Oligodeoxyribonucleotides (ODN) with repeats of the human telomeric sequence can adopt different tetrahelical conformations that exhibit similar energetic parameters. We studied the volumetric properties of the folded and unfolded states of an ODN with four repeats of the human telomeric sequence, d[A(GGGTTA)(3)GGG], by combining pressure-perturbation calorimetry (PPC), vibrating tube densimetry, ultrasonic velocimetry, and UV melting under high pressure. We carried out our volumetric measurements in aqueous buffers at pH 7 containing 20, 50, and 100 mM NaCl. All of the methods employed yielded volumetric parameters that were in excellent agreement. The molar volume changes, ΔV, of the conformational transition leading to formation of the folded state are large and positive. At 50 mM NaCl, the average transition volume, ΔV(tr), obtained from all the methods is 56.4 ± 3.5 cm(3) mol(-1) at the transition temperature of 47 °C, with ΔV(tr) decreasing with an increase in temperature. We carried out a molecular dynamics simulation of the change in the intrinsic geometric parameters of the ODN accompanying quadruplex formation. On the basis of the experimental and computational results, the folding transition of the ODN is accompanied by a release of 103 ± 44 water molecules from its hydration shell to the bulk. This number corresponds to ~18% of the net hydration of the coil conformation.

M4DOTA, [(2S,5S,8S,11S)-4,7,10-tris-carboxymethyl-2,5,8,11-tetramethyl-1,4,7,10-tetraazacyclododecan-1-yl]acetic acid (2e), and M4DOTMA, (R)-2-[(2S,5S,8S,11S)-4,7,10-tris-((R)-1-carboxyethyl)-2,5,8,11-tetramethyl-1,4,7,10-tetraazacyclododecan-1-yl]propionic acid (3e), are derivatives of ligand DOTA (1e) that form sterically crowded lanthanide chelates. M4DOTMA forms highly symmetric and totally rigid single Y(3+) and Yb(3+) species in which the ring substituents occupy corner positions in a square antiprismatic arrangement as shown by molecular mechanics calculations and by a quantitative interpretation of the relative magnitudes of the paramagnetic (1)H NMR shifts of dipolar origin. The NMR spectrum of YbM4DOTMA(-) displays two intense methyl peaks outside the 0-10 ppm range whose shift difference is strongly temperature dependent. YbM4DOTMA(-) (3d) could be a useful probe in magnetic resonance thermometric imaging. With only four methyl substituents on the tetraaza ring, M4DOTA forms three Yb(3+) species in solution. The methyl substituents prevent the inversion of configuration of the ethylenic groups but not of the acetate arms. Although the methyl groups are likely to preferably occupy ring corner positions, the dipolar equations do not allow one to distinguish with certainty between the two available corner (equatorial) orientations. Reliably applying the dipolar equations is less obvious than usually assumed. A single methyl substituent as in ligand MDOTA (5e) suffices to rigidify the tetraaza cycle but not the acetate arms. Racemic YbMDOTA(-) (5d) is present in solution as four totally asymmetric topomers with the methyl groups occupying either one of the two equatorial positions. A complete assignment of the solution structures on the basis of the dipolar equations is again uncertain. The nuclear magnetic relaxation dispersion curves of the Gd(3+) chelates of all the methylated DOTA ligands including DOTMA, (R)-2-[4,7,10-tris-((R)-carboxyethyl)- 1,4,7,10- tetraazacyclododecan-1-yl]propionic acid, are very similar, and intermolecular conformational processes appear to have no influence on the relaxivity of these small complexes for which the relaxation T(1) is mainly determined by the rotational correlation time (tau(r)). The hydration number of the Tb(3+) chelates measured by fluorescence decreases from DOTMA to M4DOTMA presumably because steric crowding leads to an increase of the metal-water distance.

This work describes the synthesis and the conformational properties of new polymethylated macrocyclic ligands of potential interest for magnetic resonance imaging. M4cyclen, (2S,5S,8S,11S)-2,5,8,11-tetramethyl-1,4,7,10-tetraazacyclododecane, was obtained by cyclotetramerization of (2S)-1-benzyl-2-methylaziridine followed by catalytic hydrogenation. The ligands M4DOTA, [(2S,5S,8S,11S)-4,7,10-tris-carboxymethyl-2,5,8,11-tetramethyl- 1,4,7,10-tetraazacyclododecan-1-yl]acetic acid, and M4DOTMA, (R)-2-[(2S,5S,8S,11S)-4,7,10-tris-((R)-1-carboxyethyl)-2,5,8,11-tetramethyl-1,4,7,10-tetraazacyclododecan-1-yl]propionic acid, were prepared by carboxyalkylation of M4cyclen in the presence of Na(2)CO(3). The triacetic ligand M4DO3A, [(2S,5S,8S,11S)-4,7-bis-carboxymethyl-2,5,8,11-tetramethyl-1,4,7,10-tetraazacyclododecan-1-yl]acetic acid, was obtained in good yields without traces of M4DOTA if NaHCO(3) was the acid scavenger when adding the carboxylic arms. In the same conditions, cyclen yielded M4DOTA in 82% yield. The difference between the reactivity of cyclen and M4cyclen is assigned to the high basicity of the substituted tetraamine as estimated by NMR titration. The one- and two-dimensional (1)H and (13)C NMR spectra of M4DOTA and M4DOTMA in the H(4)L or H(6)L(2+) forms are interpreted as arising from a slow exchange between two elongated geometries in which the methyl substituents are in one of the two possible equatorial-like positions, either close to or away from the carboxylic arms. The axial-like positions are sterically too crowded and cannot be occupied by the methyl groups. An elongated conformation is also adopted by DOTMA, (R)-2-[4,7,10-tris-((R)-carboxyethyl)-1,4,7,10-tetraazacyclododecan-1-yl]propionic acid, in the H(6)L(2+) form. The rigidification of the polymethylated ligands allows a detailed NMR analysis that cannot be carried out on the parent unsubstituted ligand DOTA.

The effective charge of membrane-active molecules such as the fungicidal lipopeptide surfactin (SF) is a crucial property governing solubility, membrane partitioning, and membrane permeability. We present zeta potential measurements of liposomes to measure the effective charge as well as membrane partitioning of SF by utilizing what we call an equi-activity analysis of several series of samples with different lipid concentrations. We observe an effective charge of -1.0 for SF at pH8.5 and insignificantly lower at pH7.4, illustrating that the effective charge may deviate strongly from the nominal value (-2 for 1 Asp, 1 Glu). The apparent partition coefficient decreases from roughly 100 to 20/mM with increasing membrane content of SF in agreement with the literature. Finally, by comparing zeta potentials measured soon after the addition of peptide to liposomes with those measured after a heat treatment to induce transmembrane equilibration of SF, we quantified the asymmetry of partitioning between the outer and inner leaflets. At very low concentration, SF binds exclusively to the outer leaflet. The onset of partial translocation to the inner leaflet occurs at about 5mol-% SF in the membrane. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

We present the application of pressure perturbation calorimetry (PPC) as a new method for the volumetric characterization of the micelle formation of surfactants. The evaluation is realized by a global fit of PPC curves at different surfactant concentration ranging, if possible, from below to far above the CMC. It is based on the knowledge of the temperature dependence of the CMC, which can for example be characterized by isothermal titration calorimetry. We demonstrate the new approach for decyl-β-maltopyranoside (DM). It shows a strong volume increase upon micelle formation of 16 ± 2.5 mL/mol (+4%) at 25 °C, and changes with temperature by -0.1 mL/(mol K). The apparent molar expansivity (E(S)) decreases upon micelle formation from 0.44 to 0.31 mL/(mol K) at 25 °C. Surprisingly, the temperature dependence of the expansivity of DM in solution (as compared with that of maltose) does not agree with the principal behavior described for polar (E(S)(T) decreasing) and hydrophobic (E(S)(T) increasing) solutes or moieties before. The results are discussed in terms of changes in hydration of the molecules and internal packing of the micelles and compared with the volumetric effects of transitions of proteins, DNA, lipids, and polymers.