Probing proton-proton proximities in the solid state: high-resolution two-dimensional 1H-1H double-quantum CRAMPS NMR spectroscopy

A new 1H DQ (double-quantum) CRAMPS (combined rotation and multiple-pulse sequence) solid-state nuclear magnetic resonance experiment incorporating DUMBO homonuclear 1H dipolar decoupling is presented. The major resolution enhancement enables DQ peaks corresponding to all 22 close (<3.5 A) proton-proton proximities in the dipeptide beta-AspAla to be observed. In particular, the DQ CRAMPS spectrum provides access to the alkyl region of the spectrum and yields a clear assignment of the two CH and two diastereotopic CH2 proton resonances.     

Identification of the products of reaction between pyridoxal phosphate and amiclenomycin and other related 1-amino-cyclohexa-2,5-dienes

Amiclenomycin, a natural product containing the 1-amino cyclohexa-2,5-diene moiety is an inhibitor of 7;8-diaminopelargonic acid aminotransferase, a pyridoxal phosphate (PLP) dependent enzyme involved in biotin biosynthesis. The postulated mechanism implies the aromatisation of the Schiff base formed between PLP and amiclenomycin. Aromatic adducts have been obtained by heating PLP with amiclenomycin and other related 1-amino cyclohexa-2,5-dienes. They were fully characterized by UV-visible and ESI mass spectrometry and provide standards for identification of the enzyme-derived products.     

Transverse dephasing optimized solid-state NMR spectroscopy

It is shown how coherence lifetimes in solid-state NMR experiments can be controlled. New decoupling schemes are introduced which actively optimize dephasing times, providing increases of up to a factor of 2 with respect to the best existing schemes. The new schemes are implemented in transverse-dephasing-optimized (TDOP) NMR experiments for the disorded solid cellulose, and for a microcrystalline protein, where sensitivity improvements of up to a factor of 5 are obtained.     

Improved resolution in proton NMR spectroscopy of powdered solids

We present a new solid-state nuclear magnetic resonance experiment that yields, under CRAMPS decoupling conditions, a significant reduction in proton line widths for powdered organic solids. This experiment which relies on a constant-time acquisition of the proton transverse magnetization, removes the contribution of nonrefocusable broadening from the proton line widths. Although this new technique suffers from relatively low sensitivity, we demonstrate in this paper its feasibility on two model samples, L-alanine and the dipeptide Ala-Asp. In both cases a factor of between 2 and 3 in line width reduction is obtained for most of the proton resonances.     

Synthesis of deuterium-labeled cryptophane-A and investigation of Xe@cryptophane complexation dynamics by 1D-EXSY-NMR experiments

We present the synthesis of a series of deuterated cryptophanes 2-6 by a slightly modified procedure used for cryptophane-A. We show that for [Xe@cryptophane] complexes the use of variable-temperature one-dimensional 129Xe magnetization transfer (1D-EX-SY) allows the measurement of exchange rates. From these data the decomplexation activation energy Ea has been estimated to be 37.5+/-2 kJ mol(-1). The decomplexation activation enthalpy, deltaH(++) = 35.5+/-2 kJ mol(-1), and entropy, deltaS(++) = -60+/-5 J mol(-1) K(-1), have also been calculated. The calculated negative activation entropy suggests that the activated complex associated with decomplexation is conformationally more strained than the complex in its ground state.     

A general method for the computation of probabilities in systems of first order chemical reactions

We present a general method for the computation of molecular population distributions in a system of first-order chemical reactions. The method is based on the observation that the molecules in first-order reactions do not interact with each other. Therefore, the population distributions are a convolution of densities for one molecule. With this method one can study reactions involving any number of molecules. Such analysis is demonstrated on several examples, including an enzyme catalyst reaction and a first-order reaction chain.     

Chiral Phosphate in Rhodium‐Catalyzed Asymmetric [2+2+2] Cycloaddition: Ligand,Counterion, or Both?

Investigations based on NMR spectroscopy, mass spectrometry, and DFT calculations shed light on the metallic species generated in the rhodium‐catalyzed asymmetric [2+2+2] cycloaddition reaction between diynes and isocyanates with the chiral phosphate TRIP. The catalytic mixture comprising [{Rh(cod)Cl}₂], 1,4‐diphenylphosphinobutane (dppb), and Ag(S)‐TRIP actually gives rise to two species, both having an effect on the stereoselectivity. One is a rhodium(I) complex in which TRIP is a weakly coordinating counterion, whereas the other is a bimetallic Rh/Ag complex in which TRIP is a strongly coordinating X‐type ligand.