Dynamic studies of proton diffusion in mesoscopic heterogeneous matrix

Abstract

The thin water layer, as found in chloroplast or mitochondria, is confined between low dielectric amphypathic surfaces a few nm apart.The physical properties of this mesoscopic space, and how its dimensions affect the rate of chemical reactions proceeding in it, is the subject for this study.The method selected for this purpose is time resolved fluorometry which can monitor the reversible dissociation of a proton from excited molecule of pyranine (8 hydroxy pyrene 1,3,6 tri sulfonate) trapped in thin water layers of a multilamellar vesicle made of neutral or slightly charged phospholipids.The results were analyzed by a computer program of N. Agmon (Pines, E., D. Huppert, and N. Agmon. 1988. J. Am. Chem. Soc. 88:5620-5630) that simulates the diffusion of a proton, subjected to electrostatic attraction, in a thin water layer enclosed between low affinity, proton binding surfaces. The analysis determines the diffusion coefficient of the proton, the effective dielectric constant of the water and the water accessibility of the phosphomoieties of the lipids.These parameters were measured for various lipids [egg-phosphatidylcholine (egg PC), dipalmitoyl phosphatidylcholine (DPPC), cholesterol + DPPC (1:1) and egg PC plus phosphatidyl serine (9:1)] and under varying osmotic pressure which reduces the width of the water layer down to approximately 10 approximately across.WE FOUND THAT: (a) The effective dielectric constant of the aqueous layer, depending on the lipid composition, is approximately 40. (b) The diffusion coefficient of the proton in the thin layer (30-10 approximately across) is that measured in bulk water D = 9.3 10(-5) cm(2)/s, indicating that the water retains its normal liquid state even on contact with the membrane. (c) The reactivity of the phosphomoiety, quantitated by rate of its reaction with proton, diminishes under lateral pressure which reduces the surface area per lipid.We find no evidence for abnormal dynamics of proton transfer at the lipid water interface which, by any mechanism, accelerates its diffusion.

Solvent modulation of the structural heterogeneity in FeIII myoglobin samples: a low temperature EPR investigation

“…In general, it seems that every molecular mechanism leading to a stabilization of the protein structure can also lead to a reduction in the structural heterogeneity. Moreover, while glycerol molecules insert into the water structure, in a water-sucrose mixture, clusters of bulk water are separated by sucrose molecules (Gutman et al 1992). Consequently, the freezing strain in solutions with sucrose is expected to be different from that occurring in a water-glycerol solution.…”

Protein dynamics control proton transfer from bulk solvent to protein interior: A case study with a green fluorescent protein

“…Process 1 is unlikely to be the rate‐determining step since protonation of free fluorescein (wherein the protonation site is exposed) occurs within 10 μsec (Mallik et al 2003) in contrast to EGFP protonation which occurs in ∼300 μsec. Process 2 is also an unlikely candidate since proton transfer at interfaces have been shown to occur without any major kinetic barrier (Gutman et al 1992; Maity and Krishnamoorthy 1995), and is expected to occur on the submicrosecond timescale. In fact, protonation of fluorescein covalently attached to the surfaces of proteins such as barstar and bovine serum albumin occurred within ∼10 μsec in our setup (Fig.…”

Exact solution for the geminate ABCD reaction

“…28,29 To address this question, Gutman et al have studied geminate proton-base recombination for a photoacid trapped in the interbilayer space between phospholipid membranes. 30,31 This two-dimensional geometry could also be applied to the exchange ͑ABCD͒ reaction, for which results in this section would be relevant.…”