摘要:

AbstractThe aqueous and hydroalcoholic solutions of the heptenic macrolide amphotericin B display strong and variable signals in CD and absorption spectroscopies in the range of the π* ← π transition. An interpretation of the spectroscopic changes is proposed based on the equilibrium between two forms of the intermolecular organization: the aggregated one (A) with strong excitonic interaction and the nonaggregative one (B) whose spectra are like those of linear conjugated polyenes in true solution with a well‐developed vibrational structure. The intermediate spectra are fitted by linear combination of the A‐ and B‐form spectra. A two‐level organization of the aggregates is proposed for the A‐form: (1) a close packing of few molecules, which is the origin of the absorption maxima hypsochromic shift; and (2) interaction between the preceding small units inside the aggregates, which is spectroscopically expressed by the inten

ISSN:0006-3525    

DOI:10.1002/bip.1981.360200802

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

年代:1981

数据来源: WILEY

摘要:

AbstractInvestigations of DNA using CD spectroscopy show that the P‐form is available in a wide variety of methanol–ethanol mixtures when the water content is low. Increasing the temperature or the ethanol content of a 95% methanol solution causes DNA to undergo a cooperative transition to the P‐form. However, this transition cannot be reversed on cooling, or on adding methanol. Thus P‐form DNA appears to be stable at high methanol concentrations, but it is usually not observed because the DNA is trapped by a kinetic barrier. P‐form DNA will instantaneously assume the native B‐form on addition of water, confirming earlier reports that P‐form DNA is not strand separated [E. Kay (1976)Biochemistry15, 5241]. CD spectra extended to 190 nm show that there is no base–base interaction in the P‐form. However, the P‐form is extremely stable to heat denaturation in solvents which promote hydrogen bonding between the base pairs. A number of models that can account for the properties of P‐f

ISSN:0006-3525    

DOI:10.1002/bip.1981.360200803

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

年代:1981

数据来源: WILEY

摘要:

AbstractThe structure of poly(aspartic acid) prepared by thermal polycondensation has been studied by means of nmr spectroscopy. The analysis of the13C‐nmr spectra of the polymer at various pH values and comparison with the spectrum of poly(α‐L‐aspartic acid) revealed that the polymer contained aspartic acid linked in α‐ and β‐peptide bonds. The mole fraction of the β‐peptide bonds has been found to be 0.8 ± 0.1. The significance of the results for the evolutionary theory of S. W.

ISSN:0006-3525    

DOI:10.1002/bip.1981.360200804

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

年代:1981

数据来源: WILEY

摘要:

AbstractThe nature of peptide bonds in poly(aspartic acid) prepared by debenzylation of poly(β‐benzyl‐L‐aspartate) under various conditions has been studied by means of nmr spectroscopy. It was established that the majority of the polymers prepared, as well as the commercially obtained polymer, contained aspartic acid linked in both α‐ and β‐peptide bonds. The purest polymer, having practically undetectable amounts of β‐bond, was prepared by debenzylation by HBr in trifluoroacetic acid. It was established that the β‐bonds are forme

ISSN:0006-3525    

DOI:10.1002/bip.1981.360200805

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

年代:1981

数据来源: WILEY

摘要:

AbstractPolypeptides of dicarboxylic amino acids having the monomer units linked in α‐ and ω‐peptide bonds contain two kinds of carboxyls of different acidity. How well potentiometric titration can distinguish these two carboxyls and so characterize the nature of the peptide bonds is evaluated critically. An analysis of the equation describing the dependence of pH on the degree of neutralization based on neglecting the polymer effect and a discussion of the dissociation behavior of polyanions show that the method of evaluating experimental data found in the literature is incorrect. Nevertheless, if a conformational transition does not interfere, some useful and reliable information may be gained by this method; namely, an indication of the presence of two different peptide bonds, their mole ratio, and an approximate pKvalue for the carboxyl of the amino acid linked in the ω‐peptide bond. The presence of two types of carboxyls complicates the evaluation of the titration curves in the conformation

ISSN:0006-3525    

DOI:10.1002/bip.1981.360200806

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

年代:1981

数据来源: WILEY

摘要:

AbstractThe ir absorption and x‐ray diffraction analysis ofN‐tert‐butyloxycarbonyl‐DL‐alanine (t‐Boc‐DL‐Ala‐OH) in the solid state has revealed a new mode of self‐association for aN‐urethanyl‐α‐amino acid, i.e., ribbons of hydrogen‐bonded cyclic dimers formed through the —COOH groups. In contrast to the recemate, a water molecule, incorporated into the crystal of the chiralt‐Boc‐D‐Ala‐OH, alters in part that hydrogen‐bonding scheme. In the two independent molecules of the unit cell of the optically active alanine derivative, as in that of the racemic derivative: (i) the conformation of the —CONH group istrans, also a new observation for aN‐urethanyl‐α‐amino acid, and (ii) the overall conformation isquasi‐extended. These findings exclude the occurrence of an oxy‐C7peptide conformation. In solvents of high polarity, strongly solvated species predominate, as shown by ir absorption spectroscopy. In deuterochloroform nonassociated and associated species occur simultaneously. No differences were observed between the optically active and racemic derivatives. The type of self‐association near saturation seems to differ, at least in part, from that found

ISSN:0006-3525    

DOI:10.1002/bip.1981.360200807

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

年代:1981

数据来源: WILEY

摘要:

AbstractApproximate methods are developed and evaluated for treating the rate of binding ligands that cover several contiguous sites to a homogeneous one‐dimensional lattice, which represents a nucleic acid or other linear biopolymer. The model requires as input only the number of lattice sites necessary for binding, the total number (possibly infinite) of lattice sites, and elementary rate constants for the cooperative and noncooperative association and dissociation of the ligand on the lattice. The computational methods employed are an extension of the triplet closure approximation from the helix–coil (single‐site ligand) problem to the large ligand binding problem. It is found that consideration of clusters ofn+ 2 lattice sites, where each ligand coversnsites, gives a surprisingly accurate description of the kinetics. The approximation is implemented by an extension of the matrix‐iteration approach proposed by Craig and Crothers. The effects of the finite lattice length, as well as the capability to treat ligand motion along the lattice, are incorporated. When all symmetries are taken into consideration, the time required for the matrix iteration calculation rises only linearly with the ligand lengthnand is considerably less than that of the Monte Carlo method, which is used as a standard for com

ISSN:0006-3525    

DOI:10.1002/bip.1981.360200808

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

年代:1981

数据来源: WILEY

摘要:

AbstractProton magnetic resonance data have been obtained for 6‐methyl‐2′‐deoxyuridine (dT*), its 3′‐ and 5′‐monophosphates, and its 3′,5′‐diphosphate, as well as for the corresponding thymine derivatives. The synthesis of the dideoxynucleoside monophosphates—d(TpT), d(T*pT), d(TpT*), and d(T*pT*)—was accomplished, and spectral data were obtained for these four dimers. The data show that the 6‐methyluracil base prefers thesynconformation about theN‐glycosyl bond at the monomer and dimer levels. The presence of thesynbase leads to increases in theciscouplings of the sugar ring,J1′2″andJ2′3′, which indicate a trend towards eclipsing of the substituents on the C1′‐C2′ and C2′‐C3′ fragments. This trend is discussed in terms of changes in the pseudorotational parameters which describe the pucker of the ring. Thesynbase destabilizes theg+conformer about the C4′‐C5′ bond, leading to a preference for thetconformer in all dT* residues at the monomer and dimer levels. Preliminary work on the formation of cyclobutane‐type photodimers in d(T*pT) and d(T*pT*) is discussed and presented as evidence for the capability of

ISSN:0006-3525    

DOI:10.1002/bip.1981.360200809

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

年代:1981

数据来源: WILEY

摘要:

AbstractThe hydrogen–deuterium exchange reaction for the tryptophan residues in lysozyme have been followed in 4.5MLiBr at pH 7.2 in the temperature range of the unfolding transition by measuring the transmittance change at 293 nm. The exchange reaction proceeded in three phases at low temperature for native protein. The first and the second phases were ascribed to the H‐D exchange reactions of three relatively exposed tryptophan residues on the molecular surface. The third phase corresponded to the H‐D exchange reaction of the three tryptophan residues buried in the interior of the molecule. The H‐D exchange reaction proceeded in two phases near the melting temperature and in a single phase at high temperature, where almost all molecules are unfolded. The H‐D exchange of three tryptophan residues buried in folded molecules was caused by fluctuation between the folded and unfolded structure of the protein molecule. The rates of such a fluctuation were determined from the rates of the exchange reaction at various temperatures. These rates agreed very well with those determined from the temperature‐jump method. This means that a protein molecule in solution fluctuates between the N‐ and D‐states at every temperature within the transition region, where the N‐form is the tightly folded native structure and the D‐form the randomly coiled chain. From measurements of thermal unfolding of ester‐108‐lysozyme and the binding constant of (NAG)3to ester‐108‐lysozyme, it was found that almost all cross‐linked molecules are in the folded state near 50°C and pH 7.2 in 4.5MLiBr, where intact molecules are unfolded. We also studied the H‐D exchange reaction of ester‐108‐lysozyme. In the temperature region of 43–50°C, about 70% of the exchangeable tryptophan residues of ester‐108‐lysozyme were exchanged within 1 s immediately after the mixing of D2O, in spite of the fact that almost all molecules are in the folded state. This was considered the premeltin

ISSN:0006-3525    

DOI:10.1002/bip.1981.360200810

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

年代:1981

数据来源: WILEY

摘要:

AbstractConformational analysis of antiparallel double‐helical polynucleotides with Watson‐Crick base pairing was reduced to a four‐dimensional problem using original mathematical methods. In the four‐dimensional conformational space the family of structures, characterized by the base‐pair stacking with the most stable conformations in water solution as well as in the solid state, was localized. For the C′2‐endosugar pucker, both right‐handed and left‐handed structures were found; right‐handed structures only, however, seem to be allowed for the C′3‐endopucker, the only possible one for ribonucleot

ISSN:0006-3525    

DOI:10.1002/bip.1981.360200811

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

年代:1981

数据来源: WILEY