Molecular rivet

A close relationship exists between physicochemical properties of pigment molecules and their ability to be absorbed and thus to exhibit biological functions. Carotenoids are hydrophobic molecules that require a lipophilic environment. In vivo, they are found in precise locations and orientations within biological membranes. For example, the dihydroxycarotenoids such as lutein and zeaxanthin orient themselves perpendicularly to the membrane surface as molecular rivets in order to expose their hydroxyl groups to a more polar environment. 

Carotenoids are hydrophobic molecules and thus are located in lipophilic sites of cells, such as bilayer membranes. Their hydrophobic character is decreased with an increased number of polar substitutents (mainly hydroxyl groups free or esterified with glycosides), thus affecting the positioning of the carotenoid molecule in biological membranes. For example, the dihydroxycarotenoids such as LUT and zeaxanthin (ZEA) may orient themselves perpendicular to the membrane surface as molecular rivet in order to expose their hydroxyl groups to a more polar environment. In contrast, the carotenes such as (3-C and LYC could position themselves parallel to the membrane surface to remain in a more lipophilic environment in the inner core of the bilayer membranes (Parker, 1989 Britton, 1995). Thus, carotenoid molecules can have substantial effects on the thickness, strength, and fluidity of membranes and thus affect many of their functions. 

Figure 4. Representation of the arrangement of lipid lamellae between adjacent comeocytes and role as a molecular sieve . Lipid layers are attached to adjacent comeocytes via long-chain ceramide (which act as a molecular rivet )...

Polar carotenoids rigidify the lipidmembrane in its fluid state by restrictions to different kind of molecular motions oflipid. This type ofinteraction influences mechanical properties of the membrane (reinforcement) and is consistent with the idea of a xanthophyll pigment as a molecular rivet to the membrane. 

Kolchinski, A.G. Busch, D.H. Molecular riveting High yield preparation of a [3]-rotaxane. J. Chem. Soc, Chem. Commun. 1998. 1437. 

Although most of the ceramides in the stratum bilayer appear to play a role in establishing barrier function, one stmcturally unusual ceramide appears to function as a molecular rivet. Downing and co-workers [30-32] developed this concept to explain their inability to quantitatively recover ceramides by chloroform-methanol extraction techniques without prior alkaline hydrolysis [31]. Accordingly, this unusual ceramide may link adjacent bilayers and/or provide a covalent linkage between bilayers and comeocytes. 

Fabricating and repairing complex materials systems. Mechanical methods currently in use (e g., riveting of metals) cannot be applied reliably to the composite materials of the future. Chemical methods (e g., adhesion and molecular self-assembly) will come to the fore. 

Advanced adhesives are composite liquids that can be used, for example, to join aircraft parts, thus avoiding the use of some 30,000 rivets that are heavy, are labor-intensive to install, and pose quality-control problems. Adhesives research has not involved many chemical engineers, but the generic problems include surface science, polymer rheology and thermodynamics, and molecular modeling of materials... 

Toudic B, Gallier J, Rivet P, Ddugeard Y (1985) Proton magnetic resonance study of some crystalline p-polyphenyls Evidence of rdaxation by slow molecular motions above room temperature. Chem. Phys. 99 275... 

Aiken G.R., Brown P.A., Noyes T.I., Pinckney D.J. (1989), Molecular size and weight of fulvic and humic acids from the Suwannee Rivet, in Averett R.C., Leenheer J.A., McKnight D.M., Thom K.A. (Eds.), Humic Substances in the Suwannee Rivet, Georgia Interactions, Properties, and Proposed Structures, U.S. Geological Survey, Open-File Report 87-557,163-178. 

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