Dispersion of linear and nonlinear optical susceptibilities and the hyperpolarizability of 3-methyl-4-phenyl-5-(2-pyridyl)-1,2,4-triazole.

As a starting point for our calculation of 3-methyl-4-phenyl-5-(2-pyridyl)-1,2,4-triazole we used the XRD data obtained by C. Liu, Z. Wang, H. Xiao, Y. Lan, X. Li, S. Wang, Jie Tang, Z. Chen, J. Chem. Crystallogr., 2009 39 881. The structure was optimized by minimization of the forces acting on the atoms keeping the lattice parameters fixed with the experimental values. Using the relaxed geometry we have performed a comprehensive theoretical investigation of dispersion of the linear and nonlinear optical susceptibilities of 3-methyl-4-phenyl-5-(2-pyridyl)-1,2,4-triazole using the full potential linear augmented plane wave method. The local density approximation by Ceperley-Alder (CA) exchange-correlation potential was applied. The full potential calculations show that this material possesses a direct energy gap of 3.4 eV for the original experimental structure and 3.2 eV for the optimized structure. We have calculated the complex's dielectric susceptibility ε(ω) dispersion, its zero-frequency limit ε(1)(0) and the birefringence. We find that a 3-methyl-4-phenyl-5-(2-pyridyl)-1,2,4-triazole crystal possesses a negative birefringence at the low-frequency limit Δn(0) which is equal to about -0.182 (-0.192) and at λ = 1064 nm is -0.193 (-0.21) for the non-optimized structure (optimized one), respectively. We also report calculations of the complex second-order optical susceptibility dispersions for the principal tensor components: χ(ω), χ(ω) and χ(ω). The intra- and inter-band contributions to these susceptibilities are evaluated. The calculated total second order susceptibility tensor components at the low-frequency limit |χ(0)| and |χ(ω)| at λ = 1064 nm for all the three tensor components are evaluated. We established that the calculated microscopic second order hyperpolarizability, ß(ijk), the vector component along the dipole moment direction, at the low-frequency limit and at λ = 1064 nm, for the dominant component |χ(ω)| is 4.99 × 10(-30) esu (3.4 × 10(-30) esu) and 7.72 × 10(-30) esu (5.1 × 10(-30) esu), respectively for the non-optimized structure (optimized structure).

Linear and nonlinear optical susceptibilities of 3-phenylamino-4-phenyl-1,2,4-triazole-5-thione.

We report first-principles calculations of the linear and nonlinear optical susceptibilities of 3-phenylamino-4-phenyl-1,2,4-triazole-5-thione crystals. The X-ray diffraction structural data of Wang et al. (Molecules 2009, 14, 608.) was used as the starting point of the computational optimization calculation by minimization of the forces acting on the atoms, and the optimized geometry was used to calculate the linear and nonlinear optical susceptibilities. We have employed the full potential linear augmented plane wave (FPLAPW) method within density functional theory (DFT) along with the Engel-Vosko exchange correlation potential. The full potential calculations show that this crystal possesses an indirect energy gap of 3.1 eV. The compound exhibits some uniaxial dielectric anisotropy resulting in a strong birefringence. The 3-phenylamino-4-phenyl-1,2,4-triazole-5-thione crystal possesses high second harmonic generation with several nonzeroth components, but only one component, chi(111)((2))(omega), is dominant and its second-order optical susceptibility of the total real part and the total absolute value at zero frequency is equal to 0.097 x 10(-7) esu.

Phosphorylation controls the three-dimensional structure of plant light harvesting complex II.

The most abundant chlorophyll-binding complex in plants is the intrinsic membrane protein light-harvesting complex II (LHC II). LHC II acts as a light-harvesting antenna and has an important role in the distribution of absorbed energy between the two photosystems of photosynthesis. We used spectroscopic techniques to study a synthetic peptide with identical sequence to the LHC IIb N terminus found in pea, with and without the phosphorylated Thr at the 5th amino acid residue, and to study both forms of the native full-length protein. Our results show that the N terminus of LHC II changes structure upon phosphorylation and that the structural change resembles that of rabbit glycogen phosphorylase, one of the few phosphoproteins where both phosphorylated and non-phosphorylated structures have been solved. Our results indicate that phosphorylation of membrane proteins may regulate their function through structural protein-protein interactions in surface-exposed domains.

Complex formation in plant thylakoid membranes. Competition studies on membrane protein interactions using synthetic peptide fragments.

Thylakoid membranes of pea were used to study competition between extra-membrane fragments and their parental membrane-bound proteins. Phosphorylated and unphosphorylated fragments of light harvesting complex II (LHC II) from higher plants were used to compete with LHC II for interactions with itself and with other thylakoid protein complexes. Effects of these peptide fragments of LHC II and of control peptides were followed by 80 K chlorophyll fluorescence spectroscopy of isolated thylakoids. The phosphorylated LHC II fragment competes with membrane-bound phosphoproteins in the phosphatase reaction. The same fragment accelerates the process of dark-to-light adaptation and decreases the rate of the light-to-dark adaptation when these are followed by fluorescence spectroscopy. In contrast, the non-phosphorylated LHC II peptide does not affect the rate of adaptation but produces results consistent with inhibition of formation of a quenching complex. In this quenching complex we propose that LHC II remains inaccessible to the LHC II kinase, explaining an observed decrease in LHC II phosphorylation in the later stages of the time-course of phosphorylation. The most conspicuous protein which is steadily phosphorylated during the time-course of phosphorylation is the 9 kDa (psbH) protein. The participation of the phosphorylated form of psbH in the quenching complex, where it is inaccessible to the phosphatase, may explain its anomalously slow dephosphorylation. The significance of the proposed complex of LHC II with phospho-psbH is discussed.

Structural and functional studies in vitro on the p6 protein from the HIV-1 gag open reading frame.

Protein p6 from HIV-1 gag open reading frame is reported to affect both the final phase of assembly of the viral particle and the early stage of the gag polyprotein maturation in vitro. Two separate hypotheses have been proposed, on only one of these reported effects. We think that both observations may be eventually explained if p6 protein strongly inhibits the HIV-1 proteinase. Protein p6 was synthesised by solid-phase peptide synthesis. Several methods of folding the p6 protein were tested, each resulting in the random structure according to both CD and 1D proton NMR spectra. A uniformly high exposure of NH protons to the solution was confirmed by temperature-dependent NMR spectra and isotope exchange experiments. Thus the p6 protein does not have any rigid conformation in solution. A rigid structure is not formed after further cleavage by HIV-1 proteinase as neither the protein nor its fragments are cleaved by this proteinase. In addition, the p6 protein itself does not act as inhibitor of HIV-1 proteinase. This excludes a direct role of p6 protein and supports the hypothesis that p6 is involved in forming the appropriate structure of gag polyprotein precursor. The role of slowly cleaved tight gag-proteinase in the final stage of maturation may be to slow down maturation of the precursor polyproteins prior to their transport to final location in the membrane.

15gag proteinase of myeloblastosis-associated virus: specificity studies with substrate-based inhibitors.

The specificity of the proteinase of myeloblastosis-associated virus (MAV) was studied with (a) 21 substrate-based inhibitors, (b) 9 inhibitors with pseudopalindrome sequences, (c) 8 chimeric inhibitors, and (d) 3 compounds designed as human immunodeficiency virus 1 (HIV-1) proteinase inhibitors. The central inhibitory unit (transition state or cleaved bond analog) and the role of the inhibitor side chains from P4 to P4' were investigated. MAV proteinase prefers an aromatic side chain in P1 and a small aliphatic nonpolar chain in P2 and P2'. Residues in P5 and P4 positions are outside of the short catalytic cleft of the enzyme, but still influence binding considerably. The data obtained provide evidence that the MAV proteinase has generally lower specificity and poorer binding than the HIV proteinase.

Specificity studies on retroviral proteinase from myeloblastosis-associated virus.

The specificity of the p15 proteinase of myeloblastosis-associated virus (MAV) was tested with nonviral high molecular weight substrates and with synthetic peptides. Peptides with sequences spanning known cleavage sites in viral polyproteins of Rous sarcoma virus (RSV) and avian leukemia viruses, as well as in BSA and HSA, were synthesized, and the rate of their cleavage by the MAV proteinase was compared. Synthetic peptides require for successful cleavage at least 4 residues at the N-terminal side and 3 residues at the C-terminal side. The proteinase shows a preference for hydrophobic residues with bulky side chains (Met, Tyr, Phe) in P3, although Arg and Gln can also be accepted. Small hydrophobic residues are required in P2 and P2', and large hydrophobic residues (Tyr, Met, Phe/p-nitro-Phe) are preferred in both P1 and P1'. The difference between the specificity of the p15 proteinase and that of the HIV-1 proteinase mostly pertains to position P2' of the substrate, where bulkier side chains are accepted by the HIV-1 proteinase (Richards et al., 1990). A good chromogenic substrate for the MAV and RSV proteinases was developed and used to further characterize the MAV proteinase activity with respect to ionic strength and pH. The activity of the proteinase is strongly dependent on ionic strength and pH. Both the kcat and Km values contribute to a higher cleavage efficiency at higher salt concentrations and show a bell-shaped pH dependence curve with a sharp maximum at pH 5.5 (kcat) and 6.5 (Km).