A Water-soluble Polylysine-Retinaldehyde Schiff Base
نویسندگان
چکیده
In order to improve the existing models of retinalprotein Schiff bases, a water-soluble polylysine-retinaldehyde imine has been synthesized and its stability assessed under a variety of conditions through changes in the visible absorption spectrum. The compound absorbs at 342 nm and consists of a 90-kDa poly-L-lysine containing a retinal Schiff base in about 2% of the lysyl c-amino ends. Retinal is mostly in the all-tram form; under no conditions is more than 15% of the 13-cis isomer detected. The absorption maximum exhibits a pa-dependent reversible shift to 402 nm, with an apparent PK. = 3.4. In the presence of the anionic surfactant sodium dodecyl sulfate, this PK. is shifted to z8.9, probably because of electric neutralization of lysyl c-amino groups. Other detergents (cetyltrimethylammonium bromide, Triton X-100) do not modify the Schiff base pK,, but rather promote its hydrolysis; in this case detergents act in the same way as certain solvent mixtures, by providing an amphiphillic environment to the imine that in turn stabilizes the products of hydrolysis. Our results suggest that once the surfactant reaches the Schiff base, preferential partition of retinal into detergent micelles is the main factor facilitating imine bond breakdown. The response of our synthetic Schiff base to changes in pH or solvent polarity point together to an important role of the supporting polypeptide in providing a suitable environment to the chromophore.
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