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451.
452.
A dielectric relaxation study of aqueous solutions of the amphiphilic model peptide N‐acetyl‐leucine amide (NALA) at 298 K over a wide range of hydration levels is presented. The experiments range from states where water builds up several hydration layers to states where single water molecules or small water clusters are shared by several NALA molecules. The dielectric spectra reveal two modes on the 10 and 100 ps timescales. These are largely broadened with regard to the Lorentzian shape caused by simple Debye‐type relaxation, and are well described by the Kohlrausch–Williams–Watts stretched exponential function. The fast mode is assigned to water reorientation comprising bulk water as well as hydration water. Even when all water molecules are in contact with the solute, this fast component is dominant, and its mean relaxation time is retarded by less than a factor of two relative to neat water. The amplitude of the slow process is far higher than expected for the dipolar reorientation of the solute. The observations are consistent with results from molecular dynamics simulations for a similar model peptide reported in the literature. They suggest that the slow relaxation mode is mainly founded in peptide–water dipolar couplings, with some additional contribution from slowly reorienting hydration water molecules. The results are discussed with regard to the hydration dynamics of proteins and the interpretation of dielectric spectra of protein solutions. 相似文献
453.
G. Srinivas Rao Neela Sudhakar B. Venkateswara Rao S. Jeelani Basha 《Tetrahedron: Asymmetry》2010,21(16):1963-1970
The formal synthesis of 3-epi jaspine B was achieved by using a stereoselective intramolecular oxa-Michael addition. The diacetate derivative of 3-epi jaspine B was also synthesized. 相似文献
454.
Suman S. Thakur Padmanabhan Balaram 《Journal of the American Society for Mass Spectrometry》2009,20(5):783-791
Peptide disulfides are unstable under alkaline conditions, resulting in the formation of products containing lanthionine and
polysulfide linkages. Electrospray ionization mass spectrometry has been used to characterize major species obtained when
cyclic and acyclic peptide disulfides are exposed to alkaline media. Studies on a model cyclic peptide disulfide () and an acyclic peptide, oxidized glutathione, bis (γGlu - Cys - Gly - COOH), are described. Disulfide cleavage reactions are initiated by the abstraction of CαH or CβH protons of Cys residues, with subsequent elimination of H2S or H2S2. The buildup of reactive thiol species which act on intermediates containing dehydroalanine residues, rationalizes the formation
of lanthionine and polysulfide products. In the case of the cyclic peptide disulfide, the formation of cyclic products is
facilitated by the intramolecular nature of the Michael addition reaction of thiols to the dehydroalanine residue. Mass spectral
evidence for the intermediate species is presented by using alkylation of thiol groups as a trapping method. Mass spectral
fragmentation in the negative ion mode of the peptides derived from trisulfides and tetrasulfides results in elimination of
S2. 相似文献
455.
We prove that an equational class of Hilbert algebras cannot be defined by a single equation. In particular Hilbert algebras
and implication algebras are not one-based. Also, we use a seminal theorem of Alfred Tarski in equational logic to characterize
the set of cardinalities of all finite irredundant bases of the varieties of Hilbert algebras, implication algebras and commutative
BCK algebras: all these varieties can be defined by independent bases of n elements, for each n > 1.
相似文献
456.
Sudhakar S. Dhondge Chandrashekhar P. Pandhurnekar Dilip V. Parwate 《The Journal of chemical thermodynamics》2009,41(5):577-585
Experimental results of density (ρ), speed of sound (u), and refractive index (nD) have been obtained for aqueous solutions of ethylene glycol monomethyl ether (EGMME), ethylene glycol monoethyl ether (EGMEE), diethylene glycol monomethyl ether (DEGMME), and diethylene glycol monoethyl ether (DEGMEE) over the entire concentration range at T = 298.15 K. From these measurements, the derived parameters, apparent molar volume of solute (?V), excess molar volume (VE), isentropic compressibility of solution (βS), apparent molar isentropic compressibility of solute (?KS), deviation in isentropic compressibility (ΔβS), molar refraction [R]1,2 and deviation in refractive index of solution (ΔnD) have been calculated. The Redlich–Kister equation has been fitted to the calculated values of VE, ΔβS and ΔnD for the solution. The results obtained are interpreted in terms of hydrogen bonding and various interactions among solute and solvent molecules. 相似文献
457.
An absorption law is an identity of the form p = x. The ternary function x+y+z (ring addition) in Boolean algebras satisfies three absorption laws in two variables. If a term satisfies these three identities
in a variety, it is called a minority term for that variety. We construct a minority term p for orthomodular lattices such the identity defines Boolean algebras modulo orthomodular lattices. (The dual of p is denoted by .) Consequently, having a unique minority term function characterizes Boolean algebras among orthomodular lattices. Our main
result generalizes this example to arbitrary arity and arbitrary consistent sets of 2-variable absorption laws.
Presented by J. Berman. 相似文献
458.
Molecular dynamics simulations have been employed to study the formation of a physical gel by semiflexible polymer chains. The formation of a geometrically connected network of these chains is investigated as a function of temperature and rate of cooling. The stiffness of the molecules is controlled via a potential between beads separated by two bonds. As the temperature is lowered, a percolated homogeneous solution phase separates to form a high-density, non-percolated nematic fluid and a low-density gas phase. On further decreasing the temperature, the chains are dynamically arrested preventing the completion of the vapor-liquid (VL) phase separation. As a result, the chains are stuck in a three-dimensional network of nematic bundles forming a percolated gel. Apart from temperature, the rate of cooling also plays an important role in the formation of the gel. Cooling the system at a faster rate yields gel while slower rates result in complete VL phase separation. 相似文献
459.
Roy DR Parthasarathi R Padmanabhan J Sarkar U Subramanian V Chattaraj PK 《The journal of physical chemistry. A》2006,110(3):1084-1093
The philicity concept [J. Phys. Chem. A 2003, 107, 4973] is put in proper perspective. In the present work we analyze different physicochemical problems using philicity. It provides satisfactory results in all such cases. We also compare the relative electro(nucleo)philicity with philicity to show that philicity is better than relative electro(nucleo)philicity when the intermolecular reactivity trends are considered and there is hardly any preference of one above the other as far as the intramolecular reactivities are concerned. On the contrary, the philicity concept has some advantages over the other concept. 相似文献
460.
Quantum theories constructed on the noncommutative spacetime called the Groenewold–Moyal plane exhibit many interesting properties
such as Lorentz and CPT noninvariance, causality violation and twisted statistics. We show that such violations lead to many
striking features that may be tested experimentally. These theories predict Pauli forbidden transitions due to twisted statistics,
anisotropies in the cosmic microwave background radiation due to correlations of observables in spacelike regions and Lorentz
and CPT violations in scattering amplitudes. 相似文献