3-Hydroxy Retinal, a New Chromophore Identified in Insect Eyes: HPLC Separation and NMR Spectroscopic Identification of the Oxime Forms

11 -eis Retinal ((1) see Fig. 1) and its 3,4-dehydro derivative (11-ds A2-retinal, (2)) were believed to be the sole chromophores in the visual pigments. Recently, another retinal compound — 3-hydroxy retinal (3) — was identified as the functional chromophore in the eyes of several related insect orders [1—3]. It was demonstrated that the adaptation of 3-hydroxy retinal as visual chromophore provides an advantage, since it allows — e.g. in the flies — the additional use of the alcohol compound, 3-hydroxy retinol (4), as antenna pigment [2]. The hydroxy group at position 3 of the cyclohexyl ring renders the molecule much more polar than retinal or a substitution at position 2 or 4. Such increased polarity requires long separation times and makes a complete separation of the isomers of 3-hydroxy retinal very difficult. The unambiguous identification of