E and Z Secondary Amides in Supramolecular Nanotubes and Nanospheres - Molecular Self-Assembly: Advances and Applications - page 38

[12-14]; they can also be used as drugs [15]. Hollow nanospheres
can be used either as nanocontainers in drug delivery systems or as
enzyme-like catalytic active sites [16-34].
Ifcovalentmoleculesofdiferentshapescouldbecomparedto
the letters of an alphabet, words could be assimilated to the simplest
supramolecules and further assembly of these words into sentences,
paragraphs, chapters, and ultimately topics would correspond to
supramolecules of increasing complexity as in enzymes, organelles,
cells, and living beings.
Eachlanguageisacollectionofspecificwordsthatcanbeused
according to rules, its grammar. Without good knowledge of both,
vocabulary (making of words from letters) and grammar (making
of sentences from words), one cannot expect to compose anything
but useless babbling or confusing odd sentences at best. Likewise,
molecular self-assembly into useful and functional objects is governed
by rules that must be understood. Chemists can invent as many new
building blocks or letters as they want but they are attempting to
grasp the intrinsic rules that govern their assembly. As a preliminary
endeavorwehavefocusedourresearchefortsonthe3Dcontrol
of specially designed subunits into simple supramolecular objects
like rods, tubes, spheres, and so forth[35], which could eventually
become parts of much more complex functional assemblies like
molecular machines [4-9].

2.2 Hydrogen Bonds

The hydrogen bond constitutes the gluing motif used in nature at
largeandforproteinsinparticular.Itcanobviouslybeusedefficiently
in non-covalent synthesis [37,38], although its true nature is still the
subject of numerous debates [39]. However, it is generally accepted
that several forces are included in the hydrogen bond interaction,
amongwhichareelectrostaticswithlong-rangeefects,polarization
and charge transfer whose contribution diminishes very rapidly with
distanceandfinallyisotropicvanderWaalsinteractions[40].For
optimal short distances it appears that charge transfers are likely the
maincontributor,whereaselectrostaticsstayinfluentialovermuch
longerdistances.Despitethepredominantefectofelectrostatics,
hydrogen bonds are directional both at the donor (N-H) and at the
acceptor(C=O)sides,thepreferredvaluesbeing180°and120°for