The<unk> s<unk> 1 and<unk> 2) are chemical derivatives of the dietary supplement vitamin A (see retinol) formed through oxidation reactions.<unk> 1 is better known as retinaldehyde or simply retinal and is fundamental in the transduction of light into visual signals in the photoreceptor level of the retina (known as the visual cycle).<unk> 2 is more formally known as dehydroretinaldehyde.
The energy of impinging photons will convert retinaldehyde from an 11-cis isomer into an all-trans form.  In the retina, this conversion induces a conformational change in the surrounding photopsin protein pigment, leading to signaling through the G protein transducin. Retinaldehyde also forms a part of bacteriorhodopsin, a light-induced proton pump found in some archaea.
Experimentally, it is possible to replace 11-cis retinaldehyde by perfusing retinal tissue preparations with retinaldehyde derivatives.  Selective modification of the retinaldehyde structure, particularly the density of electrons in the π-orbitals, can lead to insights into the interaction between the retinaldehyde moiety and the surrounding pigment protein.
Retinene
Pyritinol
Gedocarnil
Retinol
Bioallethrin
Adapalene
Tetrahydrozoline
Epoetin