Rhodopsin

From the Greek ῥόδον(rose) and ὄψις(view) is a reddish pigment contained in retinal rods, which is also called retinal purple or visual purple on which depends the mechanism of twilight and night vision. 

It consists of a simple protein and a prosthetic group, the retinal, aldehyde form of vitamin A that, with the stimulus of light, breaks down and causes the nervous impulse. More in detail, this protein is a membrane protein with 7 domains transmembrane to α-helical, is mainly found in the stick cells of the human retina that allow black and white vision. These cells have an elongated shape and in their apical part they have several membrane discs with many rhodopsins, formed by a pigment, the 11-cis-retinal, sensitive to the light, bound to the opsin, a protein of the retina. In the retinal rods we also have numerous channels for the Na+ open in dark conditions, because in the cell there are high concentrations of cGMP which keep the channels open. When the light interacts with the 11-cis-retinal, it catalyzes its conversion to all-trans-retinal, which determines a conformational change in the "opsin", transforming it into an active opsin. This activated protein activates a second protein, a Gt protein, transducin, which in turn activates a specific GTP Phosphodiesterase, which splits cGMP into GMP and causes a lowering of cGMP concentration: the channels for Na+ along the membrane of the cell stick close, resulting in a change in membrane potential.

According to recent studies, anthocyanins stimulate the regeneration of rhodopsin in the rods of the frog's retina.¹ 

DOI: 10.3390/molecules24183311

Notes:

1 Therapeutic Effects of Anthocyanins for Vision and Eye Health. 

Nomi Y, Iwasaki-Kurashige K, Matsumoto H.