The formation of a highly stable inclusion complex between 2,7-dimethyldiazapyrenium (Me(2)DAP(2+)) and the cucurbit[8]uril host (CB8) was demonstrated by X-ray crystallography; MALDI-TOF mass spectrometry; and (1)H NMR, electronic absorption, and emission spectroscopy. The equilibrium association constant was determined to be 8.9(+/-0.2)x10(5) L mol(-1) from UV-visible data and 8.4(+/-1.5) x 10(5) L mol(-1) from fluorescence data. The Me(2)DAP(2+).CB8 inclusion complex acted as a host to bind compounds containing aromatic pi-donor moieties (D), such as catechol and dopamine. This point was demonstrated by (1)H NMR spectroscopy, and electrochemical and emission measurements. Fluorescence detection of the Me(2)DAP(2+).D.CB8 ternary complexes was evident in aqueous solution and on the surface of silica particles, to which fluorescent diazapyrenium units had been covalently immobilized.

Collagens are integral structural proteins in animal tissues and play key functional roles in cellular modulation. We sought to discover collagen model peptides (CMPs) that would form triple helices and self-assemble into supramolecular fibrils exhibiting collagen-like biological activity without preorganizing the peptide chains by covalent linkages. This challenging objective was accomplished by placing aromatic groups on the ends of a representative 30-mer CMP, (GPO)10, as with L-phenylalanine and L-pentafluorophenylalanine in 32-mer 1a. Computational studies on homologous 29-mers 1a-d (one less GPO), as pairs of triple helices interacting head-to-tail, yielded stabilization energies in the order 1a > 1b > 1c > 1d, supporting the hypothesis that hydrophobic aromatic groups can drive CMP self-assembly. Peptides 1a-d were studied comparatively relative to structural properties and ability to stimulate human platelets. Although each 32-mer formed stable triple helices (CD) spectroscopy, only 1a and 1b self-assembled into micrometer-scale fibrils. Light microscopy images for 1a depicted long collagen-like fibrils, whereas images for 1d did not. Atomic force microscopy topographical images indicated that 1a and 1b self-organize into microfibrillar species, whereas 1c and 1d do not. Peptides 1a and 1b induced the aggregation of human blood platelets with a potency similar to type I collagen, whereas 1c was much less effective, and 1d was inactive (EC50 potency: 1a/1b Ͼ Ͼ 1c > 1d). Thus, 1a and 1b spontaneously self-assemble into thrombogenic collagen-mimetic materials because of hydrophobic aromatic interactions provided by the special end-groups. These findings have important implications for the design of biofunctional CMPs.biomaterial ͉ platelets ͉ structure-function ͉ supramolecular triplex T he self-association of peptides and proteins into well ordered supramolecular structures is of pivotal importance in normal physiology and pathophysiology, such as in the assembly of collagen fibrils (1), actin filaments (2), and amyloid fibrils (3, 4). Collagens, which constitute a ubiquitous protein family in animals, contribute an essential matrix component to soft tissues and bones (5, 6). A structural hallmark of many collagens is a rope-like triple helix, the architecture of which derives from the interplay of three proline-rich polypeptide strands (e.g., two ␣1 and one ␣2 for type I collagen) (6-8). In the core domain of the triple helix, the amino acid sequence G-X-Y is repeated multiple times, and each glycine amide NH forms a hydrogen bond with the X-position amide carbonyl on an adjacent strand. The X-and Y-positions are often populated by L-proline and 4(R)-hydroxy-L-proline (O; Hyp), respectively, with the latter stabilizing the triple helix by stereoelectronic effects (9) and water-bridged hydrogen bonds (10).To investigate collagen's structure and function, researchers have resorted to using synthetic collagen model peptides (CMPs)

Collagen, the most abundant protein in mammals, has fascinated scientists because of its extraordinary structural features and biological importance. In 1961, Rich and Crick suggested that collagen possesses a triple-helical structure, 1 which was later confirmed by X-ray analysis of 30-mer collagen-related peptides (CRPs), such as (Pro-Hyp-Gly) 4 Pro-Hyp-Ala-(Pro-Hyp-Gly) 5 and (Pro-Pro-Gly) 10 . 2 The large triple-helical domains of collagen consist of three peptide strands with Gly-X-Y repeating motifs, 300 nm in length (vs ∼9 nm for CRPs), with the X and Y mainly populated by Pro and Hyp, respectively. In addition, there are short telopeptide regions at the N-and C-termini, which are important for fibril assembly. 3 The rigidity of the ropelike super-helix and the assembled fibril helps provide mechanical strength to tissues, such as skin, tendons, ligaments, and blood vessels. Following vascular injury, the exposed collagen in the vessel wall promotes tissue repair by activating platelets for aggregation and adhesion. However, excessive platelet activity, such as from rupture of an atherosclerotic plaque, can lead to pathological thrombosis with attendant arterial obstruction, as in myocardial infarction or stroke. 4 The study of the structure, stability, and function of collagen triple helices has been facilitated by the use of synthetic collagen model peptides. 5 Although oligomerized CRPs, via dendrimer assembly or covalent crosslinking, can effectively induce platelet aggregation, less organized CRPs have lacked this property. 6 We sought to identify a single-stranded CRP that could spontaneously self-assemble into a bioactive material. Recently, two research groups obtained micrometer-scale CRP-based materials from the self-assembly of covalently attached triple-stranded entities by employing a cysteine knot. 7 We report on the design, synthesis, and characterization of novel single-stranded CRP 1a (Figure 1a), which self-assembles by noncovalent interactions into a triplehelical, micrometer-scale, composite fibrillar substance with noteworthy biological activity.The requirement of the telopeptide regions of collagen, 3 and specific Tyr and Phe residues within the C-terminal telopeptide chain, 8 suggests the importance of aromatic residues in the "code" for collagen self-assembly. 8 Inspired by nature, we sought to explore short, single-stranded CRPs that possess the main repeat-unit of collagen, Gly-Pro-Hyp, and contain aromatic recognition units at the N-and C-termini. Phenyl and pentafluorophenyl end-groups were considered, given the strong noncovalent aromatic-stacking interaction between benzene and hexafluorobenzene. 9 In this regard, 32-mer peptide 1a, with pentafluorophenylalanine (F 5 -Phe) and Phe residues at the N-and C-termini, respectively, was of interest. We speculated that interstrand aromatic-stacking and ordered hydro-

We have identified a strategy to communicate a chemical signal between two independent molecular components. One of them is a photoactive merocyanine that switches to a spiropyran, releasing a proton, when stimulated with visible light. The other is a 4,4'-pyridylpyridinium monocation that captures the released proton, producing an electroactive 4,4'-bipyridinium dication. Under the irradiation conditions employed, the photoinduced transformation requires ca. 15 min to reach a photostationary state. In the dark, the ensemble of communicating molecules reequilibrates to the original state in ca. 5 days. These processes can be monitored following the photoinduced enhancement and thermal decay, respectively, of the current for the monolectronic reduction of the 4,4'-bipyridinium dication. The pronounced difference in time scale for the current enhancement and decay steps can be exploited to implement a memory element with a bit retention time of 11 h. A bit of information can be written optically in the chemical system and it can be read electrically and nondestructively. The memory can be reset, extending its permanence in the dark beyond the bit retention time. A binary logic analysis of the signal transduction operated by the communicating molecules reveals the characteristic behavior of sequential logic operators, which are the basic components of digital memories.