pKa of the protonated Schiff base of visual pigments.

The pKa of bovine rhodopsin is greater than 15; that of the long-wave-length-sensitive gecko P521 pigment ranges from 8.4 to 10.5 depending on chloride concentration; and that of octopus, an invertebrate, is 10.5. These pKa values are much higher than are needed just to maintain the Schiff base in its protonated state in the photoreceptor cell. The high pKa of the Schiff base may be at least partially related to a low pKa of its counterion, which would lower the frequency of thermal isomerization of the chromophore and thus lower the dark noise in the photoreceptor cell. After light absorption, the high pKa of the protonated Schiff base of a vertebrate visual pigment must get lowered enough to allow it to deprotonate, a required step in vertebrate visual excitation. This deprotonation step is not required in invertebrate visual excitation.