Biphasic Tautomerization Dynamics of Excited 7-Hydroxyquinoline in Reverse Micelles

Abstract

The excited-state tautomerization dynamics of 7-hydroxyquinoline in the water pools of reverse micelles has been investigated by monitoring time-resolved fluorescence spectra and kinetics as well as static absorption and emission spectra with a variation of water content and isotopic fractionation. The normal and the tautomeric species are found to reside preferentially in the bound- and the free-water regions of the micelles, respectively. The excited-state tautomerization of the normal species in the bound-water layers is suggested to occur via two different channels, depending on rotamers at the moment of excitation. The cis tautomerizes via proton relay from the enol group to the imino group along a hydrogen-bonded water bridge, unusual in water but common in alcohols, whereas the trans tautomerizes via the stepwise individual acid-base reactions of two prototropic groups as found in bulk water. Proton relay can take place because water in the pools has substantially reduced polarity and disrupted hydrogen-bond networks compared with bulk water.

Anomalous Acid-Base Equilibria in Biologically Relevant Water Nanopools

“…[5][6][7][8][9][10][11][12][13] For example, the dielectric constant of a water nanopool has been reported to be much lower than that of bulk water (ε = 78. 5) 17 but similar to that of an alcohol (ε = [30][31][32][33][34][35][36][37][38][39][40].…”

“…As already mentioned above, the confinement effect and the enclosing interfacial surfaces of water nanopools are the major factors to determine the properties, such as H-bonding ability, polarity, and viscosity, of waterpools. [5][6][7][8][9][10][11][12][13] Water nanopools of AOT reverse micelles have been suggested to be alcohollike in polarity; water confined in reverse micelles has a relative dielectric constant (ε) of 30-40, which is very close to ε of methanol (33) and much smaller than ε of bulk water (78). [17][18][19] Thus, the micropolarity of water near 6HQ in reverse micelles is substantially low compared with the polarity of bulk water.…”

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Water plays a crucial role in many principal biological phenomena such as enzymatic catalysis and proton pumping through a membrane protein channel.1-4 Moreover, in biological systems, water is usually contained in a small pocket of a membrane, 5-8 and such confined water, which is generally called a water nanopool, [8][9][10] shows peculiar properties differing considerably from the properties of bulk water.

8-16The confinement effect and the enclosing interfacial surfaces of waterpools are the main factors to determine the properties, such as polarity, viscosity, and H-bonding ability, of water nanopools.5-13 For example, the dielectric constant of a water nanopool has been reported to be much lower than that of bulk water (ε = 78.5)17 but similar to that of an alcohol (ε = 30-40).18,19 On the other hand, biological processes often take place based on proton relay along a hydrogen (H)-bonded chain, [1][2][3][4][20][21][22][23][24] and the dynamics of biological proton relay is determined by the size, the structure, and the motion of a water cluster which is the prime agent in most of biological systems. 3,[13][14][15][24][25][26] Thus, for better understanding of cellular dynamics, it is necessary to investigate the properties of a biologically relevant water nanopool as a biomimetic system of water confined in a cell membrane.

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“…[1][2][3][4] Moreover, in biological systems, water is usually contained in a small pocket of a membrane, [5][6][7][8] and such confined water, which is generally called a water nanopool, [8][9][10] shows peculiar properties differing considerably from the properties of bulk water.…”

Water-wire catalysis in photoinduced acid–base reactions

“…5, the two prominent timescales are evident in the lifetime of N*. The bimodality can now be attributed to the coexistence of ground-state cis-and trans-rotamers, of which fractions are effectively frozen in S 1 due to the increased double bond character of a C-O bond, 26,27 experiencing different hydration environments. This leads to branching off tautomerization into the proton relay along the water wire in 27 AE 2 ps 38 and the acid-base reaction with independent water clusters forming the A* intermediate, whose lifetime is 160 ps, in 4.6 AE 0.5 ps.…”