Excited state proton transfer in reverse micelles: Effect of temperature and a possible interplay with solvation

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Highlights

  • Excited state proton dynamics (ESPT) of d-lufiferin is studied in AOT, DDAB and Igepal reverse micelles at different temperatures.

  • ESPT propensity is high in DDAB and Ig RMs while it is very low in AOT RM.

  • ESPT dynamics is high in Ig and is low in DDAB RMs.

  • Solvation of the protonated d-luciferin precedes ESPT in DDAB and AOT, while in Ig they co-occur.

  • While both solvation and ESPT dynamics accelerates with temperature they do not necessarily complement each other.

Abstract

Excited state proton transfer (ESPT) is a fundamental process of immense biophysical interest and considering the heterogeneity existing in real biological environments we investigate the process in a bio-mimicking reverse micellar (RM) systems. We herein report a detailed study on the ESPT process of a photo-acid d-luciferin at different temperatures in RMs composed of: anionic AOT, cationic DDAB, and neutral Igepal-520 using steady state and time resolved fluorescence measurements. We found that with increasing temperature both solvation as well as the ESPT rate accelerate, however, the extent of the increase is RM specific, and they even not complement each other. Our study clearly identifies the pivotal role of solvation, specially in micro-heterogeneous environments, to guide the ESPT process.

Introduction

Excited state proton transfer (ESPT), in which, upon electronic excitation, a fluorophore photo-acid gives up a proton to the neighboring solvent to promote its anionic form [1], is a fundamental process of much chemical and biological interest [2] and it plays key roles in several processes including photosynthesis [3], green fluorescence protein [4,5], in interchanging of the inter-base H-bond network in DNA [6] and other chemical applications [[7], [8], [9], [10]]. Considering the inherent heterogeneity in real biological environments it deems informative to investigate ESPT reactions in constrained and confined environments rather than in conventional aqueous surroundings [11]. Reverse micelles (RM) offer with such an unique bio-mimicking platform to study ESPT reaction. Owing to the varied physical nature of the water encapsulated in the RM water-pool and with the additional control of tuning the physical properties of water by changing the head group charge, dissipating solvent and water to surfactant ratio (w0), considerable interest has been paid in the recent past to investigate ESPT reaction of various photoacids in RM systems [[12], [13], [14], [15], [16], [17], [18], [19], [20]]. ESPT is found to be relatively delayed in RMs compared to that in bulk solvents which manifests the bound nature of the water present in the RMs.

While most of these studies have been carried out at ambient conditions, the effect of temperature on ESPT process has been assessed only in limited attempts [[21], [22], [23], [24]]. It has been observed that temperature does play a pivotal role in determining the ESPT dynamics, however, a general comprehension has not yet been achieved. While most of temperature dependent studies reported earlier have been carried out in neat or mixed solvents, those are sparse in RM systems. In this contribution we have made a detailed investigation on the effect of temperature on ESPT dynamics of a well-known photoacid d-luciferin in three different RM systems of various charge types (anionic: AOT; cationic: DDAB and neutral: Igepal) using steady state and ps-resolved fluorescence measurements.

ESPT process of a photoacid can efficiently be probed by monitoring the fluorescence profiles of the protonated form (ROH*) and the deprotonated form (RO*) of the fluorophore. Additionally, time-resolved fluorescence measurements enable to estimate the ESPT dynamics. d-luciferin (Scheme S1) is a light-emitting compound, found in organisms that generate bioluminescence [25]. This molecule consists of two ring 6-hydroxy-benzothiazole system that undergoes an efficient excited state deprotonation process of the hydroxyl group. ESPT of d-luciferin in water is a fast process with a rate constant of 3 × 1010 s-1, however, is associated with a noticeable quenching of the deprotonated form due to an irreversible geminate recombination process [26]. This fluorescence quenching of deprotonated form is mainly attributed to the excited state protonation of the nitrogen heteroatom of the benzothiazole moiety [27]. It has been found that this ESPT process is dependent on the solvent composition [28], pH [29] and temperature [21,30,22,31,32]. From previous result it has been observed that d-luciferin is a weak acid in its ground state with pKa value of ∼7, while in excited state it turns into a much stronger acid with pKa* value approximately 0.5. [32] A few early reports suggest that ESPT of d-luciferin is mild in AOT RMs at low hydration, however, it gets favoured as the content of water increases in the RM [14]. In a recent study [19] we have explored the effect of surfactant charge type on the ESPT dynamics of d-luciferin in three different RMs composed of AOT (ionic), DDAB (cationic) and Igepal-520 (non-ionic) and their mixtures. We found that the ESPT rate could be modulated by varying the mixing ratio of the surfactant(s). As a pertaining continuation of that study in the present paper we investigate the effect of temperature on ESPT dynamics in those RM systems. We fix the surfactant concentration at 0.1 M and w0 at 10 in order to ensure all the RMs are spherical in nature [33]. Since ESPT in a micro-heterogeneous environment is believed to be an optimization between the local concentration of water and solvent relaxation [24], hydration dynamics is expected to play a pivotal role in determining the dynamics. We measure both ESPT dynamics and solvation dynamics using ps-resolved fluorescence measurements and we observe marked distinction between the two rates depending on the surfactant charge type and also on temperature.

Section snippets

Materials and methods

Sodium bis(2-ethylhexyl) sulfosuccinate (AOT), didodecyldimethylammonium bromide (DDAB), polyoxyethylene (5) nonylphenylether (Igepal-520), cyclohexane (Cy), d-luciferin and coumarin 500 (C-500) (scheme S1) were products of Sigma-Aldrich. All the chemicals were used without further purification. AOT, DDAB, and Igepal were dissolved in Cy at a concentration of 0.1 (M). Then suitable amount of water was added into it to produce the RMs of w0 = 10. DLS measurements were carried out in a Nano-S

DLS measurements

We measure the size of the RM droplets using DLS technique and the results are depicted in figure S1 (supporting information section). The droplet size follows the order DDAB > Ig > AOT and it decreases with temperature.

Steady-state fluorescence measurements

Steady-state fluorescence spectra of d-luciferin in different RMs (at 293 K) are shown in Fig. 1a. For comparison the emission spectrum in pure water is shown in the same figure. The ESPT behaviour is rather contrasting in these RMs. In bulk water we observe only one peak at

Discussions

ESPT reaction in constrained environments of RMs is contrasting when compared to that in the bulk solvents, the reason mostly being the reduced polarity [44], higher micro-viscosity [34] of the water molecules encapsulated, which in turn also makes solvation dynamics slower in RMs [11,34]. A pioneering work by Rini et al. [41] using fs-resolved IR spectroscopy revealed that ESPT process initiates through a fast solvation stage (diffusion controlled) followed by an encounter and finally a

Conclusions

ESPT of d-luciferin in three different RM systems were studied; from the steady state measurements it was found that ESPT is most prominent in DDAB followed by Ig while in AOT the formation of the deprotonated species is highly restricted, the electrostatic (in)stability of the deprotonated species rationale the steady state findings. Time resolved anisotropy measurements revealed that both the species enjoy the ease of rotational relaxation at elevated temperatures. From the time resolved

Authorship statement

All persons who meet authorship criteria are listed as authors, and all authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in any other publication before its appearance in the J. Photochemistry Photobiology

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

S.I.I. acknowledges UGC (the University Grant Commission), India, for a research grant (Sr. no. 2061510232 ; Ref. no. 21/06/2015(i)EU-V ; Roll no. 129597). The authors acknowledge S.N. Bose Centre for experimental support.

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  • Cited by (0)

    1

    Present Address: Department of Chemistry, Jadavpur University, Jadavpur, Kolkata-700032, India.

    2

    Contributed equally.

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