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Salvador Tomas

Publications and source records attributed to Salvador Tomas.

8 recordsLinked to original sources
PubMed2006

Accurate length control of supramolecular oligomerization: Vernier assemblies.

Linear oligomeric supramolecular assemblies of defined length have been generated using the Vernier principle. Two molecules, containing a different number (n and m) of mutually complementary binding sites, separated by the same distance, interact with each other to form an assembly of length (n x m). The assembly grows in the same way as simple supramolecular polymers, but at a molecular stop signal, when the binding sites come into register, the assembly terminates giving an oligomer of defined length. This strategy has been realized using tin and zinc porphyrin oligomers as the molecular building blocks. In the presence of isonicotinic acid, a zinc porphyrin trimer and a tin porphyrin dimer form a 3:4 triple stranded Vernier assembly six porphyrins long. The triple strand Vernier architecture introduced here adds an additional level of cooperativity, yielding a stability and selectivity that cannot be achieved via a simple Vernier approach. The assembly properties of the system were characterized using fluorescence titrations and size-exclusion chromatography (SEC). Assembly of the Vernier complex is efficient at micromolar concentrations in nonpolar solvents, and under more competitive conditions, a variety of fragmentation assemblies can be detected, allowing determination of the stability constants for this system and detailed speciation profiles to be constructed.

Isonicotinic Acids↗
PubMed2006

Polymeric vesicle permeability: a facile chemical assay.

We present a simple method to characterize vesicles and determine, at the same time, the membrane permeability to specific molecules. The method is based on encapsulating highly hydrophilic 3,3',3' '-phosphinidynetris-benzenesulfonic acid (PH) into vesicles and subsequently monitoring its reaction with 5,5'-dithiobis-2-nitrobenzoic acid (DTNB). We tested the method by measuring the membrane permeability of vesicles formed from a series of poly(ethylene oxide)-co-polybutylene oxide (EB) copolymers and egg yolk phosphatidylcholine. We found that the experimental data are in good agreement with calculations based on Fick's first law. We therefore quantified the DTNB permeability across vesicle membranes, finding that polymeric EB membranes have a more selective permeability toward polar molecules compared to phospholipids membranes.

Dithionitrobenzoic Acid↗
PubMed2006

Cooperativity in the self-assembly of porphyrin ladders.

Cooperativity is a general feature of intermolecular interactions in biomolecular systems, but there are many different facets of the phenomenon that are not well understood. Positive cooperativity stabilizes a system as progressively more interactions are added, and the origin of the beneficial free energy may be entropic or enthalpic in origin. An "enthalpic chelate effect" has been proposed to operate through structural tightening that improves all of the functional group interactions in a complex, when it is more strongly bound. Here, we present direct calorimetric evidence that no such enthalpic effects exist in the cooperative assembly of supramolecular ladder complexes composed of metalloporphyrin oligomers coordinated to bipyridine ligands. The enthalpic contributions of the individual coordination interactions are 35 kJ.mol(-1) and constant over a range of free energies of self-assembly of -35 to -111 kJ.mol(-1). In rigid well defined systems of this type, the enthalpies of individual interactions are additive, and no enthalpic cooperative effects are apparent. The implication is that in more flexible, less well defined systems such as biomolecular assemblies, the enthalpy contributions available from specific functional group interactions are well defined and constant parameters.

Calorimetry↗
PubMed2005

Tailbiter: a new amide foldamer.

Hydrogen bond directed folding of a synthetic polyamide was studied in chloroform solution, and the three-dimensional structure of the foldamer determined using 1H NMR chemical shifts.

Amides↗
PubMed2003

New building blocks for the assembly of sequence selective molecular zippers.

Synthetic H-bonded molecular zippers contain no sequence information that can be used to engineer the selective binding interactions characteristic of biopolymers; reversing the sense of the amide bonds in the two binding partners generates a new orthogonal recognition motif and the mutually complementary binding partners form complexes an order of magnitude more stable than the corresponding mismatch complexes.

Amino Acid Motifs↗
PubMed2003

Cooperativity, partially bound states, and enthalpy-entropy compensation.

Efforts to develop a quantitative understanding of molecular recognition rely on the additivity of individual intermolecular interactions, and cooperativity represents one of the major potential stumbling blocks. A chemical double-mutant cycle has been used to experimentally measure cooperativity between functional group interactions within a complex framework. The interaction between two aromatic groups varies by 0.2 +/- 0.4 kJ mol(-1) in synthetic H-bonded complexes that differ by 8-13 kJ mol(-1) in overall stability. In these systems, the free energies associated with individual intermolecular interactions can therefore be reliably treated in an additive fashion. The results suggest that alternative explanations should be considered for cooperative phenomena observed in other systems, and a rationale based on the population of partially bound states in flexible molecules is proposed to account for the enthalpic chelate effect and enthalpy-entropy compensation.

Magnetic Resonance Spectroscopy↗
PubMed2002

Chemical triple-mutant boxes for quantifying cooperativity in intermolecular interactions.

Chemical double-mutant cycles have been used to quantify intermolecular functional-group interactions in H-bonded zipper complexes in chloroform. If the same interaction is measured in zippers of different overall stability, the double-mutant cycles can be combined to produce a triple-mutant box. This construct quantifies cooperativity between the functional group interaction of interest and the other interactions that are used to change the overall stability of the complexes. The sum of two edge-to-face aromatic interactions (-2.9 +/- 0.5 kJ mol-1) is shown to be insensitive to changes of up to 13.7 +/- 0.2 kJ mol-1 in the overall stability of the complex. In principle, enthalpic cooperative effects caused by entropy-enthalpy compensation could perturb the measurement of intermolecular interactions when using the double-mutant cycle approach, but these experiments show that, for this system, the magnitude of the effect lies within the error of the measurements.

Hydrogen Bonding↗