Low-temperature photoluminescence of p-nitroaniline and o-methyl-p-nitroaniline crystals

Photoemission spectra of para-nitroaniline (p-NA) and ortho-methyl-para-nitroaniline (MNA) single crystals measured between 5 and 250K revealed that the MNA crystals emit phosphorescence and fluorescence in contrast with the p-NA crystals which have only fluorescence. It is assumed that the fluorescence of p-NA crystals at 5K originates from the doublet state of negatively charged polarons while the fluorescence of MNA crystals at 60K is due to trap states. The Stokes shift between absorption and emission onsets, measured at 5K, is much larger in the p-NA crystals than in the MNA and meta-nitroaniline (m-NA) crystals. This fact is rationalized by different geometrical changes caused by excitation.     

IPr# – highly hindered,broadly applicable N-heterocyclic carbenes

IPr (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) represents the most important NHC (NHC = N-heterocyclic carbene) ligand throughout the field of homogeneous catalysis. Herein, we report the synthesis, catalytic activity, and full structural and electronic characterization of novel, sterically-bulky, easily-accessible NHC ligands based on the hash peralkylation concept, including IPr#, Np# and BIAN-IPr#. The new ligands have been commercialized in collaboration with Millipore Sigma: IPr#HCl, 915653; Np#HCl; 915912; BIAN-IPr#HCl, 916420, enabling broad access of the academic and industrial researchers to new ligands for reaction optimization and screening. In particular, the synthesis of IPr# hinges upon cost-effective, modular alkylation of aniline, an industrial chemical that is available in bulk. The generality of this approach in ligand design is demonstrated through facile synthesis of BIAN-IPr# and Np#, two ligands that differ in steric properties and N-wingtip arrangement. The broad activity in various cross-coupling reactions in an array of N–C, O–C, C–Cl, C–Br, C–S and C–H bond cross-couplings is demonstrated. The evaluation of steric, electron-donating and π-accepting properties as well as coordination chemistry to Au(i), Rh(i) and Pd(ii) is presented. Given the tremendous importance of NHC ligands in homogenous catalysis, we expect that this new class of NHCs will find rapid and widespread application.

We report novel, sterically-bulky, easily-accessible NHC ligands based on the hash peralkylation concept. The new ligands have been commercialized in collaboration with Millipore Sigma: IPr#HCl, 915653; Np#HCl; 915912; BIAN-IPr#HCl, 916420.     

Phase polymorphism of [Zn(DMSO)6](ClO4)2 studied by differential scanning calorimetry

Four solid phases of [Zn(DMSO)₆](ClO₄)₂ have been detected by differential scanning calorimetry (DSC). Specifically, the phase transitions were detected between: metastable phase KII ↔ supercooled phase K0 at , stable phase KIb ↔ stable phase KIa at , stable phase KIa ↔ stable phase K0 at . At T_m2 = 389 K crystals partially and at T_m1 = 465 K completely melts. From the entropy change values it was concluded that the phases: K0 and K0′ are the orientationally dynamically disordered phases, so called ODDIC crystals, and phases KIa, KIb and metastable KII are dynamically ordered but with some degree of positional disorder.     

Glycerol nucleoside triphosphates: synthesis and polymerase substrate activities

[Structure: see text] The synthesis of (S)-glycerol nucleoside triphosphates (gNTPs) and the analysis of their substrate activities for enzymatic polymerization is described. NTPs with simplified carbohydrate backbones such as the tNTPs (alpha-L-threose-NTPs) are polymerase substrates and offer the potential to create non-natural aptamer sequences with simplified backbones through enzymatic means. The acyclic (S)-GNA was modeled after the shortened alpha-threofuranosyl backbone. Here we describe the synthesis of (S)-glycerol NTPs and initial enzymatic testing of this further simplified nucleic acid backbone.     

Polymorphism of 2-nitroaniline studied by calorimetric (DSC), structural (X-ray diffraction) and spectroscopic (FT-IR, Raman, UV–Vis) methods

The separation and growth methods of three ortho-nitroaniline (o-NA) polymorphs were found. The irreversible character of the  → β and β → γ phase transitions was revealed by differential scanning calorimetry (DSC) measurements and microscopic hot stage observations. The X-ray structure of the β-form was determined and compared with the γ phase structure solved by Daneshwar et al. [N.N. Daneshwar et al. Acta Crystallogr., Sect. B 34 (1978) 2507]. Intramolecular hydrogen bonding (intra H-bond) interactions are dominant in both structures. The IR and Raman spectral features of the solutions and of three polycrystalline o-NA polymorphs are specific for intramolecular resonance assisted H-bonds (RAHB’s). The DFT calculations facilitated the almost complete assignments of bands to normal vibrations and the analysis of the measured spectra. The manifestations of weak inter H-bonds in the β and γ crystals and in the vibrational spectra of all polymorphs are observed as well; the strongest inter H-bonds occur in the γ polymorph. The differences in lowest electronic transition energies of three , β and γ layers explain their different colours: the yellowish-green of the form and the orange ones of the β- and γ- phases. The least stable form is probably an amorphous one with the weakest inter H-bonds. The differences in relative orientations of the –NH₂, –NO₂ groups and phenyl rings in the β- and γ-phases indicate that the o-NA polymorphism has conformational character.     

An in vitro selection system for TNA

(3'-2')-alpha-l-Threose nucleic acid (TNA) is an unnatural polymer that possesses the rare ability to base-pair with RNA, DNA, and itself. This feature, coupled with its chemical simplicity, makes TNA of interest as a possible progenitor of RNA during the early history of life. To evaluate the functional potential of TNA, we have developed a system for the in vitro selection of TNA. We identified the Therminator DNA polymerase as a remarkably efficient DNA-dependent TNA polymerase capable of polymerizing more than 50 tNTPs. We have also developed a method of covalently linking a DNA template to the TNA strand that it encodes, thus obviating the need for a TNA-dependent DNA polymerase during cycles of selection.     

Nonenzymatic, template-directed ligation of oligoribonucleotides is highly regioselective for the formation of 3'-5' phosphodiester bonds

We have found that nonenzymatic, template-directed ligation reactions of oligoribonucleotides display high selectivity for the formation of 3'-5' rather than 2'-5' phosphodiester bonds. Formation of the 3'-5'-linked product is favored regardless of the metal ion catalyst or the leaving group, and for several different ligation junction sequences. The degree of selectivity depends on the leaving group: the ratio of 3'-5'- to 2'-5'-linked products was 10-15:1 when the 5'-phosphate was activated as the imidazolide, and 60-80:1 when the 5'-phosphate was activated by the formation of a 5'-triphosphate. Comparison of oligonucleotide ligation reactions with previously characterized single nucleotide primer extension reactions suggests that the strong preference for 3'-5'-linkages in oligonucleotide ligation is primarily due to occurence of ligation within the context of an extended Watston-Crick duplex. The ability of RNA to correctly self-assemble by template-directed ligation is an intrinsic consequence of its chemical structure and need not be imposed by an external catalyst (i.e., an enzyme polymerase); RNA therefore provides a reasonable structural basis for a self-replicating system in a prebiological world.     

Evolutionary optimization of a nonbiological ATP binding protein for improved folding stability

Structural comparison of in vitro evolved proteins with biological proteins will help determine the extent to which biological proteins sample the structural diversity available in protein sequence space. We have previously isolated a family of nonbiological ATP binding proteins from an unconstrained random sequence library. One of these proteins was further optimized for high-affinity binding to ATP, but biophysical characterization proved impossible due to poor solubility. To determine if such nonbiological proteins can be optimized for improved folding stability, we performed multiple rounds of mRNA-display selection under increasingly denaturing conditions. Starting from a pool of protein variants, we evolved a population of proteins capable of binding ATP in 3 M guanidine hydrochloride. One protein was chosen for further characterization. Circular dichroism, tryptophan fluorescence, and (1)H-(15)N correlation NMR studies show that this protein has a unique folded structure.