Water structural transformation at molecular hydrophobic interfaces

Journal name:
Nature
Volume:
491,
Pages:
582–585
Date published:
DOI:
doi:10.1038/nature11570
Received
Accepted
Published online

Hydrophobic hydration is considered to have a key role in biological processes ranging from membrane formation to protein folding and ligand binding1. Historically, hydrophobic hydration shells were thought to resemble solid clathrate hydrates2, 3, 4, with solutes surrounded by polyhedral cages composed of tetrahedrally hydrogen-bonded water molecules. But more recent experimental5, 6, 7, 8 and theoretical9, 10, 11, 12, 13, 14, 15, 16 studies have challenged this view and emphasized the importance of the length scales involved. Here we report combined polarized, isotopic and temperature-dependent Raman scattering measurements with multivariate curve resolution (Raman-MCR)17, 18, 19 that explore hydrophobic hydration by mapping the vibrational spectroscopic features arising from the hydrophobic hydration shells of linear alcohols ranging from methanol to heptanol. Our data, covering the entire 0–100°C temperature range, show clear evidence that at low temperatures the hydration shells have a hydrophobically enhanced water structure with greater tetrahedral order and fewer weak hydrogen bonds than the surrounding bulk water. This structure disappears with increasing temperature and is then, for hydrophobic chains longer than ~1nm, replaced by a more disordered structure with weaker hydrogen bonds than bulk water. These observations support our current understanding of hydrophobic hydration, including the thermally induced water structural transformation that is suggestive of the hydrophobic crossover predicted to occur at lengths of ~1nm (refs 5, 9, 10, 14).

At a glance

Figures

  1. Raman spectra of aqueous n-butanol-d9.
    Figure 1: Raman spectra of aqueous n-butanol-d9.

    Shown are isotopic, polarized and SC Raman spectra at 20°C that reveal differences between hydrophobic hydration-shell and bulk water structures. a, Raman spectra of pure water (blue) and a 0.5M aqueous solution of n-butanol-d9 (green), and the corresponding SC spectrum (red) with features that arise from solute intramolecular vibrations and from hydration-shell water molecules whose vibrational spectrum is perturbed by the solute. The inset shows an expanded view of the SC OH stretch region, and the OH band of pure water scaled to the same maximum intensity. Arrows indicate hydrogen-bonded OH; the small peak on the right is the dangling OH. b, Comparison of total (H+V) and depolarized (H) SC OH spectra, revealing the polarized character of the low-frequency hydration-shell OH Raman scattering (red region) indicative of increased tetrahedral order, and the depletion of the weakly hydrogen-bonded population (blue region). c, Isotopically dilute (n-butanol-d9 in 10% HOD/D2O) SC OH spectra, confirming the depletion of weak water hydrogen bonds in the hydration shell.

  2. Effect of temperature and alcohol chain length on water structural transformation.
    Figure 2: Effect of temperature and alcohol chain length on water structural transformation.

    a, Temperature-dependent SC spectra of aqueous n-pentanol (solid curves) and the corresponding pure water Raman spectra (dashed curves), revealing the disappearance of the low-temperature structure (blue–green) and growth of the high-temperature structure (orange–red). b, c, Plots of the minimum number of perturbed hydration-shell waters against temperature (b; chain lengths: black, 3; blue, 4; green, 5; purple, 6; magenta, 7) and alcohol chain length (c; orange, 80°C; red, 100°C), revealing the strong influence of chain length on the high-temperature hydration shell structural transformation. The points in b and c each represent mean values of three or more replicate measurement and the ±30% error bars represent typical standard deviations of these replicate measurements (except for the n-heptanol results, which have a standard deviation of about ±40%).

  3. Critical size for structural transformation of hydration-shell water.
    Figure 3: Critical size for structural transformation of hydration-shell water.

    a, b, Temperature-dependent SC spectra showing that whereas the low-temperature hydration-shell structure of aqueous n-propanol disappears (a), that of aqueous n-butanol—which has a hydrocarbon chain length of ~1nm—transforms into a high-temperature structure distinct from that of bulk water structure (b). c, d, The onset of the high-temperature transformation is evident in both the depolarized (c) and isotopically dilute (d) SC spectra of aqueous n-butanol. The dashed curves in d indicate the reproducibility range of the HOD SC spectra at 100°C.

References

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  7. Buchanan, P., Aldiwan, N., Soper, A. K., Creek, J. L. & Koh, C. A. Decreased structure on dissolving methane in water. Chem. Phys. Lett. 415, 8993 (2005)
  8. Zhang, X. Y., Zhu, Y. X. & Granick, S. Hydrophobicity at a Janus interface. Science 295, 663666 (2002)
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Author information

Affiliations

  1. Purdue University, Department of Chemistry, West Lafayette, Indiana 47907, USA

    • Joel G. Davis,
    • Kamil P. Gierszal,
    • Ping Wang &
    • Dor Ben-Amotz

Contributions

J.G.D. and K.P.G. performed experimental measurements and reproducibility validations. J.G.D. further contributed to the SMCR data analysis and manuscript writing. P.W. designed and constructed the high-performance Raman system that facilitated these studies. D.B.-A. conceived and supervised the work, and contributed to the data analysis and manuscript preparation.

Competing financial interests

The authors declare no competing financial interests.

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Supplementary information

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  1. Supplementary Information (719K)

    This file contains Supplementary Figures 1-8, a Supplementary Discussion, Supplementary Methods and additional references.

Comments

  1. Report this comment #52383

    rossi andrea said:

    Cholesterol gallstones formation could be related with the hydrophobic hydration entropies changes related with the increasing biliary solutes size.
    Andrea Cariati, MD
    General Surgery, San Martino, IST, University Hospital, Genoa, Italy
    Correspondence to:
    Andrea Cariati, MD
    Via Fratelli Coda 67/5 A
    16166 Genova, Italy
    e-mail: andrea.cariati@libero.it
    Dear Editor, I read with great interest an article on water structure transformation at molecular hydrophobic interfaces (1); in particular, at figure 2 the effects of temperature and alcohol chain length have been related to water structural transformation. These observations are very useful to understand the mechanisms of cholesterol gallstones nucleation in humans that are still uncertain.
    In fact, bile is an emulsion of bile salts (6,7-6,9%), phospholipid (2,2-2,3%), cholesterol (0,4%), protein (0,4-0,5%), bilirubin (0,03%) and ions in water (90%). During the night fasting bile is concentrated in gallbladder and it is supersaturated in cholesterol, sometimes cholesterol crystals form but usually gallstones do not develop.
    Cholesterol crystals nucleate from human bile vesicles supersaturated in cholesterol. Cholesterol crystals growth have been found to occur in particular on crystals faces with imperfections such as spirals and dislocations (2). Bile is an emulsion of lipid in water at a constant temperature of nearly 36,5°C (body temperature). In human bile temperature is fixed and cannot affect water structure transformation nor the water shell on the hydrophobic lipids domains; the other variable that can influence lipid solubility in water is the hydrophobic solutes size (1). In fact, at a fixed temperature the hydrophobic hydration entropies change sign with increasing solutes size (1). The largest hydrophobic molecules in bile are cholesterol (molecular formula: C27H46O), (molar mass 386,65), (solubility in water 0,095 mg/l at 30°C) and unconjugated bilirubin (molecular formula: C33H36N4O6), (molar mass 584,66), (solubility in water at pH 8,5 is 3mmol/l and at pH 7 decrease at 1nmol/l) (3). Cholesterol supersaturate bile do not form gallstones; this means that others factors play a basic role in cholesterol gallstones nucleation. Prolonged bile storage may decrease biliary pH and activate endogenous beta glucuronidase activity favoring the formation of unconjugated bilirubin (3). The increasing biliary solute size (increase of unconjugated bilirubin) modify the hydrophobic hydration entropies and favor the precipitation of calcium bilirubin, cholesterol and mucin to form gallstones. It have been demonstrated that the 88% of cholesterol gallstones center contain visible pigment, mostly unconjugated bilirubin (Ca(HUCB)2) (2, 4). These data confirm that bilirubin glucuronide deconjugation may be an early event also in the formation of cholesterol gallstones and that, probably the persistence and the presence at higher concentration of unconjugated bilirubin in gallbladder bile is one of the main factors of the recurrence and of the failure of cholesterol gallstones oral dissolution therapy (5).
    References
    1)Davis JG, Gierszal KP, Wang P, Ben-Amotz D. Water structural transformation at molecular hydrophobic interfaces. Nature 2012; 491: 582-585
    2) Cariati A, Piromalli E. Limits and perspective of oral therapy with statins and aspirin for the prevention of symptomatic cholesterol gallstones disease. Expert Opin Pharmacother 2012; 13: 1223-1227
    3) Cariati A. Blackberry pigment (whitlokite) gallstones in uremic patient. Clin Res Hepatol Gastroenterol 2012, http://dx.doi.org/10.1016/J.clinre.2012.08.004
    4) Kaufman HS, Magnuson TH, Pitt HA, et al. The distribution of calcium salt precipitates in the core, periphery and shell of cholesterol, black pigment and brown pigment gallstones. Hepathology 1994; 19: 1124-32
    5) Cariati A, Piromalli E. Ultrastructural basis of the failure of oral dissolution therapy with bile salts and/or statin for cholesterol gallstones. Expert Opin Pharmacother 2012; 13: 1387-1388

  2. Report this comment #53054

    rossi andrea said:

    Cholesterol gallstones formation in bile. Admirand and Small triangle could be modified reporting the rules of unconjugated bilirubin and mucin.
    Running title: A new model to study in vivo and in vitro cholesterol gallstones formation.
    Andrea Cariati, MD
    General Surgery,
    San Martino, IST, University Hospital, Genoa, Italy
    Address for correspondence:
    Andrea Cariati, MD
    Via Fratelli Coda 67/5 A
    16166 Genoa, Italy
    Tel +390103724909
    E mail: andrea.cariati@libero.it
    No Conflict of interest, no fund received.
    Key words: Cholesterol gallstones, Admirand and Small triangle, unconjugated bilirubin, mucin
    Dear Editor, recent articles about lipid in water 1 and cholesterol gallstones formation and dissolution [2, 3] evidenced that the exact mechanism of cholesterol gallstones nucleation and growth is still unknown [4, 5, 6]. Also in black pigment gallstones formation the nucleation and the aggregation phases are multifactorial and involve also mucosal factors [7, 8]. In fact, cholesterol gallstones formation can be separated in two steps 9: a) when bile is supersaturated in cholesterol, cholesterol crystals can form, b) microcrystals of cholesterol, with the involvement of mucin, unconjugated bilirubin and calcium, aggregate, unite and growth to form macroscopic gallstones [2, 9]. The first step have been well studied and it is almost clear [9, 10]; the second one needs more laboratory and clinical studies to be understood [11, 12, 13, 14]. In fact, when a gallstone nidus is present, also in bile model, cholesterol can precipitate and can improve gallstones size 10. But the main question remain: how the primary nidus creates itself in bile? We have been demonstrated 2, as other authors, that calcium bilirubinate is present among cholesterol crystals and in cholesterol gallstones center; moreover, it have been shown that bilirubin mono- and diglucuronide deconjugate, at pH 7,2-7,8, spontaneously (non-enzymic hydrolysis) or can be hydrolyzed by human beta-glucoronidase at lower pH (acid) 12. In addition, it have been reported that the percentage of unconjugated bilirubin respect to the total is always over the 2% (2%-8%) in the bile of patients with gallstones and that it is less than 3% in control subjects 11. Mucin, and also other proteins, increased 18-fold, from 62 to 1100 microgram/mL in the bile of patients that underwent rapid weight loss and gallstones formation 13. Overall, mucin (at 1000 microgram/mL) and calcium stimulate cholesterol crystallization in model bile 14. For these motivations, next laboratory and clinical studies on cholesterol gallstones formation could use a modify Admirand and Small triangle model reporting also the role of unconjugated bilirubin and mucin. In this model the classic triangle is placed in a Cartesian axes system with mucin concentration (usually with a top at 1100 microgram/mL) reported on the ordinate (y) (figure 1) and with the addition of the third axe (z) (Euclidean space) to represent the percentage of unconjugated bilirubin (usually the 2-8% of the total) present in the bile system. Cholesterol, lecithin and bile salt relative percentages among cholesterol gallstones patients have been usually reported as: 5-30%; 13-26%; 61-72% respectively 9. Using this method, all the five constituent of bile (cholesterol, lecithin, biliary salt, mucin, unconjugated bilirubinate of calcium) can be well represented and are comprised in a truncated pyramid at the lower left of the Admirand and Small triangle (figure 1). The application of this model could be very useful to compare clinical and laboratory data for a better understand of cholesterol crystals aggregation and growth forming gallstones. Moreover, the extension of the triangle with the z axe, will permit us to compare all the other variable constituents of bile. In fact, for example, putting a fixed %UCB value on y axe and a fixed Calcium value on x axe, it is possible to put as a variable, the different mucin concentrations on the z axe. The fixed value on y axe is represented with the distance of the triangle from x axe and the fixed value of the second variable on the x axe is represented with the distance of the triangle from the y axe. This operation could be done putting on the z axe, in turn, all the variables that we would like to evaluate and to study as: calcium or %UCD or mucin, or other proteins, etc.
    Figure 1: Modified Admirand and Small triangle. CHOL is cholesterol percentage from 0-100%. LECIT is lecithin percentage from 0-100%. BILIARY SALT is bile salt concentration from 0-100%. On ordinate mucin is mucin concrentration (with a top at 1100 microgr/mL). On z axe UCB is the percentage of Unconjugated bilirubin respect to the conjugated one (usually 2-8). The final result is a truncated pyramid volume that contain data of the five biliary variables in patients with cholesterol gallstones.

    References
    1) Davis JG, Gierszal KP, Wang P, Ben-Amotz D. Water structural transformation at molecular hydrophobic interfaces. Nature 2012; 491: 582-582
    2) Cariati A, Piromalli E. Limits and perspective of oral therapy with statins and aspirin for the prevention of symptomatic cholesterol gallstones disease. Expert Opin Pharmacother 2012; 13: 1223-1227
    3) Ahmed MH, Hamad MA, Routh C, Connolly V. Statins as potential treatment for cholesterol gallstones: an attempt to understand the underlying mechanism of actions. Expert Opin Pharmacother 2012; 12: 2673-81
    4) Cariati A, Piromalli E. Ultrastructural basis of the failure of oral dissolution therapy with bile salts and/or statin for cholesterol gallstones. Expert Opin Pharmacother 2012; 13: 1387-8
    5) Ahmed MH, Connolly V, Hamad MA. Author?s reply: statins and cholesterol gallstones: what we know, thought we knew and hope to gain. Expert Opin Pharmacother 2012; 13: 1385-86
    6) Cariati A. Cholesterol gallstones formation could be related with the hydrophobic hydration entropies changes related with the increasing biliary solutes size. Comment on line to Davis JG, Gierszal KP, Wang P, Ben-Amotz D. Water structural transformation at molecular hydrophobic interfaces. Nature 2012; 491: 582-585 http://www.nature.com/nature/report/index.html?comment=52383&doi=10.1038/nature11570
    7) Cariati A, Piromali E. Role of parietal (gallbladder mucosa) factors in the formation of black pigment gallstones. Clin Res Hepatol Gastroenterol 2012; 36: e50-e51
    8) Cariati A. Blackberry pigment (whitlockite) gallstones in uremic patients. Clin Res Hepatol Gastroenterol 2012, in press http://dx.doi.org/10.1016/j.clinre.2012.08.004
    9) Admirand WH, Small DM. The physicochemical basis of cholesterol gallstone formation in man. J Clin Invest 1968; 47: 1043-1052
    10) Van den Berg AA, Van Buul JD, Ostrow D, Groen AK. Measurement of cholesterol gallstone growth in vitro. J Lipid Res 2000; 41: 189-194
    11) Dutt MK, Murphy GM, Thompson RPH. Unconjugated bilirubin in human bile: the nucleating factor in cholesterol cholelithiasis? J Clin Pathol 2003; 56: 596-598
    12) Spivak W, Di Venuto D, Yuey W. Non-enzymic hydrolysis of bilirubin mono- and diglucuronide to uncojugated bilirubin in model and native bile system. Biochem J 1987; 242: 323-329
    13) Shiffman ML, Sugerman HJ, Kellum JM, Moore EW. Changes in gallbladder bile composition following gallstone formation and weight reduction. Gastroenterology 1992; 103: 214 (Abstract)
    14) Van den Berg AA, Van Buul JD, Tytgat GNJ, Groen AK, Ostrow JD. Mucins and calcium phosphate precipitates additively stimulate cholesterol crystallization. J Lipid Res 1998; 39: 1744-1751

  3. Report this comment #53127

    rossi andrea said:

    Figure 1: Admirand and Small triangle modified according to Cariati: http://www.flickr.com/photos/91290231@N04/8287137279/

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