Water and life are closely linked. Liquid water is required for life to start and for life to continue. Moreover, water has a number of anomalous properties which have been related to the water’s role in the chemistry of life [1]. Water molecules are “connected” by a weak bond, and the strangeness of water is a consequence of the extensive three-dimensional hydrogen bonding of water molecules to one another. There are two main hypotheses [2] concerning the hydrogen bonding of liquid water that divide water science: water forms (i) a continuum three dimensional network of (more or less) distorted H-bonding around a near-tetrahedral local structure from one side, and (ii) the mixture of two distinct competing structural motifs, namely a low-density water (LDW) and a high-density water (HDW), on the other side. Although the water anomalies are more evident in the supercooled region, recent experimental evidences at ambient conditions, where most of important biological processes involving water occur, have been reported, as: • the formation of massive ‘exclusion zone’ (EZ), where solutes are excluded, next to various hydrophilic surface [3]; • the observation by X-ray spectroscopy of two distinct, inter-converting structural species whose ratio depends on temperature [4]; • the observation by dielectric spectroscopy measurements of a Debye-like slow relaxation, associated to structural and/or dynamical inhomogeneities on length scales as large as 0.1 mm [5]. With the aim of using new experimental techniques that can provide different and complementary information, in this work water structuring has been investigated by measuring the time-resolved photo-induced Delayed Luminescence (DL) from suitable aqueous solutions. Indeed: • DL intensity has been correlated to the dimension of the ordered structures where it has been measured [6]; • the DL phenomenon in biological systems has been be connected with the formation and dissociation of non-linear coherent self-trapped (localised) electron states (solitons and electrosolitons [7]. • the water structure (formation of a dynamic quasi-lattice) and its polymorphism have been used to explain intrinsic luminescence of water [8]. In Literature it is reported that the water ordering effect can be enhanced by adding suitable solute. In aqueous salt solutions, in fact, addition of a solute to water causes a displacement of the HDW/LDW equilibrium because ions partition differently between contiguous micro-domains [9]. On the other hand, viscous liquid has showed effects that indicate the presence of structure for a quite long period. In this respect, the SOL–GEL technique based on a tetraethoxysilane (TEOS) precursor leads to the formation of a three dimensional silica network at or near room temperature and water elimination. Moreover, a recurring hypothesis in experimental and computational studies is that cryoprotectants, as glycerol, act by modifying water structure giving rise to the formation of nano-clusters of water surrounded by the matrix of solute molecules [10]. So we performed DL measurements from aqueous solutions of some salts [11, 12], TEOS [13] and glycerol [14], also on varying the concentration and the temperature, by using the dedicated equipment for fast ultraweak luminescence analysis ARETUSA developed at the Laboratori Nazionali del Sud (LNS-INFN), in Catania [11-14]. The measurements of DL showed: • Evidence of a significant DL signal only for the aqueous solutions for which LDW clusters formation is foreseen, suggesting that LDW domains should be longer-lasting and/or larger and/or more ordered than HDW ones. • The probability distribution function of decay time presents a maximum in the microsecond range for all the solutions, letting to suppose that LDW domains have lifetime of the same order of magnitude. This surprising result appears to be the first experimental evidence of so long lasting lifetimes of water's structures. • The findings are correlated with experimental [3,4] and theoretical [10, 15] studies from other authors. So, changes in the water structuring could be used as a simple tool to store and transfer the energy that is harnessed in biochemical processes. On the other hand the many physical and chemical properties of water, that have a crucial and unique role for life as we know it, are related to such low density structuring. In this respect, the possibility to explore the water structuring by using DL measurements could constitute a powerful tool of investigation of life’s processes. References [1] Chaplin M. Water structure and science. London South Bank University. Available from: http://www.lsbu.ac.uk/water. [2] Editorial. Debated waters. Nature Mat 13: 663 (2014) [3] Zheng J-M, Chin W-C, Khijniak E, Khijniak E Jr, Pollack GH. Surfaces and interfacial water: evidence that hydrophilic surfaces have long-range impact. Adv Colloid Interface Sci 127: 19–27 (2006) [4] Huang C, Wikfeldt KT, Tokushima T, Nordlund D, Harada Y, Bergmann U, et al. The inhomogeneous structure of water at ambient conditions. PNAS 106: 15214-8 (2009) [5] Jansson H, Bergman R, Swenson J. Hidden Slow Dynamics in Water. Phys Rev Lett. 104: 017802 (2010) [6] Scordino A, Triglia A, Musumeci F. Analogous features of delayed luminescence from Acetabularia acetabulum and some solid state systems. J Photochem Photobiol B. 56: 181-186 (2000) [7] Brizhik L, Scordino A, Triglia A, Musumeci F. Delayed luminescence of biological systems arising from correlated many-soliton states. Phys Rev E 64: 031902 (2001) [8] Lobyshev VI, Shikhlinskaya RE, Ryzhikov BD. Experimental evidence for intrinsic luminescence of water. J Mol Liq. 82: 73-81 (1999) [9] . Wiggins PM. High and low density water in gels. Prog Polym Sci 20: 1121-1163 (1995) [10] Towey JJ, Dougan L. Structural Examination of the Impact of Glycerol on Water Structure. J Phys Chem B. 116: 1633−1641 (2012) [11] Gulino M, Grasso R, Lanzanò L, Scordino A, Triglia A, Tudisco S, Musumeci F. Lifetime of Low-Density Water domains in salt solutions by time-resolved Delayed Luminescence. Chem Phys Lett 497: 99-102 (2010) [12] Musumeci F, Grasso R, Lanzanò L, Scordino A, Triglia A, Tudisco S, Gulino M. Delayed luminiscence: a novel technique to get new insights into water structure. J Biol Phys. 38: 181-195 (2012) [13] Colleoni C, Esposito S, Grasso R, Gulino M, Musumeci F, Romeli D, Rosace G, Salesi G, Scordino A. Delayed luminescence induced by complex domains in water and in TEOS aqueous solutions. Phys Chem Chem Phys 18: 772-780 (2016) [14] Grasso R, Musumeci F, Gulino M, Scordino A - Exploring the behaviour of water in glycerol solutions by using Delayed Luminescence. PLoS ONE 13: e0191861 (2018) [15] Liu X, Lu W-C, Wang CZ, and Ho KM. Energetic and fragmentation stability of water clusters (H2O)n, n=2–30. Chem Phys Lett 508: 270-275 (2011)

Water structuring in aqueous solutions investigated by delayed luminescence

Agata SCORDINO
;
Rosaria GRASSO;Francesco MUSUMECI;Antonio TRIGLIA
2018

Abstract

Water and life are closely linked. Liquid water is required for life to start and for life to continue. Moreover, water has a number of anomalous properties which have been related to the water’s role in the chemistry of life [1]. Water molecules are “connected” by a weak bond, and the strangeness of water is a consequence of the extensive three-dimensional hydrogen bonding of water molecules to one another. There are two main hypotheses [2] concerning the hydrogen bonding of liquid water that divide water science: water forms (i) a continuum three dimensional network of (more or less) distorted H-bonding around a near-tetrahedral local structure from one side, and (ii) the mixture of two distinct competing structural motifs, namely a low-density water (LDW) and a high-density water (HDW), on the other side. Although the water anomalies are more evident in the supercooled region, recent experimental evidences at ambient conditions, where most of important biological processes involving water occur, have been reported, as: • the formation of massive ‘exclusion zone’ (EZ), where solutes are excluded, next to various hydrophilic surface [3]; • the observation by X-ray spectroscopy of two distinct, inter-converting structural species whose ratio depends on temperature [4]; • the observation by dielectric spectroscopy measurements of a Debye-like slow relaxation, associated to structural and/or dynamical inhomogeneities on length scales as large as 0.1 mm [5]. With the aim of using new experimental techniques that can provide different and complementary information, in this work water structuring has been investigated by measuring the time-resolved photo-induced Delayed Luminescence (DL) from suitable aqueous solutions. Indeed: • DL intensity has been correlated to the dimension of the ordered structures where it has been measured [6]; • the DL phenomenon in biological systems has been be connected with the formation and dissociation of non-linear coherent self-trapped (localised) electron states (solitons and electrosolitons [7]. • the water structure (formation of a dynamic quasi-lattice) and its polymorphism have been used to explain intrinsic luminescence of water [8]. In Literature it is reported that the water ordering effect can be enhanced by adding suitable solute. In aqueous salt solutions, in fact, addition of a solute to water causes a displacement of the HDW/LDW equilibrium because ions partition differently between contiguous micro-domains [9]. On the other hand, viscous liquid has showed effects that indicate the presence of structure for a quite long period. In this respect, the SOL–GEL technique based on a tetraethoxysilane (TEOS) precursor leads to the formation of a three dimensional silica network at or near room temperature and water elimination. Moreover, a recurring hypothesis in experimental and computational studies is that cryoprotectants, as glycerol, act by modifying water structure giving rise to the formation of nano-clusters of water surrounded by the matrix of solute molecules [10]. So we performed DL measurements from aqueous solutions of some salts [11, 12], TEOS [13] and glycerol [14], also on varying the concentration and the temperature, by using the dedicated equipment for fast ultraweak luminescence analysis ARETUSA developed at the Laboratori Nazionali del Sud (LNS-INFN), in Catania [11-14]. The measurements of DL showed: • Evidence of a significant DL signal only for the aqueous solutions for which LDW clusters formation is foreseen, suggesting that LDW domains should be longer-lasting and/or larger and/or more ordered than HDW ones. • The probability distribution function of decay time presents a maximum in the microsecond range for all the solutions, letting to suppose that LDW domains have lifetime of the same order of magnitude. This surprising result appears to be the first experimental evidence of so long lasting lifetimes of water's structures. • The findings are correlated with experimental [3,4] and theoretical [10, 15] studies from other authors. So, changes in the water structuring could be used as a simple tool to store and transfer the energy that is harnessed in biochemical processes. On the other hand the many physical and chemical properties of water, that have a crucial and unique role for life as we know it, are related to such low density structuring. In this respect, the possibility to explore the water structuring by using DL measurements could constitute a powerful tool of investigation of life’s processes. References [1] Chaplin M. Water structure and science. London South Bank University. Available from: http://www.lsbu.ac.uk/water. [2] Editorial. Debated waters. Nature Mat 13: 663 (2014) [3] Zheng J-M, Chin W-C, Khijniak E, Khijniak E Jr, Pollack GH. Surfaces and interfacial water: evidence that hydrophilic surfaces have long-range impact. Adv Colloid Interface Sci 127: 19–27 (2006) [4] Huang C, Wikfeldt KT, Tokushima T, Nordlund D, Harada Y, Bergmann U, et al. The inhomogeneous structure of water at ambient conditions. PNAS 106: 15214-8 (2009) [5] Jansson H, Bergman R, Swenson J. Hidden Slow Dynamics in Water. Phys Rev Lett. 104: 017802 (2010) [6] Scordino A, Triglia A, Musumeci F. Analogous features of delayed luminescence from Acetabularia acetabulum and some solid state systems. J Photochem Photobiol B. 56: 181-186 (2000) [7] Brizhik L, Scordino A, Triglia A, Musumeci F. Delayed luminescence of biological systems arising from correlated many-soliton states. Phys Rev E 64: 031902 (2001) [8] Lobyshev VI, Shikhlinskaya RE, Ryzhikov BD. Experimental evidence for intrinsic luminescence of water. J Mol Liq. 82: 73-81 (1999) [9] . Wiggins PM. High and low density water in gels. Prog Polym Sci 20: 1121-1163 (1995) [10] Towey JJ, Dougan L. Structural Examination of the Impact of Glycerol on Water Structure. J Phys Chem B. 116: 1633−1641 (2012) [11] Gulino M, Grasso R, Lanzanò L, Scordino A, Triglia A, Tudisco S, Musumeci F. Lifetime of Low-Density Water domains in salt solutions by time-resolved Delayed Luminescence. Chem Phys Lett 497: 99-102 (2010) [12] Musumeci F, Grasso R, Lanzanò L, Scordino A, Triglia A, Tudisco S, Gulino M. Delayed luminiscence: a novel technique to get new insights into water structure. J Biol Phys. 38: 181-195 (2012) [13] Colleoni C, Esposito S, Grasso R, Gulino M, Musumeci F, Romeli D, Rosace G, Salesi G, Scordino A. Delayed luminescence induced by complex domains in water and in TEOS aqueous solutions. Phys Chem Chem Phys 18: 772-780 (2016) [14] Grasso R, Musumeci F, Gulino M, Scordino A - Exploring the behaviour of water in glycerol solutions by using Delayed Luminescence. PLoS ONE 13: e0191861 (2018) [15] Liu X, Lu W-C, Wang CZ, and Ho KM. Energetic and fragmentation stability of water clusters (H2O)n, n=2–30. Chem Phys Lett 508: 270-275 (2011)
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