A variety of natural and synthetic compounds are known to selfassemble to give transmembrane ion channels. 1 Hydrogen-bonded macrocycles that can π-stack are a new type of channel motif. 2 Thus, folate quartets stack to give ion channels in lipid bilayers. 3 This folate assembly had a single-channel conductance of 10-20 picosiemens (pS), values consistent with the quartet's 3 Å diameter. We found that a noncovalent assembly of 16 guanosine monomers could be cross-linked to give a "unimolecular" G-quadruplex that can transport Na + across lipid membranes. 4 We now report that the ditopic guanosine-lithocholate 1 forms discrete channels in phospholipid membranes (Figure 1). These pores are large (nS conductance) and stable, with "open" times of seconds, distinguishing them from most synthetic channels, which typically conduct in the pS range with millisecond lifetimes. 1,5 Lehn and Barboiu have independently shown that ditopic monomers with guanine end groups form supramolecular polymers in cation-templated processes. 6 Possible supramolecular structures built from these ion-templated G 4 -quartets are depicted in Figure 2. In addition to the G 4 -quartet channel, such structures might well stack to form pores for transmembrane transport. Of relevance was Barboiu's demonstration that Na + and K + could be transported across films made from G 4 -quartet polymers. 6b The nucleoside-sterol conjugate 1 has two guanosine groups connected by a bis-lithocholate linker. This spacer was inspired by Kobuke's studies that showed that bis-cholic acid derivatives formed cation-selective channels with pS conductance. 7,8 We envisioned that membrane insertion of 1, followed by formation of G 4 -quartets, might well provide functional pores (Figure 2).Compound 1 was made by coupling 2′,3′-tBDMSi-5′-amino G, 9 with a bis-lithocholic acid. Compound 2, with -NMe amide end groups, was a control. The 1 H NMR spectrum of 1 gave sharp peaks in DMSO-d 6 , a polar solvent that inhibits self-assembly mediated by hydrogen bonding. In contrast, the NMR spectrum of 1 in CDCl 3 gave much broader signals, consistent with self-association in this "poor" solvent.We used CD spectroscopy to gain evidence that 1 forms stacked G 4 -quartets in a nonpolar solvent. 10 Figure 3 shows CD spectra for samples of 1 in CHCl 3 . The CD spectrum of 1 (blue) was taken after isolation from a silica gel. This sample showed a weak Cotton band in the 200-280 nm region, suggesting some stacked G 4 -quartets. 10 We added [2.2.2]-cryptand to ensure that any adventitious cations bound by 1 were sequestered. Indeed, the resulting spectrum (green trace) was inactive. We next stirred the mixture of 1 and [2.2.2]-cryptand in the presence of excess K + 2,6-dinitrophenolate (DNP). The CD spectrum of ditopic 1 was much different after extraction of K + DNP -(red trace). This sample showed a CD signature diagnostic for stacked G-quartets, with a positive band at λ ) 266 nm and a negative peak at λ ) 240 nm. 10 The complex formed by 1 and K + also showed a strong Cotton ba...

Phosphonate cavitands are an emerging class of synthetic receptors for supramolecular sensing. The molecular recognition properties of the third-generation tetraphosphonate cavitands toward alcohols and water at the gas-solid interface have been evaluated by means of three complementary techniques and compared to those of the parent mono- and diphosphonate cavitands. The combined use of ESI-MS and X-ray crystallography defined precisely the host-guest association at the interface in terms of type, number, strength, and geometry of interactions. Quartz crystal microbalance (QCM) measurements then validated the predictive value of such information for sensing applications. The importance of energetically equivalent multiple interactions on sensor selectivity and sensitivity has been demonstrated by comparing the molecular recognition properties of tetraphosphonate cavitands with those of their mono- and diphosphonate counterparts.

A Si(100) surface featuring molecular recognition properties was obtained by covalent functionalization with a tetraphosphonate cavitand (Tiiii), able to complex positively charged species. Tiiii cavitand was grafted onto the Si by photochemical hydrosilylation together with 1-octene as a spatial spectator. The recognition properties of the Si-Tiiii surface were demonstrated through two independent analytical techniques, namely XPS and fluorescence spectroscopy, during the course of reversible complexation-guest exchange-decomplexation cycles with specifically designed ammonium and pyridinium salts. Control experiments employing a Si(100) surface functionalized with a structurally similar, but complexation inactive, tetrathiophosphonate cavitand (TSiiii) demonstrated no recognition events. This provides evidence for the complexation properties of the Si-Tiiii surface, ruling out the possibility of nonspecific interactions between the substrate and the guests. The residual Si-O(-) terminations on the surface replace the guests' original counterions, thus stabilizing the complex ion pairs. These results represent a further step toward the control of self-assembly of complex supramolecular architectures on surfaces.