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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