐S‐carboxymethylcysteine, polylysine and silk fibroin ofBombyx moriin heavy water solutions and in organic solvents. Assuming that the optical spectra of these two types of structure are additive, the method was proposed for determining relative contents and molar absorption coefficients of amide I band. Integral intensity and maximum frequency values of the main components of the amide I band for all the samples in the β‐form turned out to be specific. In contrast to this the integral intensity of the band of amide I in the random coil form varied within certain limits, while the main maximum frequency for all samples remai

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120610

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

年代:1973

数据来源: WILEY

摘要:

AbstractThe far ultraviolet absorptions of aliphatic amino acids such as glycine, alanine, aminobutyric acid, valine, and leucine were measured with their solid thin films down to 120 nm. Four absorption bands arising from the carboxyl group were observed in the 200–140 nm region. It was found that the band positions and intensities vary systematically according to the number of carbon atoms in the aliphatic side chain. This result may be interpreted as a phenomenon similar to the hyper‐and hypochromisms of aliphatic homopolypeptides in far ultraviolet reg

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120611

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

年代:1973

数据来源: WILEY