摘要:

AbstractSemi‐empirical energy calculations for an internal Pro‐Pro dimer are presented that take into account the nature of the flexibility of the proline ring due to its puckering. Calculations show that three stable conformations are available for the dimer: thecis(ω = 0°, ψ = 160°); thetrans(ω = 180°, ψ = 160°, also referred to astrans′); and thecis′ (ω = 180°, ψ = −40°) conformations. The best conformational pathways between these stable conformations are determined. Calculations also show that the barrier forcis′–trans′ conversion is of the same order of magnitude as

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140602

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

年代:1975

数据来源: WILEY

摘要:

AbstractThe cross‐sectional radius of gyration of the deoxyribonucleoprotein (DNP) threads was measured by small‐angle X‐ray scattering in a wide range of ionic strengths (from 0.0005 to 2MNaCl). For DNP in a solution of low ionic strength, this value is 30 Å. The increase of ionic strength results in partial deproteinization of DNP, while the cross‐sectional radius of gyration varies from 25 Å for DNP in 0.7MNaCl to 10 Å for DNP in 2MNaCl. It is suggested that gradual deproteinization by the increase of NaCl concentration causes conformational changes, which are associated with the alteration of the DNP superstructure. The data are interpreted on the basis of the superhelical model of DNA packing in DNP; however, the coexistence of superhelical and unfolded regions in the DNP structure is also a p

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140603

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

年代:1975

数据来源: WILEY

摘要:

AbstractThe electron‐mediated spin–spin coupling constantJbetween the amide NH and the α‐CH protons in the dipeptide fragment CαCO(NHCαH)RC′ONHCαis dependent on the dihedral angle of rotation (Φ) around the NC bond. Measurement ofJin a series of zwitterionic dipeptides H3N+CHR1CONHCHR2CO2−(which is conformationally similar to the dipeptide fragment) in TFA solution shows thatJis independent of R1, but dependent on the steric bulk of R2. The data are interpreted in terms of a model that assumes that what we measure is an average value ofJa thermal average over all the possible rotamers. The groups R1and R2are, in most cases, sterically kept apart by thetransand planar amide bonds, and hence the independence ofJof R1. This model is consistent with the theoretical calculations done on the dipeptide fragment. The effect of the structural characteristics of the side chains (e.g., the effect of lengthening and branching the side chains) on theJvalues in dipeptides is discussed in the light of the existing results of theoretical calculations.Study of 〈J〉 values in tripeptides (C6H5CH2OCONHCHR1CONHCHR2CO2CH3, essentially three linked peptide units) shows that electrostatic interaction between the two amide bonds modifies the potential energy surface and the 〈J〉 value of a dipeptide subunit in the tripeptides. Also in some cases, direct steric interaction between the two side chains in the two adjacent dipeptide subunits in the tripeptide affects the potential energy surfaces of the individual dipeptide subunits and hence the 〈J〉 values. The influence of the structural characteristics of the side chains of individual amino acids on structure formation at or beyond the dipeptide level is discussed at various points.TheJ(NHαCH) values of CH3CONHCHRCONH2and CH3CONHCHRCO2CH3with the same R are quite different for R = valine, leucine, phenylalanine, methionine, but equal for R = glycine. This, coupled with the fact that one of the carboxamide NH resonances has a chemical shift different from its counterpart in simple amides like CH3CONH2and the other carboxamide NH has the same chemical shift as its counterpart in CH3CONH2, suggest the presence of a hydrogen bond in dipeptide CH3CONHCHRCONH2with carboxamide NH as the donor. Theoretical evidence for two seven‐membered hydrogen‐bonded rings with the carboxamide NH as donor and the acetyl oxygen as acceptor is summarized. Our data cannot suggest the number of such hydrogen‐bonded rings, nor can they conclude the relative proportion of these rings in a particular dipeptide. A discussion of the difficulty of interpretation is

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140604

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

年代:1975

数据来源: WILEY

摘要:

AbstractThe buoyant density and potentiometric hydrogen ion titration curves of human immuno‐gamma globulin in 3Mcesium chloride have been recorded. In addition, the amino acid analysis of the IgG employed has been completed. The hydration of the protein and the variation of the hydration with pH have been calculated from the buoyant density data. The potentiomtric hydrogen ion titration curve has been employed to estimate the intrinsic pK′s of the acidic and histidyl residues of the molecule, and to confirm the hypothesis that it does in fact conform to the oil drop model of protein conformation. Correlations have been drawn between the three sets of data in the following manner. The results of the potentiometric hydrogen ion titration have been checked against the amino acid analysis to determine whether the numbers of groups observed to titrate and the numbers of groups observed in the amino acid analysis do correspond. Second, previous hypotheses as to the direct correlation between potentiometric hydrogen ion titration behaviour and buoyant density titration behaviour have been investigated and substantially confir

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140605

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

年代:1975

数据来源: WILEY

摘要:

AbstractA basic theory for equilibrium properties of cooperative transitions (particularly helix–coil transformations) of sufficiently long linear biopolymers under the influence of an electric field is developed. General relations for the calculation of the distribution of uninterrupted sequences of elementary subunit states (e.g., helical fragments) as well as the overall degree of transition θ are given. Strong cooperativity is found to permit simplifications. It is shown that only in this case can a practically significant field effect be expected. Numerical results are presented for a special example of experimental intere

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140606

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

年代:1975

数据来源: WILEY

摘要:

AbstractAn electric birefringence apparatus with an He–Ne laser is described, which permits a high sensitivity of measurement for field strengths up to 20 kV/cm and pulse durations between 10−5to 1 sec. It is applied to poly(γ‐benzyl‐L‐glutamate) and poly(β‐benzyl‐L‐aspartate) in a nonaqueous solvent mixture, especially under the conditions of the helix‐coil equilibrium. A theoretical treatment of the problem shows that the usual neglect of end effects is only justified for comparatively long chains. On the basis of a computational procedure taking into account the finite chain lengths, the experimental data for 15 kV/cm reveal an obvious enhancement of the coil‐to‐helix transition caused by the electric field. This effect is quantitatively confirmed by a pertinent theory. The cooperative parameter σ may be best determined from the initial linear increase of the bire‐fringence versus the square of the field strength where the conformational e

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140607

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

年代:1975

数据来源: WILEY

摘要:

AbstractAn X‐ray study of the synthetic polypeptide poly(L‐homoarginine hydrochloride) has been made to investigate whether, like the chemically related polypeptides poly(L‐lysine hydrochloride), poly(L‐arginine hydrochloride), and poly(L‐ornithine hydrobromide), it can undergo conformational transitions merely from variations in its degree of hydration. X‐ray photographs of powder and oriented specimens containing one to 15 molecules of water perL‐homoarginine hydrochloride residue showed that this polymer forms only a β‐pleated‐sheet structure. The pleated sheets, formed by antiparallel polypeptide chains hydrogen‐bonded to each other, are piled up along thebaxis in an alternating sequence (“sandwich structure”). This structure did not appreciably change with variations of the degree of hydration, and the observed reflections at 56% relative humidity (1.8 molecules of water per residue) could be indexed satisfactorily in terms of a monoclinic unit cell, of space group P21, witha= 9.34 Å,b= 40.07 Å,c= 6.94 Å, and γ = 106°. These dimensions are shown by models to be compatible with the proposed structure, and the calculated density of 1.27 g/cm3agrees well with the experimental value of 1.29 g/cm3. Removal of the last molecule of water results in a very diffuse pattern, while specimens containing 20 molecules of water per residue show

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140608

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

年代:1975

数据来源: WILEY

摘要:

AbstractThe first uv absorption band hypochromism of poly(dA) · poly(dT), poly(dG) · poly(dC), poly(dA), poly(dT), poly(dG), and poly(dC) is calculated with the help of perturbation theory on the basis of monomer characteristics computed by the Pariser‐Parr‐Pople method taking into account all singly excited configurations. The theoretical results obtained are in good agreement with experimental values of hypochromism. The origin of the hypochromic effect in the double‐stranded polynucleotides is investigated. It is shown that intrastrand interactions between the bases make the main contribution to hypochromism (60–76%), while the contribution of the Watson–Crick‐pair formation is small (2–12%). The essential part of hypochromism (22–28%) is due to the interstrand interactions between the bases that are not coupled by hydrogen bonds. The discussion of the experimental data shows that the present theoretical investigation could serve as a basis for the correct treatment of

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140609

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

年代:1975

数据来源: WILEY

摘要:

AbstractCircular dichroism and absorption spectra are measured on mixed solutions of acridine orange and poly(S‐carboxymethyl‐L‐cysteine) at different pH and P/D mixing ratios. The observed circular dichroism spectra are classified into several types, mainly based on the number and sign of circular dichroic bands in the visible region. Three of them are associated with the absorption spectra characteristic of dimeric dye or higher aggregates of dye. Type I is observed with solutions, of which the pH is acid and P/D is higher than 4, and it has an unsymmetrical pair of positive and negative dichroic bands at 470 and 430 nm. This type is induced on the dye bound to the polymer in the β‐conformation. Types II and III are considered to be characteristic of randomly coiled polymers. Type II is exhibited by solutions of P/D higher than 1 at pH 5–7 and has two dichroic bands around the same wavelengths as Type I but with opposite signs and an additional positive band at 560 nm. Type III, shown by solutions of P/D 2–0.6 at pH 6–10.5, has three dichroic bands around the same wavelengths as Type II but with signs opposite to it. The other two types of circular dichroism, induced for the solutions of P/D less than 1 at slightly acid pH, are associated with the absorption spectra of monomeric dye and are observed with disordered or randomly coiled polymer. They have a pair of dichroic bands at 540 and 425 nm, and the signs of these bands are opposite to each other in t

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140610

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

年代:1975

数据来源: WILEY

摘要:

AbstractThe helix–coil conformational transition undergone by poly(γ‐benzyl‐L‐glutamate) in solutions of trifluoroacetic acid and deuterated chloroform was studied by proton and carbon‐13 nmr. The results indicate that in the case of the solvent‐induced helix–coil transition, the side chain assumes a helical conformation before the backbone. In the thermally induced helix–coil transition, the results indicate the existence of an intermediate state, which is between the α‐helix and random coil and is free from intramolecul

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140611

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

年代:1975

数据来源: WILEY