A fast perturbation in proton concentration can be induced in aqueous solution using a pulsed ultraviolet laser and suitable photolabile compounds which, upon photoexcitation, irreversibly release protons. The volume change and the rate constant for the reaction of the photodetached protons with proton-accepting groups in solution can be monitored using time resolved photoacoustics. A typical proton concentration jump of 1 microM can be obtained with a 200-microJ laser pulse at 308 nm. Reaction dynamics from 20 ns to 5 micros can be easily followed. The methodology we establish represents a direct, time-resolved measurement of the reaction volume in proton transfer processes and an extension to the nanosecond-microsecond range of traditional relaxation techniques, such as stopped-flow. We report example applications to reactions involving simple molecules and polypeptides.
"Time-resolved photoacoustics was used to monitor the structural response of the systems (Braslavsky and Heibel, 1992). We have recently applied this experimental methodology to proton transfer reactions in aqueous solutions, characterizing the solvation of photoinduced charges (Bonetti et al., 1997; Viappiani et al., 1998; Losi and Viappiani , 1998), the formation of water molecules from proton and hydroxide (Viappiani et al., 1998; Bonetti et al., 1997) and the reaction of protons with poly-L-lysine (Viappiani et al., 1998) and apomyoglobin (Abbruzzetti et al., 2000). Volume changes in proton transfer reactions arise from electrostrictive effects due to the changes in the net number of charges present in solution and to the alterations in the specific interactions between solute and solvent molecules (Van Eldick et al., 1989). "
[Show abstract][Hide abstract] ABSTRACT: Photoactivatable caged protons have been used to trigger proton transfer reactions in aqueous solutions of acetate, glutamate, and poly-L-glutamic acid, and the volumetric and enthalpic changes have been detected and characterized by means of time-resolved photoacoustics. Neutralization of carboxylates in aqueous solutions invariably results in an expansion of the solution due to the disappearance of two charges and is accompanied by little enthalpic change. The reactions occur with thermally activated, apparent bimolecular rates on the order of 10(10) M(-1)s(-1). In the case of aqueous solutions of poly-L-glutamic acid at pH around the pK(a) of the coil-to-helix transition, diffusional binding of a proton by carboxylates is followed by a sequential reaction with rate 1.06 (+/- 0.05) x 10(7)s(-1). This step is not thermally activated in the temperature range we have investigated and is likely related to local formation of hydrogen bonds near the protonation site. This structural event may constitute a rate-limiting step in helix propagation.
"ϭ 80 M, [GuHCl] ϭ 0.2 M. The structural volume changes have been determined by means of Eq. 3 and dividing the values by the deprotonation quantum yield 0.4 (George and Scaiano, 1980; Viappiani et al., 1998a; Pelagatti et al., 1998). Plots report ⌬V 1 / "
[Show abstract][Hide abstract] ABSTRACT: Ultrafast, laser-induced pH jump with time-resolved photoacoustic detection has been used to investigate the early protonation steps leading to the formation of the compact acid intermediate (I) of apomyoglobin (ApoMb). When ApoMb is in its native state (N) at pH 7.0, rapid acidification induced by a laser pulse leads to two parallel protonation processes. One reaction can be attributed to the binding of protons to the imidazole rings of His24 and His119. Reaction with imidazole leads to an unusually large contraction of -82 +/- 3 ml/mol, an enthalpy change of 8 +/- 1 kcal/mol, and an apparent bimolecular rate constant of (0.77 +/- 0.03) x 10(10) M(-1) s(-1). Our experiments evidence a rate-limiting step for this process at high ApoMb concentrations, characterized by a value of (0. 60 +/- 0.07) x 10(6) s(-1). The second protonation reaction at pH 7. 0 can be attributed to neutralization of carboxylate groups and is accompanied by an apparent expansion of 3.4 +/- 0.2 ml/mol, occurring with an apparent bimolecular rate constant of (1.25 +/- 0.02) x 10(11) M(-1) s(-1), and a reaction enthalpy of about 2 kcal/mol. The activation energy for the processes associated with the protonation of His24 and His119 is 16.2 +/- 0.9 kcal/mol, whereas that for the neutralization of carboxylates is 9.2 +/- 0.9 kcal/mol. At pH 4.5 ApoMb is in a partially unfolded state (I) and rapid acidification experiments evidence only the process assigned to carboxylate protonation. The unusually large contraction and the high energetic barrier observed at pH 7.0 for the protonation of the His residues suggests that the formation of the compact acid intermediate involves a rate-limiting step after protonation.
[Show abstract][Hide abstract] ABSTRACT: The molecular structural volume change (as determined by laser-induced optoacoustics), ΔVR −18 Å3, accompanying triplet state formation of free base 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphine (TSPP4-) in aqueous solutions of pH > 5.5, was markedly decreased to −5 Å3 for the dimer, to −3.5 Å3 for ZnTSPP2-, and to −4 Å3 for the nonionic micelle-included free base tetraphenyl porphyrin. For the free bases of other meso-substituted porphyrins with cationic side-groups such as 5,10,15,20-tetrakis-(4-methylaminophenyl)-porphine (TMAPP4+) and 5,10,15,20-tetrakis-(4-methylpyridyl)-porphine, similar large contractions as for monomeric TSPP4- were determined upon triplet formation, that decreased also on lowering pH to −5 Å3 for H2TSPP2- (pH 4) and to −4 Å3 for H2TMAPP6+ (pH 2.2). The triplet state quantum yield was not markedly affected by pH changes, oligomerization (mostly as a dimer), or complexation. The value of pKa = 5.1−5.2 derived from the titration of ΔVR for formation (and decay) of triplet TSPP4- is identical to that for the equilibrium of free base and monoprotonated forms, derived from fittings of the pH-dependent absorption and fluorescence data. For TMAPP4+ the pKa = 3.5 from titration of the contraction (triplet formation) coincided with that for the transition mono-/diprotonated species, whereas upon triplet decay the expansion showed a pKa = 4.0 similar to the value for monoprotonated/free-base equilibrium. The contraction upon triplet formation in the free base porphyrins mainly originates in the rearrangement of water around the excited macrocycle nitrogen atoms and to a minor extent in a contraction due to bonds shortening upon excitation. The contribution of the chromophore-solvent interactions is thus reduced upon impairment of hydrogen bridges between the nitrogen lone electron pairs and water by dimer formation, metal complexation, and protonation.
Journal of the American Chemical Society 10/1999; 121(45). DOI:10.1021/ja9913885 · 12.11 Impact Factor
Data provided are for informational purposes only. Although carefully collected, accuracy cannot be guaranteed. The impact factor represents a rough estimation of the journal's impact factor and does not reflect the actual current impact factor. Publisher conditions are provided by RoMEO. Differing provisions from the publisher's actual policy or licence agreement may be applicable.