Synthesis of Meso‐Substituted Subphthalocyanine–Subporphyrin Hybrids: Boron Subtribenzodiazaporphyrins

The first syntheses of hybrid structures that lie between SubPhthalocyanines and SubPorphyrins are reported. The versatile, single-step synthetic protocol uses a preformed aminoisoindolene to provide the bridging methine unit and its substituent, while trialkoxyborates simultaneously act as Lewis acid/template and provider of the apical substituent. Selection of each component therefore allows controlled formation of diverse, differentially functionalised systems. The new hybrids are isolated as robust, pure materials that display intense absorption and emission in the mid-visible region. They are further characterised in solution and solid state by variable temperature NMR spectroscopy and X-ray crystallography respectivel

Studies in the mechanism of transmission of non-conjugative substituent effects

M.S.Charles L. Liott

Mechanisms of base selection by human single-stranded selective monofunctional uracil-DNA glycosylase

hSMUG1 (human single-stranded selective monofunctional uracil-DNA glyscosylase) is one of three glycosylases encoded within a small region of human chromosome 12. Those three glycosylases, UNG (uracil-DNA glycosylase), TDG (thymine-DNA glyscosylase), and hSMUG1, have in common the capacity to remove uracil from DNA. However, these glycosylases also repair other lesions and have distinct substrate preferences, indicating that they have potentially redundant but not overlapping physiological roles. The mechanisms by which these glycosylases locate and selectively remove target lesions are not well understood. In addition to uracil, hSMUG1 has been shown to remove some oxidized pyrimidines, suggesting a role in the repair of DNA oxidation damage. In this paper, we describe experiments in which a series of oligonucleotides containing purine and pyrimidine analogs have been used to probe mechanisms by which hSMUG1 distinguishes potential substrates. Our results indicate that the preference of hSMUG1 for mispaired uracil over uracil paired with adenine is best explained by the reduced stability of a duplex containing a mispair, consistent with previous reports with Escherichia coli mispaired uracil-DNA glycosylase. We have also extended the substrate range of hSMUG1 to include 5-carboxyuracil, the last in the series of damage products from thymine methyl group oxidation. The properties used by hSMUG1 to select damaged pyrimidines include the size and free energy of solvation of the 5-substituent but not electronic inductive properties. The observed distinct mechanisms of base selection demonstrated for members of the uracil glycosylase family help explain how considerable diversity in chemical lesion repair can be achieved

Mechanisms of base selection by the E.coli mispaired uracil glycosylase

The repair of the multitude of single-base lesions formed daily in the cells of all living organisms is accomplished primarily by the base-excision repair (BER) pathway that initiates repair through a series of lesion-selective glycosylases. In this paper, single-turnover kinetics have been measured on a series of oligonucleotide substrates containing both uracil and purine analogs for the E. coli mispaired uracil glycosylase, MUG. The relative rates of glycosylase cleavage have been correlated with the free energy of helix formation, and with the size and electronic inductive properties of a series of uracil 5-substituents. Data is presented that MUG can exploit the reduced thermodynamic stability of mispairs to distinguish U:A from U:G pairs. Discrimination against the removal of thymine results primarily from the electron-donating property of the thymine 5-methyl substituent, while the size of the methyl group relative to a hydrogen atom is a secondary factor. A series of parameters have been obtained that allow prediction of relative MUG cleavage rates that correlate well with observed relative rates that vary over five orders of magnitude for the series of base analogs examined. We propose that these parameters may be common among DNA glycosylases, however, specific glycosylases may focus more or less on each of the parameters identified. The presence of a series of glycosylases which focus on different lesion properties, all coexisting within the same cell, would provide a robust and partially redundant repair system necessary for the maintenance of the genome

Determination of the degree of substitution (DS) of mixed cellulose esters by elemental analysis

Equations for transforming the results of elemental analysis (EA) into degree of substitution (DS) values for aliphatic cellulose esters were developed. The equations allow to determine the DS of cellulose derivatives bearing not only one but also two different acyl substituents. Error transmission studies revealed that the accuracy of the DS-values for a pure sample depends on both the number of carbon atoms in the substituents and on the uncertainty of the EA, especially from the hydrogen content. This method provides accurate DS-values (± 0.10 in most cases) if H-content determinations are within ± 0.1

Synthesis and Characterization of Three-Coordinate Ni(III)-Imide Complexes

A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. Oxidation of nickel(II) complexes led to the formation of both aryl- and alkyl-substituted nickel(III)-imides, and examples of both types have been isolated and fully characterized. The aryl substituent that proved most useful in stabilizing the Ni(III)-imide moiety was the bulky 2,6-dimesitylphenyl. The two Ni(III)-imide compounds showed different variable-temperature magnetic properties but analogous EPR spectra at low temperatures. To account for this discrepancy, a low-spin/high-spin equilibrium was proposed to take place for the alkyl-substituted Ni(III)-imide complex. This proposal was supported by DFT calculations. DFT calculations also indicated that the unpaired electron is mostly localized on the imide nitrogen for the Ni(III) complexes. The results of reactions carried out in the presence of hydrogen donors supported the findings from DFT calculations that the adamantyl substituent was a significantly more reactive hydrogen-atom abstractor. Interestingly, the steric properties of the 2,6-dimesitylphenyl substituent are important not only in protecting the Ni═N core but also in favoring one rotamer of the resulting Ni(III)-imide, by locking the phenyl ring in a perpendicular orientation with respect to the NiPP plane

Unraveling substituent effects on frontier orbitals of conjugated molecules using an absolutely localized molecular orbital based analysis.

It is common to introduce electron-donating or electron-withdrawing substituent groups into functional conjugated molecules (such as dyes) to tune their electronic structure properties (such as frontier orbital energy levels) and photophysical properties (such as absorption and emission wavelengths). However, there lacks a generally applicable tool that can unravel the underlying interactions between orbitals from a substrate molecule and those from its substituents in modern electronic structure calculations, despite the long history of qualitative molecular orbital theory. In this work, the absolutely localized molecular orbitals (ALMO) based analysis is extended to analyze the effects of substituent groups on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of a given system. This provides a bottom-up avenue towards quantification of effects from distinct physical origins (e.g. permanent electrostatics/Pauli repulsion, mutual polarization, inter-fragment orbital mixing). For the example case of prodan (a typical dye molecule), it is found that inter-fragment orbital mixing plays a key role in narrowing the HOMO-LUMO gap of the naphthalene core. Specifically, an out-of-phase mixing of high-lying occupied orbitals on the naphthalene core and the dimethylamino group leads to an elevated HOMO, whereas an in-phase combination of LUMOs on the naphthalene core and the propionyl group lowers the LUMO energy of the entire molecule. We expect this ALMO-based analysis to bridge the gap between concepts from qualitative orbital interaction analysis and quantitative electronic structure calculations