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Letter
Water-Like Behavior of Formamide: Jump Reorientation Probed by Extended Depolarized Light Scattering Stefania Perticaroli, Lucia Comez, Paola Sassi, Assunta Morresi, Daniele Fioretto, and Marco Paolantoni J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02943 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 17, 2017
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Water-Like Behavior of Formamide: Jump Reorientation Probed by Extended Depolarized Light Scattering †
‡
S. Perticaroli, L. Comez, P. Sassi,
§,⊥
§
A Morresi, D. Fioretto,
#,⊥
§
M. Paolantoni*,
†
Shull Wollan Center, a Joint Institute for Neutron Sciences, Oak Ridge National Laboratory, Oak Ridge,
Tennessee 37831, United States ‡
#
IOM-CNR c/o Dipartimento di Fisica e Geologia and Dipartimento di Fisica e Geologia, Università degli
Studi di Perugia, Via Pascoli, I-06123 Perugia, Italy §
⊥
Dipartimento di Chimica, Biologia e Biotecnologie and Centro di Eccellenza sui Materiali Innovativi
Nanostrutturati (CEMIN), Università degli Studi di Perugia, Via Elce di Sotto 8, I-06123 Perugia, Italy
ABSTRACT: Water is a strong self-associated liquid with peculiar properties that crucially depend on H-bonding. As regards its molecular dynamics, only recently water reorientation has been successfully described based on a jump mechanism, which is responsible for the overall H-bonding exchange. Here, using high-resolution broad-band depolarized light scattering, we have investigated the reorientational dynamics of formamide (FA) as a function of concentration from the neat liquid to diluted aqueous solutions. Our main findings indicate that in the diluted regime the water rearrangement can trigger the motion of FA solute molecules, which are forced to reorient at the same rate as water. This highlights an exceptional behavior of FA, which perfectly substitutes water within its network. Besides other fundamental implications connected with the relevance of FA, its water-like behavior provides rare experimental evidence of a solute whose dynamics is completely slaved to the solvent.
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Water is ubiquitous and simple, but presents amazing properties. The main player is hydrogen bonding: the presence of an extensive H-bonding network and its rapid rearrangement are indeed considered responsible for many peculiar properties of this liquid. Despite many years of theoretical and experimental investigations on its molecular dynamics, only recently water reorientational motions have been rationalized in terms of the extended jump model (EJM).1,2 This model has led to a sound description of the molecular mechanism behind the H-bond reorganization at picosecond timescales, successfully explaining a number of spectroscopic findings.3-5 Here we demonstrate that water can induce a simple solute, such as formamide (FA), to reorient in response to its own dynamics. This provides novel insights on the mechanism of molecular reorientation and dynamics coupling which occur in structured systems formed by different species with high hydrogenbonding capabilities. This scenario has not been deeply investigated so far. In its pure liquid phase, FA is highly self-associated through directional H-bonds, whose molecular organization and dynamics have recently been compared to those of liquid water.6-10 Similarly to water, FA is a small molecule (MW 45) with a large dipole moment (3.8 D)9 that can form up to four hydrogen bonds with two H-donor NH groups and a double H-acceptor O site. In fact, liquid FA can be considered to some extent a water-like solvent, in which, due to hydrophobiclike (i.e. solvophobic) effects, surfactant self-assembly phenomena may occur.7,11 However, specific differences in their intermolecular organization8,9 and ultrafast dynamics6 have also been shown. FA and water are fully miscible, their mixing is almost ideal12 and the two species are expected to form micro-homogenous percolated networks.13 FA contains a peptide linkage and can be regarded as a simple model to explore fundamental processes in biophysics where H-bonding involving water, -CO-NH- groups and their mutual interplay, are significant.14 These include conformational fluctuations of proteins – essential for their functions – that have proven to be controlled (or slaved) by the dynamics of surrounding water.15,16 As recently pointed out, this slaving action is also responsible for the conformational rearrangement of small peptides.17 Thus, a detailed analysis of FA dynamics and its aggregation properties in aqueous solutions might be helpful to achieve a molecular understanding of a number of important biophysical phenomena. Moreover, FA is considered an important precursor of prebiotic species, relevant for research on the origin of life.18 It has recently been proposed that, due to thermophoresis and convections, FA can efficiently accumulate in hydrothermal pores leading to the formation of prebiotic nucleobases. This accumulation propensity has been connected with its hydration and selfaggregation properties,19 derived from numerical simulations.14 FA is also employed as a cellpenetrating cryoprotectant agent (CPA), to inhibit ice growth in cryoconservation procedures of biological material (cells, tissues and organs).20 Unlike other CPAs, FA does not facilitate the
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vitrification of its aqueous solutions,20 a property that might be related to the effect exerted by this molecule on water structure and mobility. In this context, we carry out Extended Depolarized Light Scattering (EDLS) experiments,2123
in order to specifically investigate the rotational dynamics of FA in water mixtures, in the entire
concentration range, looking for possible clustering and coupling effects between solvent (Hbonding) reorganization and solute motion. The capability of probing a broad spectral range with high resolution, together with the intrinsic large scattering activity of FA, allows straightforward spectral analysis and application of an almost model-free method. Our results describe a scenario in which the solute is completely slaved to water dynamics.
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Figure 1. EDLS susceptibility obtained for FA-water solutions at different mole fractions (XFA), at T=25°C (left). The main contribution peaks at 20-50 GHz and is due to the rotational dynamics of FA. Schematic 2D representation of FA-water mixtures, emphasizing the large polarizability anisotropy of FA compared to water (right). As a result, the rotational contribution of FA dominates the EDLS spectrum below ∼200 GHz in the entire concentration range (see Figure 2). Figure 1 shows EDLS susceptibilities obtained in the frequency range from 0.3 GHz to 36 THz (0.01 to 1200 cm-1) for pure FA and FA-water mixtures at different mole fractions (XFA). Details on the experimental equipment and on the spectral reduction procedure have been described
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elsewhere.24,25 The FA spectrum is dominated by an intense band at ∼ 20 GHz due to the collective rotational relaxation, while signals observed within 1-10 THz are assigned to molecular librations.26-33 The spectra depicted have been rescaled to the water libration signals in the 10-36 THz region (see Figure 2), where FA contributions only arise from rather sharp intramolecular bands peaked at 18 and 33 THz. The intensity of these bands has been employed to verify the suitability of the normalization procedure adopted. Due to the great anisotropic polarizability (γ) of FA compared to water,31 its rotational contribution dominates the spectrum below ∼200 GHz in the whole concentration range. As shown in Figure 1, the band shifts to higher frequencies with decrease in XFA, indicating acceleration of FA motion upon water addition. The relaxation time (τEDLS) of the FA collective reorientation can be estimated directly from these EDLS profiles through a simple evaluation of the peak frequency (νmax) of the main band: τEDLS = (2πνmax)-1. To eliminate solvent contributions that might be of some relevance at higher water contents, corresponding peak frequencies were determined after subtraction of the spectrum of pure water. This procedure was validated by comparing the results obtained for pure FA at different temperatures with available data resulting from recent time-resolved optical Kerr effect (TR-OKE) experiments, which represent the time-domain counterpart of EDLS ones (see Supporting Information (SI)). An example of subtraction procedure is shown in Figure 2 for the least favorable case considered (XFA=0.01). We note that, due to the dominating rotational component of FA, possible contributions arising from hydration water, commonly found below 100 GHz,22,23,25 can be safely neglected. It is worth mentioning that the optical polarizability of water is almost isotropic, such that rotational fluctuations do not contribute to the EDLS relaxation component. Hence, unlike most molecular liquids, the relaxation term in pure water (∼300 GHz) is essentially ascribed to fast (τW∼ 0.5 ps) and local intermolecular translations; these cause polarizability fluctuations through “second order” interaction-induced (II) effects with low scattering activity.31,34 As a matter of fact, ultrafast time-resolved infrared experiments that probe a 2-rank rotational relaxation time (τ2), as EDLS does, give a τ2 value of ∼2.5 ps for pure water.35 Thus a rotational component, if present, should be located at ∼ 60 GHz in the EDLS spectrum, but in fact no signatures of this contribution can be observed (Figure 2). On the other hand, an intense rotational component is evident in the dielectric spectrum of water. 36 Since, in this case, the first-rank rotational relaxation time τ1 (dipole fluctuations) is probed, the contribution is found at significantly lower frequencies (∼ 20 GHz).36
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Figure 2. EDLS susceptibility obtained for the most diluted sample (orange area) after subtracting the spectrum of pure water (gray line) from that of the solution at XFA=0.01 (green line). Experimental spectra have been normalized to the water libration signal (10-36 THz). The FA relaxation times are reported in Figure 3a as a function of XFA, together with those derived by Castner et al.28 in a former TR-OKE study. τEDLS values decrease progressively from 12 ps (XFA=1) to 3.2 ps (XFA=0.01) with the addition of water, in close agreement with MD simulation results31 and with the drop of the system viscosity.37 Remarkably, the value of τEDLS at infinite dilution, extrapolated from the water-rich concentration region (XFA
