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PubMed1978

Hydrogen-deuterium substitution and solvent effects on the nitrogen-15 nuclear magnetic resonance of gramicidin S: evaluation of secondary structure.

Complete assignments of nitrogen-15 resonances of gramicidin S have been made in dimethyl sulfoxide, trifluoroethanol, and in a solvent mixture of dimethyl sulfoxide (50%) and methanol (50%). The assignments are achieved by utilizing the secondary structure of gramicidin S, by comparing the nitrogen-15 spectrum of gramicidin S with that of di-N-methylphenylalanine-gramicidin S and by taking into account the distinguishable value of nitrogen-15 chemical shift for valine in model compounds. Deuterium substitution for labile peptide protons was performed to delineate solvent shielded and deshielded peptide nitrogens and to substantiate further the signal assignments. The solvent titration on going from dimethyl sulfoxide to trifluoroethanol was also performed and shown to have a large deshielding effect on the peptide nitrogen whose corresponding peptide carbonyl, within the peptide moiety, was accessible to the trifluroethanol solvent.

Deuterium↗
PubMed2006

Scope and mechanistic insights into the use of tetradecyl(trihexyl)phosphonium bistriflimide: a remarkably selective ionic liquid solvent for substitution reactions.

A survey of substitution reactions conducted in a phosphonium bistriflimide ionic liquid is presented. The results demonstrate high selectivity favoring substitution over typically competitive elimination and solvolytic processes even when challenging secondary and tertiary electrophiles are employed. The first reports of Kornblum substitution reactions in an ionic liquid are described that proceed with very high chemoselectivity in favor of nitro over nitroso products and elimination side products. The structure-reactivity study indicates that these reactions proceed through a narrow spectrum of pathways ranging from straight S(N)2 to a preassociation pathway along a saddle point that approaches the S(N)1 limit. The barrier to the formation of dissociated carbocations is attributed to the structural features of this ionic liquid that favor intervention of the associated nucleophile over dissociation, also preventing cross over to E1 processes. The lack of any basic entity in the phosphonium bistriflimide ionic liquid appears to prevent any potential base-mediated elimination reactions, which makes this a highly selective medium for use in general substitution reactions.

Journal Article↗
PubMed2003

Mechanism of an alcohol-trapping reaction in the direct and benzophenone-sensitized photodenitrogenation of a spiroepoxy-substituted azoalkane: solvent effects and stereochemical deuterium labeling.

The spin-state-dependent reactivity, singlet versus triplet, of the 2-spiroepoxy-1,3-cyclopentane-1,3-diyl DR2 has been assessed through alcohol-trapping reactions for which the effect of solvent acidity on the product distribution of the alcohol trapping products 2 versus 3 + 4 and stereochemical deuterium-labeling studies have been performed. The proposed mechanism for the solvent effect on the product ratio (2/3 + 4) reveals the importance of the hydrogen-bonded intermediates I1 and I2 in the trapping reactions; the stereochemical deuterium-labeling results clarify the dipole structure trapped by the alcohol. The dipoles DP1 and DP2, in which the configuration between the epoxide oxygen and the deuterium atoms is retained, are inferred for the direct photodenitrogenation reactions (singlet state), whereas for the benzophenone-sensitized photoreactions (triplet state), after ISC, the ring-opened dipole DP3 is implied as the intermediate that is trapped by the alcohol.

Journal Article↗
PubMed1981

Morphological correlations with dimensional change during SEM specimen preparation.

We have previously reported details of the dimensional changes taking place during the processing of soft tissue specimens for scanning electron microscopy. Mouse embryo limbs were used for many of these measurements and the present paper deals with the associated morphological findings. Effects of fixation, dehydration and drying are considered. Freeze drying and critical point drying of glutaraldehyde and glutaraldehyde and osmium fixed samples give perfectly acceptable results for scanning electron microscopy. The best volume retention with freeze dried material is matched by the best morphological appearance of the specimen surface, except when ice crystal damage occurs due to a failure to freeze the tissue rapidly enough. For CPD tissues, perforation of the plasmalemma may occur in glutaraldehyde-only fixed tissue, this being prevented by post-osmication if the glutaraldehyde fixation is not unduly prolonged. This perforation may be due to the extraction of some plasmalemma component during dehydration or further solvent substitution on critical point drying. Solvent evaporation drying usually causes recognizable distortion due to shrinkage: this is minimal in the case of solvent evaporation drying in a nearly saturated atmosphere of the same solvent if this is a very volatile solvent. The examples of Freon 113 and diethyl ether are given here. Swelling during early stages of ethanol dehydration can be prevented by using 70% or 100% ethanol as the first step, with marginal reduction in the post CPD volume and no apparent differences in the SEM. The severe swelling causing sample disruption which can occur with GA + OsO4 fixed tissue can also be prevented by treating the sample with divalent cations, such as Ca++ or Cu++, at any stage.

Animals↗
PubMed2026

Evaluating Substitution of Hazardous Solvents in USP Monograph HPLC Methods.

BACKGROUND: Hazardous solvents, such as dichloromethane (DCM), n-hexane, and acetonitrile (ACN), are widely used in HPLC methods, posing significant health and environmental risks. OBJECTIVE: To evaluate the feasibility and impact of substituting hazardous solvents with greener alternatives in USP monograph methods. METHODS: Six high-impact solvents were identified from USP-NF monographs. Two representative monographs per solvent were selected. Substitution strategies were assessed, and performance was compared using system suitability and sample acceptance criteria. Greenness improvement was evaluated using the Analytical GREEnness (AGREE) metric. RESULTS: Performance remained equivalent across all twelve substitution cases. In almost every instance, only the mobile phase required modification, either by direct substitution or by adjusting the solvent-to-buffer ratio, except for one case that required a minor adjustment in column temperature. The AGREE Greenness metric increased by 18-65% in ten out of twelve cases; for n-hexane, improvements were modest at just 6% when replaced with n-heptane but exceeded 40% when substituted with supercritical CO₂. CONCLUSIONS: Greener solvents are highly likely to replace hazardous solvents used in compendial chromatography methods without loss of performance. HIGHLIGHTS: Demonstrated performance equivalency for greener solvent substitutions; Quantified greenness improvements using AGREE; Discussed strategies to implement greener solvents in USP monograph methods.

HPLC↗
PubMed1986

The gap junction channel. Its aqueous nature as indicated by deuterium oxide effects.

The effects of temperature and solvent substitution with deuterium oxide (D2O) on axoplasmic (ga) and gap junctional (gj) conductances were examined in the earthworm septate median giant axon (MGA). The temperature coefficients (Q10) for ga and gj were 1.4 and 1.5, respectively, between 5 and 15 degrees C. Substitution with D2O rapidly reduced both ga and gj by 20% and increased the Q10's to 1.5 and 1.8, respectively. The reduction in ga upon substitution with D2O and with cooling in either solvent reflects the changes that occur in solvent viscosity, which indicates that ion mobility in axoplasm, as in free solution, is primarily governed by viscous properties of the solvent. The similar initial reduction observed for gj suggests that solvent occupies the gap junction channel volume and influences transjunctional ion mobility. With time there was a further reduction in gj at 20 degrees C and a larger Q10 in D2O. The enhanced effects of D2O on gj cannot be accounted for by solvent viscosity alone and may be due to an increased hydration of the channels and/or the transport ions and by isotope effects of hydrogen-deuterium exchange on the channel protein that reduce gj.

Animals↗