Liu, W., Caffrey, M., Interactions of tryptophan, tryptophan peptides and tryptophan alkyl esters at curved membrane interfaces., Biochemistry, 45, 39, 2006, 11713-11726
Biochemistry; 45; 39;
Motivated by on-going efforts to understand the mechanism of membrane protein crystallogenesis
and transport in the lipidic cubic phase, the nature of the interaction between tryptophan and the
bilayer/aqueous interface of the cubic phase has been investigated. The association was quantified
by partitioning measurements which enabled the free energy of interaction to be determined.
Temperature-dependent partitioning was used to parse the association free energy change into its
enthalpic and entropic components. As has been observed with tryptophan derivatives interacting
with glycerophospholipid bilayers in vesicles, tryptophan partitioning in the cubic phase is enthalpy
driven. This is in contrast to partitioning into apolar solvents which exhibits the classic hydrophobic
effect whose hallmark is a favorable entropy change. These results with tryptophan are somewhat
surprising given the simplicity, homogeneity and curvature of the interface that prevails in the case
of the cubic phase. Nevertheless, the interaction between tryptophan and the mesophase is very slight
as revealed by its low partition coefficient. Additional evidence in support of interaction was obtained
by electronic absorption and fluorescence spectroscopy and fluorescence quenching. Partitioning
proved insensitive to the lipid composition of the membrane, examined by doping with
glycerophospholipids. However, the interaction could be manipulated in meaningful ways by the
inclusion in the aqueous medium of salt, glycerol or urea. The effects seen with tryptophan were
amplified rationally when measurements were repeated using tryptophan alkyl esters and with
tryptophan peptides of increasing length. These findings are interpreted in the context of the insertion,
folding and function of proteins in membranes.
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