15N-NMR spectroscopy. III. Neighboring residue effects in sequence polymers containing glycylglycine units

The tripeptides Phe-Gly-Gly, β-Ala-Gly-Gly, and ?-Aca-Gly-Gly as well as the peptide derivatives δ-isothiocyanatovaleroyl-Gly-Gly and ?-isothiocyanatocaproyl-Gly-Gly were synthesized by using known methods so that the peptide nitrogen between the two glycyl residues was isotopically enriched in ¹⁵N to a level of 0.8–0.9%. These monomer units were then used to produce the sequence polymers (Phe-Gly-Gly)_n, (β-Ala-Gly-Gly)_n, (δ-Ava-Gly-Gly)_n, and (?-Aca-Gly-Gly)_n. The 18.24 MHz ¹⁵N-NMR spectra of the oligo- and polypeptides were obtained by using trifluoroacetic acid as solvent, since the solutions have relatively low viscosity and exhibit a strong negative nuclear Overhauser enhancement of the ¹⁵N signals. For comparison, ¹⁵N-NMR spectra of the homopolymers (Gly)_n, (β-Ala)_n, (γ-Abu)_n, (δ-Ava)_n, and (?-Aca)_n were also recorded. The ¹⁵N signals from the ω-aminoacyl residues in the sequence polymers appear up to 11 ppm upfield of the signals observed for the homopolyamides. The ¹⁵N signals from the two glycyl residues are separated by 3–7 ppm. Comparison with the ¹³C-NMR spectra of the same polymers indicates that ¹⁵N-NMR is better suited for the characterization and sequence analysis of these types of polymers.     

四氟乙烯-全氟4-甲基-3,6-二氧杂△~7辛基磺酰氟共聚物的分子运动

本工作用振簧仪,在-150—80℃范围内,测试了四氟乙烯-全氟4-甲基-3,6-二氧杂△~7辛基磺酰氟共聚物的动态力学性能。样品是含有不同侧基数目的部分结晶共聚物。观察到四个松弛过程,按温度下降次序记为α、γ_1-、γ_2-、γ_3-。α峰(室温附近)系玻璃化转变。γ_1峰(-50℃,～215Hz)为主链有阻碍的短链段分子运动。γ_2-峰(-60℃,～230Hz)为侧基的分子运动。γ_3-峰(-77℃,～260Hz)为的链段运动,并且γ_3-峰的强度随结晶度增加而增强。     

INTERACTION OF VIOLOGEN DYES WITH CHROMATOPHORES AND REACTION-CENTER PREPARATIONS FROM RHODOSPIRILLUM RUBRUM

Abstract —Various low-potential viologen dyes enhance light-induced absorption changes in the near-infrared region (ΔA870) in chromatophore preparations from Rhodospirillum rubrum in the presence of dichlorophenol indophenol and a high concentration of ascorbate (DCPIP + asc). An increase in ΔA870 was also observed in large reaction-center preparations from R. rubrum with viologens in the presence of ascorbate. Our results indicate that E'₀, of the primary electron acceptor X may be as low as -0.37 V, as suggested recently by P. A. Loach (1973, personal communication).     

Vertical excitation as a transient phenomenon

For a particular model with two electronic states, each with two vibrations, the dipole correlation function governing electronic absorption is e ^?iωOt cosγt with spectrum ω ₀±γ. The function starts as e ^?iωOt (γ?ω ₀), with Fourier transform peaking around ω ₀ instead of ω ₀±γ, and this is associated with vertical excitation. After a time t～2/γ the spectrum goes over into the normal one. As a generalization, a procedure is outlined for characterizing the state reached first after interaction with light is initiated. Finally it is suggested that one can understand aspects of internal conversion by analogy with the case of vertical excitation.     

Triazine chemistry VIII. 2,5-dihydro-5-oxo-1,2,4-triazines

2,5-Dihydro-5-oxo-1,2,4-triazine and some of its alkylated derivatives have been prepared. Nmr spectroscopic analysis has established that the 2,5-dihydro-5-oxo tautomers are preferred over the 4,5-dihydro-5-oxo ones. This preference, and the behavior of 1,2,4-triazines in some other chemical reactions has been interpreted in terms of electron-electron repulsions between the lone pairs of electrons of N₁ and N₂ in this ring system.     

The foam analogy: from phases to elasticity

By mapping the interactions of colloidal particles onto the problem of minimizing areas, the physics of foams can be used to understand the phase diagrams of both charged and fuzzy colloids. We extend this analogy to study the elastic properties of such colloidal crystals and consider the face-centered cubic, body-centered cubic and A15 lattices. We discuss two types of soft interparticle potentials corresponding to charged and fuzzy colloids, respectively, and we analyze the dependence of the elastic constants on density as well as on the parameters of the potential. We show that the bulk moduli of the three lattices are generally quite similar, and that the shear moduli of the two non-close-packed lattices are considerably smaller than in the face-centered cubic lattice. We find that in charged colloids, the elastic constants are the largest at a finite screening length, and we discuss a shear instability of the A15 lattice.     

Engineering cytochrome P-450s

Cytochrome P-450 isozymes represent a critical component of nature’s spectrum of detoxification catalysts that could be exploited for bioremediation. The ethanol-inducible human cytochrome P-450 2E1 serves as a model eukaryotic P-450 that complements the bacterial P-450 cam in dehalogenation and detoxification of environmental pollutants. We explored the construction of novel chimeric P-450s using cytochrome P-450 camC and 2E1 genes. For construction of chimera 1 (478 amino acids, 55.14 kDa), 145 amino acids from the N-terminus of P-450 2E1 protein (493 amino acids, 56.84 kDa) were replaced with 130 amino acids from the N-terminus of P-450 camC protein (415 amino acids, 46.66 kDa). In chimera 2 (525 amino acids, 60.24 kDa) the strategy involves replacement of 28 amino acids in the C-terminus of chimera 1 with 75 amino acids from the C-terminus of P-450 camC gene. Homology models of both the chimeric proteins were developed using SWISS-MODEL based on the known crystal structure of cytochrome P-450 camC, BM-3, 1DT6A, and 2C17A. The models indicated that the proposed heme-binding site was intact, which is inevitable for catalytic activity of cytochrome P-450s. The expression of chimera 1 and 2 genes in Escherichia coli DH5α was evident from light-pink cell pellets, protein band in sodium dodecyl sulfate polyacrylamide gel electrophoresis, and diagnostic carbon monoxide-difference spectra. Our studies show that strategies can be developed to exploit the natural diversity of the P-450 superfamily to generate chimeric biocatalysts that would provide new templates amenable to directed evolution.     

Metal-thiophen derivatives. Organometallic compounds of palladium and platinum

Thienylmercury(II)chloride reacts with [Pd(PPh₃)₂Cl₂], [Pd(PPh₃)₄] and [Pt(PPh₃)₄] to afford new compounds containing a metal-2-thienyl linkage. The compound [Pd(PPh₃)₂(2-C₄H₃S)Cl] probably has trans stereochemistry.2-Bromothiophen undergoes oxidative addition with [Pd(PPh₃)₄] and [Pt(PPh₃)₄], probably via a radical mechanism. With [Pd(CO)(PPh₃)₃], a carbonyl inserted product is obtained. The bromo-metal(II) complexes have trans stereochemistry. The course of the reaction between 3-methyl-2-bromothiophen and Pd(PPh₃)₄ is more complex. Thus, there is evidence of some cis bromopalladium(II) compounds amongst the products, also there is good evidence to support the view that some isomerisation of 3-methyl-2-thienyl to 4-methyl-2-thienyl occurs during the reaction, thus giving greater molar quantities of [Pd(PPh₃)₂(4-CH₃-2-C₄H₂S)Br] than can be accounted for from any initial 4-methyl-2-bromothiophen impurity.The metallation of the thiophen ring, probably in the 4-position, with palladium(II) is described for 3-theylidene-4-methylaniline.     

The crystal structure of 1,1-bis(η5-cyclopentadienyl)-2,3,4,5-tetraphenylzirconole

The crystal and molecular structure of (η⁵-C₅H₅)₂Zr[C₄(C₆H₅)₄] has been determined by single crystal X-ray diffraction methods. The compound is isostructural with its titanium and hafnium analogues, and crystallizes in the monoclinic space group P2₁/n with unit cell parameters a = 13.790(5), b = 11.136(5), c = 18.692(7) », β = 92.82(4)°, and ?_calc = 1.34 g cm^?3 for Z = 4. Full-matrix least-squares refinement converged with a conventional R value of 0.049 for 2986 observed reflections. The metallocyclic ring is planar to within 0.05 », and the π-electron density is largely localized. The two independent ZrC(σ) bond lengths are 2.250(5) and 2.265(6) ». The ZrC(η⁵) distances range from 2.482(6) to 2.546(7) », and average 2.521(20) ». A comparison of zirconium- and hafnium-carbon bonds based on the data available shows that for M = Hf the MC bonds are shorter for all cases: C(sp), C(sp²), C(sp³), C(η⁵).     

CNDO/2 calculations of some polyazaindenes

The theoretically estimated dipole moments of indolizine, imidazo[1,2-a]pyridine, imidazo-[1,5-a]pyridine and of pyrazolo[ 1,5-a]pyridine obtained by the CNDO/2 approximation have been compared with the experimental values. The bond angles and bond distances for these polyazaindenes have been estimated.