NMR studies of active-site properties of human carbonic anhydrase II by using (15) N-labeled 4-methylimidazole as a local probe and histidine hydrogen-bond correlations.

By using a combination of liquid and solid-state NMR spectroscopy, (15) N-labeled 4-methylimidazole (4-MI) as a local probe of the environment has been studied: 1) in the polar, wet Freon CDF3 /CDF2 Cl down to 130 K, 2) in water at pH 12, and 3) in solid samples of the mutant H64A of human carbonic anhydrase II (HCA II). In the latter, the active-site His64 residue is replaced by alanine; the catalytic activity is, however, rescued by the presence of 4-MI. For the Freon solution, it is demonstrated that addition of water molecules not only catalyzes proton tautomerism but also lifts its quasidegeneracy.

How short is the strongest hydrogen bond in the proton-bound homodimers of pyridine derivatives?

Hydrogen bond geometries in the proton-bound homodimers of ortho-unsubstituted and ortho-methylsubstituted pyridine derivatives in aprotic polar solution were estimated using experimental NMR data. Within the series of homodimers studied the hydrogen bond lengths depend on the proton affinity of pyridines and--at least for the ortho-methylsubstituted pyridines--on the pKa of the conjugate acids in an approximately quadratic manner. The shortest possible hydrogen bond in the homodimers of ortho-unsubstituted pyridines is characterized by the N···N distance of 2.

Intrinsic proton-donating power of zinc-bound water in a carbonic anhydrase active site model estimated by NMR.

Using liquid-state NMR spectroscopy we have estimated the proton-donating ability of Zn-bound water in organometallic complexes designed as models for the active site of the metalloenzyme carbonic anhydrase (CA). This ability is important for the understanding of the enzyme reaction mechanism. The desired information was obtained by (1)H and (15)N NMR at 180 K of solutions of [Tp(Ph,Me)ZnOH] [1, Tp(Ph,Me) = tris(2-methyl-4-phenylpyrazolyl)hydroborate] in CD(2)Cl(2), in the absence and presence of the proton donors (C(6)F(5))(3)BOH(2) [aquatris(pentafluorophenyl)boron] and Col-H(+) (2,4,6-trimethylpyridine-H(+)).

Photoinduced proton transfer in a pyridine based polymer gel.

We describe an experimental and theoretical consideration of photoexcited proton transfer in a poly(4-vinyl pyridine)/pyridine gel. Evidence was found for two states of a multiple state process analyzed by DFT modeling. According to the latter, following irradiation at 385 nm, the proton donor is the CH group of the polymer main chain and the proton acceptor is the nitrogen of the polymeric pyridine side chain.

Counteranion-dependent mechanisms of intramolecular proton transfer in aprotic solution.

Using the freon mixture CDF(3)/CDClF(2) as solvent we have been able to measure the (1)H and (15)N NMR spectra of the doubly (15)N labeled 2,2'-bipyridinium cation (BpyH(+)) at temperatures down to 115 K. The obtained NMR parameters strongly depend on the type of counteranions indicating the formation of ion pairs. In the case of the bulky poorly coordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]-borate as the counteranion a strong intramolecular NHN hydrogen bond was observed in BpyH(+) exhibiting a degenerate intramolecular proton transfer which is of the order of 10(6) s(-1) even at 120 K.