The thermodynamics of proton dissociation of adenine

The thermodynamic quantities associated with ionization of the N₁ and N₉ protons of adenine have been calorimetrically determined as a function of temperature. The H values for proton dissociation of these groups, with pK values of 4.19 and 9.92, were found to be 5.1 and 9.1 kcal/mole, respectively, at 25°C, =0.025. The C _p values for proton dissociation of these groups were estimated to be –11 and –17 cal/mole-deg. These results indicate that the large heat capacity changes observed during conformational transitions of polynucleotides are not the result of ionization of the bases.     

MO-LCAO-Calculations on polymethines. V. PPP-type calculations and configuration analyses of simple prototype dyes in terms of molecular subsystems

In order to elucidate certain controversies in interpreting the π-electronic structure of some simple quinone and indigo dyes the Pariser–Parr–Pople SC _β,γ-wave-functions have been subjected to configuration analyses. Whereas 2,5-diamino-quinone (1) can be excellently represented by coupling of two trimethine-merocyanine chains, the analogous consideration is less appropriate with bispyrrolindigo (2). In this case the results of the configuration analyses indicate clearly the limited applicability of the Longuet/Higgins–Murrell–method.     

Substituted γ-lactones: On the structural elucidation of the reaction products from 4-aroyl-3-hydroxy-2(5H)-furanones and 1,2-diamines

The reaction pathway of 4-aroyl-3-hydroxy-2(5H)-furanones 1 with diamines depends on the nature of the amine as well as on the applied reaction conditions. Thus, the reaction of 1a-d with 5,6-diamino-1,3-dimethyluracil 5 led to the formation of two isomeric Schiff bases 7a-d and 8a-d . Conversely type 1 compounds reacted with 4,5-diaminopyrimidine 9 or 2,3-diaminopyridine 10 to form the mono acid-base adducts 11a and 11b respectively. When type 1 compounds were reacted with aliphatic diamines 13a-d or p-phenylenediamine and p-xylenediamine, respectively also an immediate formation of acid-base adducts 15a-f was observed. The reaction of a number of O-methylated type 1 compounds with 1,2-ethylenediamine afforded the novel seven-membered ring compounds 18a-d in good yields. The analogous reaction of O-alkylated 1a with o-phenylenediamine 2 or 2,3-diaminonaphthalene gave the expected tricyclic ring systems 19 or 20 .     

A facile synthesis of novel triazoloquinoxahnones and triazinoquinoxalinones

A series of 1-aryl-s-triazolo[4,3-a]quinoxalin-4-ones, 3 , were synthesized via the pyrolysis of the corresponding hydrazones, 6 . Thus, the cyclodehydrogenation occurred by refluxing them in an inert solvent (e.g. ethylene glycol) to give the triazoloquinoxalin-4-ones in a satisfactory yield. Using DMSO as a solvent for the above transformation afforded as a minor by-product an S-ylid. In contrast to earlier findings, annelation of a six-membered ring was successful and achieved through the pyrolysis of the pyruvate hydrazones derived of the quinoxalin-4-ones at ?230° to give the as-triazino[4,3-a]quinoxalin-5-ones, 4 . The reaction of 5 with acetylacetone afforded 3-(3′,5′-dimethylpyrazol-1-yl)-2(1H)-quinoxalinone, 10 . The structural assignments for the new compounds were based on their elemental analysis and spectroscopic data as well as an independent synthesis.     

Pericyclic and Heteroelectrocyclic Mechanisms for the Cyclization of 1,3,5-Hexatrien-1-one and Its 6-Aza Analog

The cyclization of 1,3,5-hexatrien-1-one, 1, and the Z- and E-isomers of 1-aza-1,3-butadienylketene 3 were studied using the semiempirical AM1 and PM3 methods. Cyclizations of compounds 1 and Z-3 are shown to occur via a mono-rotation mechanism with barriers of 15.49 and 32.85 kcal/mol respectively. The reactions proceed via non-planar transition states which result from rotation of the methylene group for compound 1 and the imino group for compound Z-3. Cyclization of E-3 proceeds via a non-rotatory mechanism through a planar transition state. The activation barrier is 4.83 kcal/mol (AM1). The electronic structures of the initial and final states, and of some intermediate structures, including the transition states for the cyclization of compounds 1 and 3, were analyzed by the natural orbital method using HF/6-31G*//AM1 calculations. Energetic, structural, and orbital criteria indicate the heteroelectric mechanism for the cyclization of compound E-3 and the pericyclic mechanism for the cyclization of compounds 1 and Z-3.     

Dibenzotetraaza crown ethers: a new family of crown ethers based on o-phenylenediamine

Dibenzotetraaza (DBTA) crown ethers possess two o-phenylenediamine moieties. They are homologues of dibenzo crown ether phase-transfer catalysts and were prepared from the condensation of benzimidazoles with oligo(ethyleneglycol) dichlorides and oligo(ethyleneglycol) ditosylates. Compounds with ring sizes ranging from 18-crown-6 to 42-crown-14 were prepared. In addition, various altered benzimidizoles were used to produce DBTA crown ethers with modified substituents and ether bridges, as well as benzimidazolidine crown ethers. The synthetic approach presented here proved to be a convenient route to a new family of crown ethers with overall yields of up to 48% based on the benzimidazole. Yields for the ring-closing step were generally high, ranging from 51% to 94%, without the need for high-dilution conditions. Reaction of the DBTA crown ethers with alkyl and benzyl halides was found to be a facile way to obtain the corresponding tetra(N-organyl) compounds. Picrate extraction studies were carried out to determine phase-transfer catalytic capabilities. Extraction efficiencies for alkali-metal ions were lower than those for dibenzo-18-crown-6. Efficiencies were higher for other metal ions, with some selectivity for Pb(2+). Tetra(N-methyl) DBTA-18-crown-6 generally exhibited higher extraction efficiencies than its N-H analogue, but the selectivity was lower.     

ZIF-8 degrades in cell media,serum, and some—but not all—common laboratory buffers

ABSTRACT

Drug delivery using metal-organic frameworks (MOF) has elicited interest in their biocompatibility; however, few studies have been conducted on their stability in common buffers, cell media, and blood proteins. In particular, the use of ZIF-8, a MOF interconnected by Zn and methylimidazole, has been frequently employed. In this study, we tested single crystals of ZIF-8 with common laboratory buffers, cell media, and serum, and noted several issues. Buffers containing phosphate and bicarbonate alter the appearance and composition of ZIF-8; however, these buffers do not appear to cause cargo to leak out even when the ZIF-8 itself is displaced by phosphates. On the other hand, serum dissolves ZIF-8, causing premature cargo release. Our results show that ZIF-8 undergoes surface chemistry changes that may affect the interpretation of cellular uptake and cargo release data. On the other hand, it provides a rational explanation as to how ZIF-8 neatly dissolves in vivo.     

Gaucher disease-associated glucocerebrosidases show mutation-dependent chemical chaperoning profiles

Gaucher disease is a lysosomal storage disorder caused by deficient glucocerebrosidase activity. We have previously shown that the cellular activity of the most common Gaucher disease-associated glucocerebrosidase variant, N370S, is increased when patient-derived cells are cultured with the chemical chaperone N-nonyl-deoxynojirimycin. Chemical chaperones stabilize proteins against misfolding, enabling their trafficking from the endoplasmic reticulum. Herein, the generality of this therapeutic strategy is evaluated with other glucocerebrosidase variants and with additional candidate chemical chaperones. Improved chemical chaperones are identified for N370S glucocerebrosidase. Moreover, we demonstrate that G202R, a glucocerebrosidase variant that is known to be retained in the endoplasmic reticulum, is also amenable to chemical chaperoning. The L444P variant is not chaperoned by any of the active site-directed molecules tested, likely because this mutation destabilizes a domain distinct from the catalytic domain.