Phospholipid—Retinaldehyde Complex absorbing at 500 nm

Abstract

COMPLEXES between retinaldehyde and ethanolamine phosphoglyceride (EPG)¹ have been reported by several workers^2–5, and Daemen and Bonting⁴ have shown one of these (λ_max 450 nm in anhydrous chloroform) to be an internally protonated Schiff's base. We call this complex I. Adams² described an additional product, formed by combining retinaldehyde with EPG, the λ_max of which is 500 nm (Fig. 1). This substance, which we call complex II, is characterized here, EPG is one of the two principal phospholipids in bovine rhodopsin⁶, so such complexes may well exist in the visual system and, indeed, have been identified in retinal extracts^3,5,7. Our experimental observations indicate a direct relationship between the complex absorbing near 450 nm (I) and that absorbing at 500 nm (II). The molecular features required to form complex II are enumerated.