Schiff base counterion distance

The structural information on such a short distance between the Schiff base and the counterion has led to confusion among researchers. This is because many researchers believed that a water molecule should be present between the Schiff base and the coimterion, as in the case of bacteriorhodopsin. In general, pKa of a Schiff base is higher than that of a glutamate, so that a proton on the Schiff base should be transferred to the glutamate if these two molecules interact directly with each other. Moreover, there is ample evidence that a water molecule is present and forms a hydrogen-bonding network near the Schiff base and the counterion. - Therefore, a water molecule was speculated to be located between the Schiff... 

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