Improved matrix-assisted laser desorption/ionisation sample preparation of a partially depolymerised cellulose derivative by continuous spray deposition and interfacing with size-exclusion chromatography

Continuous spray deposition (CSD) of aqueous solutions of partially depolymerised methyl cellulose was found to improve matrix-assisted laser desorption/ionisation (MALDI) sample preparation. One feature was that the sensitivity in MALDI time-of-flight mass spectrometry increased up to an order of magnitude compared with the standard sample preparation method. Another feature was that CSD provided targets for MALDI with homogeneously distributed analyte. This resulted in a more even signal intensity and a higher reproducibility than in the standard method. High-mass discrimination was more pronounced in CSD than in the standard method. Size-exclusion chromatography with aqueous eluent was coupled online to CSD onto matrix-precoated foils. The suitability for determination of the molar mass distribution of methyl cellulose was investigated.     

Coarse-graining protein energetics in sequence variables

We show that cluster expansions (CE), previously used to model solid-state materials with binary or ternary configurational disorder, can be extended to the protein design problem. We present a generalized CE framework, in which properties such as energy can be unambiguously expanded in the amino-acid sequence space. The CE coarse grains over nonsequence degrees of freedom (e.g., side-chain conformations) and thereby simplifies the problem of designing proteins, or predicting the compatibility of a sequence with a given structure, by many orders of magnitude. The CE is physically transparent, and can be evaluated through linear regression on the energies of training sequences. We show, as example, that good prediction accuracy is obtained with up to pairwise interactions for a coiled-coil backbone, and that triplet interactions are important in the energetics of a more globular zinc-finger backbone.     

The NMR spectra of porphyrins: 22—Ring current effects in chlorins versus porphyrins

The proton NMR spectra of tetraphenylporphyrin, octaethylporphyrin and the analogous chlorins (7,8-dihydroporphyrins) are presented, and the chemical shift changes on chlorin formation are interpreted using a ring current model. In these compounds a general 10% reduction in the ring current occurs upon chlorin formation. Similar comparison of the chemical shifts of the corresponding dications and also of the protonated form of 2-vinylphylloerythrin methyl ester with the corresponding chlorin, methyl pyropheophorbide-a, shows that chlorin formation now has a much larger effect on the ring current, this reflecting the increased steric effects within the macrocycle which occur upon protonation. Variable temperature studies on the porphyrins and chlorins examined show clearly the effect of NH exchange processes and, in particular, novel intermolecular exchange processes with trifluoroacetic acid in the protonated species are recorded.     

Total synthesis of (-)-nakadomarin A

A highly diastereoselective bifunctional organocatalyst controlled Michael addition, a nitro-Mannich/lactamization cascade, a furan N-acyliminium cyclisation, a sequential alkyne RCM/syn-reduction and an alkene RCM has allowed a 19 step, highly stereoselective synthesis of (-)-nakadomarin A.     

Tuning hyperfine fields in conjugated polymers for coherent organic spintronics

An appealing avenue for organic spintronics lies in direct coherent control of the spin population by means of pulsed electron spin resonance techniques. Whereas previous work has focused on the electrical detection of coherent spin dynamics, we demonstrate here the equivalence of an all-optical approach, allowing us to explore the influence of materials chemistry on the spin dynamics. We show that deuteration of the conjugated polymer side groups weakens the local hyperfine fields experienced by electron-hole pairs, thereby lowering the threshold for the resonant radiation intensity at which coherent coupling and spin beating occur. The technique is exquisitively sensitive to previously obscured material properties and offers a route to quantifying and tuning hyperfine fields in organic semiconductors.