NMR-STUDY OF KINETIC HH HD DH DD ISOTOPE EFFECTS ON THE TAUTOMERISM OF ACETYLPORPHYRIN - EVIDENCE FOR A STEPWISE DOUBLE PROTON-TRANSFER

The kinetic HH/HD/DH/DD isotope effects of an intramolecular reversibl e nondegenerate double proton transfer reaction are described. The mol ecule studied is l)ethyl]-2,7,12,18-tetramethyl-(21H,23H)-porphyrin (a cetylporphyrin, ACP) dissolved in CD2Cl2. As a thermodynamic and kinet ic method we used dynamic H-1 and C-13 NMR spectroscopy of suitably H- 2, N-15, and C-13 labeled ACP. The N-15 label was introduced at the 21 , 23, and 24 positions, the C-13 label at the 6- and 7-methyl position s, and H-2 at the central proton sites, respectively. The syntheses of these compounds are reported. ACP exists in two tautomeric forms of d ifferent energy which interconvert rapidly at room temperature and slo wly at 200 K with respect to the NMR time scale. The tautomer with a p roton located on the acetylpyrrole ring has the higher energy. The equ ilibrium constant for interconversion of the tautomers is given by K = 1.14 x exp(-5.82 kJ mol 1/RT). Equilibrium isotope effects on the tau tomerism of ACP are not observed within the margin of error. Whereas t he tautomerism of ACP-HH and of ACP-DD can each be described by one ra te constant, we find two different rate constants for ACP-HD: one for the HD and one for the DH reaction, respectively. In the HD reaction, an H atom jumps to the acetyl-substituted pyrrole ring; in the DH reac tion, the D atom migrates. We obtain the following kinetic results: k( HH) = 10(10.4) exp(-40 kJ mol-1/RT), 230 K less-than-or-equal-to T les s-than-or-equal-to 327 K, k(HH)(298) congruent-to 2840 s-1, k(HD) = 10 (11.3) exp(-52 kJ mol-1/RT), 266 K less-than-or-equal-to T less-than-o r-equal-to 311 K, k(HD)(298) congruent-to 180 s-1, k(DH) = 10(10.0) ex p(-41 kJ mol-1/RT), 252 K less-than-or-equal-to T less-than-or-equal-t o 312 K, k(DH)(298) congruent-to 670 s-1, k(DD) = 10(11.5) exp(-53.5 k J mol-1/RT), 273 K less-than-or-equal-to T less-than-or-equal-to 380 K , k(DD)(298) congruent-to 150 s-1, with the kinetic isotope effects of k(HH)/k(HD) congruent-to 16, k(HH)/k(DH) congruent-to 4, k(HH)/k(DD) congruent-to 19, k(HD)/k(DD) congruent-to 1.2, k(DH)/k(DD) congruent-t o 4.5 at 298 K. These results are modeled in terms of a stepwise proto n transfer. The HD and the DD reaction are characterized by similar ra te constants because in both cases, a deuterium is transferred in the rate-determining step. The implications of these results to other chem ically and biologically relevant multiple proton-transfer systems are discussed.