Cryptophanes bearing OCH(2)COOH groups in place of the methoxy groups represent a new class of xenon-carrier molecules soluble in water at biological pH. By using (1)H and (129)Xe NMR (thermally- and laser-polarized dissolved gas), the structural and dynamical behaviors of these host molecules as well as their interaction with xenon are studied. They are shown to exist in aqueous solution under different conformations in very slow exchange. A saddle form present for one of these conformations could explain the (1)H NMR spectra. Whereas the cryptophanes in such a conformation are unable to complex xenon, unprecedented high binding constants are found for cryptophanes in the other canonical crown-crown conformation. These host molecules could therefore be valuable candidates for biosensing using (129)Xe MRI.

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Nanofabrication by molecular self-assembly involves the design of molecules and self-assembly strategies so that shape and chemical complementarities drive the units to organize spontaneously into the desired structures. The power of self-assembly makes it the ubiquitous strategy of living organized matter and provides a powerful tool to chemists. However, a challenging issue in the self-assembly of complex supramolecular structures is to understand how kinetically efficient pathways emerge from the multitude of possible transition states and routes. Unfortunately, very few systems provide an intelligible structure and formation mechanism on which new models can be developed. Here, we elucidate the molecular and supramolecular self-assembly mechanism of synthetic octapeptide into nanotubes in equilibrium conditions. Their complex hierarchical self-assembly has recently been described at the mesoscopic level, and we show now that this system uniquely exhibits three assembly stages and three intermediates: (i) a peptide dimer is evidenced by both analytical centrifugation and NMR translational diffusion experiments; (ii) an open ribbon and (iii) an unstable helical ribbon are both visualized by transmission electron microscopy and characterized by small angle X-ray scattering. Interestingly, the structural features of two stable intermediates are related to the final nanotube organization as they set, respectively, the nanotube wall thickness and the final wall curvature radius. We propose that a specific self-assembly pathway is selected by the existence of such preorganized and stable intermediates so that a unique final molecular organization is kinetically favored. Our findings suggests that the rational design of oligopeptides can encode both molecular- and macro-scale morphological characteristics of their higher-order assemblies, thus opening the way to ultrahigh resolution peptide scaffold engineering.

The known xenon-binding (±)-cryptophane-111 (1) has been functionalized with six [(η(5)-C(5)Me(5))Ru(II)](+) ([Cp*Ru](+)) moieties to give, in 89% yield, the first water-soluble cryptophane-111 derivative, namely [(Cp*Ru)(6)1]Cl(6) ([2]Cl(6)). [2]Cl(6) exhibits a very high affinity for xenon in water, with a binding constant of 2.9(2) × 10(4) M(-1) as measured by hyperpolarized (129)Xe NMR spectroscopy. The (129)Xe NMR chemical shift of the aqueous Xe@[2](6+) species (308 ppm) resonates over 275 ppm downfield of the parent Xe@1 species in (CDCl(2))(2) and greatly broadens the practical (129)Xe NMR chemical shift range made available by xenon-binding molecular hosts. Single crystal structures of [2][CF(3)SO(3)](6)·xsolvent and 0.75H(2)O@1·2CHCl(3) reveal the ability of the cryptophane-111 core to adapt its conformation to guests.

Nonspecific lipid transfer protein from wheat is studied by liquid-state NMR in the presence of xenon. The gas-protein interaction is indicated by the dependence of the protein proton chemical shifts on the xenon pressure and formally confirmed by the first observation of magnetization transfer from laser-polarized xenon to the protein protons. Twenty-six heteronuclear nOes have allowed the characterization of four interaction sites inside the wheat ns-LTP cavity. Their locations are in agreement with the variations of the chemical shifts under xenon pressure and with solvation simulations. The richness of the information obtained by the noble gas with a nuclear polarization multiplied by ∼12,000 makes this approach based on dipolar cross-relaxation with laser-polarized xenon promising for probing protein hydrophobic pockets at ambient pressure.Keywords: Laser-polarized xenon; SPINOE; wheat nonspecific lipid transfer protein; protein hydrophobic cavity Supplemental material: See www.proteinscience.org.The catalytic sites of many enzymes are located in hydrophobic pockets. The content of these cavities in the absence of substrate is still subject to debate because the presence of water molecules may play a fundamental role in the thermodynamics of the enzymatic activity (Ernst et al. 1995;Matthews et al. 1995;Quillin et al. 2000). Currently, only two physical methods allow direct characterization of these hydrophobic cavities at the atomic level: i) X-ray diffraction of a protein crystal under noble gas pressure (xenon, krypton,. . .; Montet et al. 1997;Prangé et al. 1998;Quillin et al. 2000). The disadvantages of this method are that mediumto-high pressures must be used, and the dynamic properties of the interaction cannot easily be understood; ii) hydration studies using liquid-state 1 H NMR, which involve solventprotein protons cross-relaxation. These measurements can be delicate because exchange between water and hydroxyl protons can mask the through-space interactions between the solvent and the protein protons (Otting et al. 1991). A variation of this method is the use of small organic molecules that can probe cavities. Then the dependence of the internuclear distances on the intermolecular cross-relaxation rates between the protons of the protein and of the organic compound can be exploited (Otting et al. 1997). However, as these nOe signals are usually proportional to the concentration of the small organic compounds (fast-exchange conditions), improving the sensitivity of this approach requires very high pressures (e.g., 200 bars of methane). This may induce structural modifications of the protein (Prangé et al.Reprint requests to: Dr. Hervé Desvaux, Service de Chimie Moléculaire, CEA/Saclay, F-91191 Gif sur Yvette, France; e-mail: hdesvaux@Cea.fr; fax: 33-1-69-08-98-06.Abbreviations: FID, Free Induction Decay; Ns-LTP, nonspecific lipid transfer protein; nOe, nuclear Overhauser effect; NOESY, nuclear Overhauser effect spectroscopy; TOCSY, total correlation spectroscopy; SPI-NOE, spin polarization-...

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