The gecko visual pigment: a pH indicator with a salt effect
Authors: Crescitelli Frederick 
Published: December 2014

1. Unlike rhodopsin, the extracted 521‐pigment of the Tokay gecko (Gekko gekko) is pH‐sensitive and changes its spectral absorbance in the pH range of 4.5‐7.3. The colour change is reversible and pH can be employed to adjust the spectral maximum anywhere between 490 nm and its native location at 521 nm.

2. The hypsochromic shift with increasing acidity is opposite to that expected for the protonation of the Schiff base nitrogen and suggests an action on the secondary system of interacting charges that have long been postulated to adjust vertebrate visual pigment colour within the visible spectrum.

3. Chloride ions modulate this pH effect in a systematic and significant manner. For the pigment extracted in the chloride‐deficient state the colour change occurs in the pH range of 6.0‐7.0, the midpoint being close to 6.5, suggesting the possible participation of the imidazole group of histidine as the functional moiety. With added NaCl the colour shifts to the region below pH 6.2.

4. The modulating action of chloride is postulated to be a conformational change of the opsin leading to a shift of the secondary interacting site from one functional group to another or else to a change in pK of a single group due to the conformational alteration of the electrostatics of the system.

5. At pH values between 7.5 and 9.0 a different mechanism becomes apparent. In this region a decrease occurs in the photopigment density as well as a shift in absorbance toward the blue. This alkaline effect is readily reversed either by adding NaCl or else by lowering the pH. Along with the other protective effects of chloride these ions serve to reduce or prevent this alkaline loss in density.

6. Associated with this reversible photopigment loss is a reversible appearance of a product with a maximum at about 366 nm. The spectrum of this product is like that produced by the addition of 11‐cis retinal to the extract. Acidification of the alkaline preparation leads to a restitution of the photopigment as well as to a reduction of the 366‐product.

7. Addition of hydroxylamine to the alkaline extract in appropriate concentration inhibits the restitution of pigment‐521 with acid or NaCl, but adding 11‐cis retinal to the system leads to restoration of the photopigment after acidification. All the evidence suggests that product‐366 is either free 11‐cis retinal or else held to the opsin in a form that does not alter its spectral absorbance. The alkaline effect is therefore a disruption of the aldimine bond of the visual pigment.

8. In many respects the gecko 521‐pigment behaves like the chicken cone pigment, iodopsin, suggesting that an investigation of the latter in terms of pH may be a worthy project for future study.

9. With its ability to change colour with pH, with chloride, with nitrate, etc. the extractable gecko pigment offers possibilities for the investigation of mechanisms responsible for adjusting visual pigment absorbance throughout the visible spectrum. The techniques of circular dichroism, Raman spectroscopy, infra‐red spectroscopy, etc. may find here a suitable material for these studies.