Proton counterion-facilitated

Dynamic nuclear polarisation (DNP) enhanced 15N CP MAS NMR has been exploited by Mark-Jurkauskas et al.79 in the studies of intermediates of the bacteriorhodopsin photocycle. The data for L intermediate were similar to those found for 13-ds,15-anti retylidene chloride, while those for K intermediate were similar to those of acid blue bacteriorhodopsin in which the Schiff base counterion was neutralised (Table 3). The 15N chemical shifts observed have shown that for bacteriorhodopsin, the Schiff base in K intermediate state loses contact with its counterion and establishes a new one in L intermediate state. The proton energy stored at the beginning in the electrostatic modes has been transformed to torsional modes. The transfer of energy is facilitated by the reduction of bond order alternation in the polyene chain when the counterion interaction is initially broken and is driven by the attraction of the Schiff base to a new counterion. 3D CP MAS experiments of NCOCX, NCACX, CONCA and CAN(CO)CA types have been used in studies of proteorhodopsin.71... 

The authors finish by predicting a quantity that has not yet been measured, the secondary KIE that would be obtained with -ND2 in place of -NI Q as the reactive base. They also note the critical role of the Mg counterions in facilitating the reaction. The binding to the two cations of the phosphoenolpyruvate tetraanion compared to the reactant trianion is predicted to be increased by some 240 kcal mol this offsets the highly unfavorable pKa of the C-2 proton sufficiently to render the overall reaction thermochemistry only 2.8 kcal mol endergonic. 

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