On Press:
An Al(i)pha(tic) Helix?
By Jason Socrates
Bardi
The alpha helix is a common enough structural motif in nature,
but how about the aliphatic helix? Can aliphatic hydrocarbons
be made to fold into helices in solution?
Conventional wisdom holds that purely hydrophobic compounds
like alkanes and other aliphatic hydrocarbons do not make
good candidates for helical folding, which help partially
hydrophobic structures such as proteins exist in an aqueous
environment.
In fact, the opposite is true—alkanes, which are chains
of carbon atoms connected by single bonds and decorated with
hydrogen atoms, tend to extend to their full length.
Alkanes constitute a class of compounds that, at room temperature,
include: colorless gasses, like the stove-burner methane (CH4);
clear liquids, like the organic solvent hexane (C6H14), which
is used to extract essential oils from plant seeds; and waxy
solid compounds with longer carbon chains, like paraffins,
which contain more than 16 carbon atoms in their chains. These
diverse chemicals are all completely insoluble in water and
could share the designation, least likely to twist into
a helix.
But a stunning exception was described in the journal Science
last month in a report by Laurent Trembleau, who is a former
research associate at The Scripps Research Institute (TSRI),
and TSRI Professor Julius Rebek, Jr., who is director of the
Skaggs Institute for Chemical Biology.
In their paper, Trembleau and Rebek describe using nuclear
magnetic resonance to observe the helical folding of the alkyl
end of the common detergent sodium dodecyl sulphate (SDS).
Trembleau and Rebek observed how this alkane-like end of the
SDS molecule folded into a helix in order to achieve a better
fit into a synthetic receptor called a "cavitand." The cavitand
is a chemical that naturally forms a cavity-shape into which
other molecules can bind.
Interestingly enough, four carboxylates on the cavitand
make a water-soluble surface on the outside and eight benzene
rings make a hydrophobic surface on the inside. Into this
hydrophobic cavity, Trembleau and Rebek found, the alkyl chains
of the SDS, coiled into helices.
The helix forms because the coiled alkyl chain makes favorable
interactions with the benzene rings and because the coiled
alkane chain fills the space of the cavity more efficiently.
Also favoring the coiling is the fact that the hexane molecules
shed water on their surface as they coil—a situation
Rebek says is analogous to wringing dry a wet towel by twisting
it.
To read the article, "Helical Conformation of Alkanes in
a Hydrophobic Cavitand" by Laurent Trembleau and Julius Rebek,
Jr., please see the August 29, 2003 issue of the journal Science
(301, 1219-1220) or go to: http://www.sciencemag.org/cgi/content/abstract/301/5637/1219.
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