Adsorption of Explosive Molecules on Human Hair Surfaces - The

Jul 25, 2007 - Jimmie C. Oxley , James L. Smith , Louis J. Kirschenbaum , Suvarna Marimiganti , Irena Efremenko , Raya Zach , Yehuda Zeiri. Journal of...
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J. Phys. Chem. C 2007, 111, 11903-11911

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Adsorption of Explosive Molecules on Human Hair Surfaces Irena Efremenko,† Raya Zach,‡ and Yehuda Zeiri*,‡,§ Department of Organic Chemistry, Weizmann Institute of Science, 76100 RehoVot, Israel, Department of Biomedical Engineering, Ben Gurion UniVersity, Beer SheVa 84105, Israel, and Department of Chemistry, NRCN, P.O. Box 9001 Beer SheVa, Israel ReceiVed: February 27, 2007; In Final Form: May 19, 2007

The molecular mechanics (MM) and density functional theory (DFT) methods were used to study the role of hair structure on the adsorption of various explosive molecules (TATP, TNT, NG, EGDN, and RDX). The present study is limited to adsorption of explosives onto the hairs outer surface while possible diffusion into deeper layers in the hair is neglected. The adsorption properties of the hair surface are estimated from changes in the Gibbs free energy. The calculations suggest that the molecular adsorption of all explosives examined is due mainly to interaction between the molecule and the lipid layer that covers the hair surface. The binding of explosive molecules to the lipid layer consists of interplay between dispersive and Coulomb interactions as well as the distortion of the lipid layer induced by the molecular adsorption. The relative importance of these effects depends on the chemical nature, the size, and the shape of the adsorbed molecule. Several possible adsorption positions, along the lipid molecules, were found for all adsorbates examined. The theoretical prediction that explosive molecules adsorption is mainly due to the interaction with the lipid layer was examined and partially proved experimentally. Moreover, comparison of the calculated results with available experimental data allowed us to obtain the temperature-dependent sticking probability of the various explosives to the hair surface.

1. Introduction The pharmacological and toxicological implications of drug1 and metals2 accumulation in hair are widely acknowledged although not fully understood. Recently, the viability of human hair to reversibly sorb detectable amounts of explosives has been demonstrated.3 This study demonstrated that the rate of explosives sorption and accumulation on black hair was significantly greater than that on blond, brown, or bleached hair. In contrast to drugs, that could be transferred by blood and thus accumulated inside the core of the hair shaft, contamination by explosives is only due to direct adsorption from the atmosphere or due to secondary contact by hand. The detailed understanding of explosives adsorption onto hair is very important since it may lead to a significant improvement in sampling techniques used by law reinforcement agencies in their fight against terrorism. Moreover, detailed understanding of the interaction between hair surface and various organic molecules may have importance in a number of areas. Some examples are cosmetics and drugs used in hair treatments, exposure to toxic substances by workers, and so forth. In the present study, we apply atomistic and electronic structure calculations to obtain a deeper insight into the nature of the interaction of explosives with the hair surface. To model processes involving small molecules interaction with human hair, at the atomic level, information related to chemical composition and structure of the hair surface is essential. Structurally, hair consists of the medulla, an inner cortex, and an outer shell called the cuticle. Melanin granules, * Corresponding author. E-mail: [email protected] or [email protected]. † Weizmann Institute of Science. ‡ Ben Gurion University. § NRCN.

responsible for the hair color, are found only in the inner cortex layer. The surface layer, the cuticle, is a hard transparent shingle like layer made up of sheet-like cells (scales). These are 0.5 µm thick and 5-10 µm in length and width, stacked over each other. Each scale has a laminar structure with an outer and an inner layer, named respectively exo- and endocuticle.4 Chemically, the cuticle consists of fibrous protein known as keratin.5 The cuticle is rich in cystein, which can cross-link via disulfide bonds to form cystine. These cystine bonds occur in hair approximately every four turns of the R-helix. The diameter of a human hair ranges from about 18 µm to 180 µm depending on the number of cuticle layers. The surface of the scales is covered by a thin layer, approximately 3 nm wide, of lipids named the β-layer.5 The lipids constitute a monolayer of saturated fatty acids (FA), mainly 18-methyleicosanoic acid (noted as 18-MEA).6,7 The 18-MEA molecules are covalently bound, via thioester linkage, to the high cystein-containing proteinaceous outer surface of the keratin fibers.8-10 The role of this unusual branched-chain FA monolayer is thought to disrupt monolayer packing and thus impart beneficial tribological properties. As will be evident from the discussion below, such a structure also has a significant effect on the adsorptive properties of the lipid layer. Diffraction experiments showed that fraction of the lipid layer on the human hair appears in an organized state. Microdiffraction experiments performed at a perpendicular direction to the hair section showed a random local orientation of lipids in the cross section.11 Despite the wealth of available information, the local atomic level structure of the hair surface is far from being fully known. Moreover, analysis of structure and chemical composition of the hair surface show significant individual variations caused by gender, ethnic origin, diet, age, and environmental exposure as well as by hair treatment. Particularly, recent atomic force

10.1021/jp071616e CCC: $37.00 © 2007 American Chemical Society Published on Web 07/25/2007

11904 J. Phys. Chem. C, Vol. 111, No. 32, 2007 microscopy (AFM) studies12 exhibited a difference between the lipid compositions at the cuticle surfaces of African and Caucasian hairs. One major difference found by gas chromatography coupled to mass spectrometry (GC/MS) analysis involves the hydrocarbon squalene (2,6,10,15,19,23-hexamethyl2,6,10,14,18,22-tetracosahexaene, C30H50). This molecule was found as a marked peak in the case of Asian hair but was totally absent in Caucasian hair.13 Chemical treatment (bleaching) of the hair leads to significant changes in its surface topography. Complete loss of the scale-like structure after several repeated bleaching procedures was observed by AFM.14 Bleaching combined with daily weathering was shown to induce loss of 18-MEA and conversion of cystine to cystein, resulting in destruction of disulfide bonds.15 The complex structure of hair surface with its diverse functionality may result in differences in the nature of interaction with the various explosive molecules. The present work is aimed at the study of reversible nondissociative adsorption of explosive molecules onto different adsorption sites on the hair surface. The interaction potential in such systems is governed by dispersion, hydrogen bonding, and Coulomb interactions and does not involve chemical transformation of the adsorbate or the surface. The nature of the interaction forces together with the high complexity of the system led us to use the molecular mechanics (MM) approach, that is well-parametrized for such systems. For a more accurate representation of Coulomb interactions, the more rigorous density functional theory (DFT) method was applied. The DFT approach is suitable to yield reliable results concerning energy changes in smaller model systems. Different classes of explosives were examined: nitrate esters (nitroglycerin, NG, and ethylene glycol dinitrate, EGDN), a nitro-aromatic compound (2,4,6-trinitrotoluene, TNT), a nitroamine (cyclotrimethylenetrinitramine, RDX), and a peroxide (triacetone triperoxide, TATP). 2. Computational Methods In the MM simulations, the universal force field (UFF) parameters of Rappe et al.16,17 were used. This approach was proven to accurately represent both short- and long-range interactions in the absence of chemical changes (breakage and formation of bonds). Accurate representation of the Coulomb interactions requires a correct description of the electron density distribution on the interacting centers. To obtain reliable results of such interactions, we performed DFT calculations using the B3LYP hybrid density functional18 and cc-pVDZ basis set.19 All calculations were performed using the locally modified version of the Gaussian 03 package.20 The geometries were fully optimized using both MM and DFT calculations. The minimum energy geometries were determined to be true minima by the absence of imaginary frequencies in the calculated vibrational spectra. The adsorption energies are not corrected for the basis set superposition error. Zero point energies and thermal contributions to thermodynamic functions were computed using DFT or MM optimized structures and harmonic frequencies employing the rigid rotor/harmonic oscillator approximation and the standard expressions for an ideal gas in the canonical ensemble at 298.15 K and 1 atm. Thermodynamic parameters of adsorption were calculated as a difference between the corresponding values of the system with optimal position of an adsorbed molecule on the surface and those of the corresponding separated components. Adsorption parameters of the nth molecule (coverage dependence) were calculated relative to the “adsorbate + (n - 1)adsorbed molecules” system.

Efremenko et al.

Figure 1. 10-residue model of R-helix protein structure (a), MM optimized geometry of adsorbed TATP molecule (b) and model used to study the effect of aminoacid composition (c).

3. Adsorption on the Protein Surfaces Modeling of biological systems is difficult because of their complex structure that is not fully understood at the atomic level. Experimental measurements usually probe an overall average behavior of the system. Using theoretical methods allows one to probe different features of a system separately and independently. As a result, theoretical simulations may, in some cases, lead to detailed understanding of phenomena due to their ability to separate the influence of various parameters in the investigated system. This section describes results obtained by MM and DFT calculations that establish the relationship between the physisorption strength and the protein secondary structure and composition. 3.a. Secondary Structure: R-Helix. As the simplest representation of the protein secondary structure, R-helix models constructed of 10 nonpolar alanine or glycine amino acid molecules were used (Figure 1a). The interaction of explosive molecules with such protein models led to an optimal adsorption geometry that provides the maximal contact area in the adsorbate-protein system (Figure 1b). The resulting interaction strength varies from 1.36 to 11.56 kcal/mol with stronger interaction usually obtained for the less branched protein built of the glycine amino acids (first two rows in Table 1). However, entropy effects reduce the interaction to a much weaker one so that at room temperature only NG adsorption remains thermodynamically stable while, for other molecules considered, the interaction is characterized by ∆G > 0. 3.b. Amino Acid Composition. To study the relation between amino acid composition and adsorbate binding strength, one amino acid in the model R-helix made of alanine molecules was varied in the proximity of the adsorption site (Figure 1c). The system was optimized to obtain the most stable position of the adsorbed molecule. We found that, in general, the interaction energy between the explosive molecules and the protein strengthens in the presence of various functional groups (smaller ∆H values); however, at room temperature, the free energy change is negative only for NG. For most of the molecules studied, serine (Ser) and cystein (Cys) residues lead to the largest stabilization. Complete exploration of the relation between amino acid sequence and adsorption strength is impossible since proteins are characterized by a specific arrangement of many hundreds of amino acids. However, it has been shown22 that several repeated sequences, with minor differences among them, exist in the global sequence. The interaction of explosive molecules with model R-helix structures formed by such common repeats was examined. In general, changes in the chemical composition of the protein model yield small changes in the adsorption enthalpy. However, the ∆G298 values are higher than those obtained for the Gly10 sequence (Table 2). Finally, the tertiary structure of the protein was modeled by combining two R-helixes. The adsorption energies obtained using such twinned model proteins are slightly higher than the

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TABLE 1: Thermodynamic Data Obtained by MM Simulations of the Interaction between Explosive Molecules and Model Proteins with r-helix Structure Constructed of 10 Glycine, 10 Alanine, or 9 Alanine Together with One Different Amino Acid Residue ∆G298, kcal/mol

∆H, kcal/mol pI21

TATP

TNT

NG

RDX

EGDN

TATP

TNT

NG

RDX

EGDN

Gly10 Ala10

6.064 6.107

-9.05 -7.46

-2.19 -3.61

-11.56 -7.82

-5.46 -2.60

-1.36 -1.60

4.17 5.30

8.14 4.46

-1.42 -0.29

3.45 2.63

6.51 8.70

Ala9-Cys Ala9-Glu Ala9-Ser Ala9-Pro Ala9-Thr

5.02 3.08 5.68 6.3 5.60

-13.19 -8.74 -14.98 -8.33 -8.32

-5.12 -2.95 -7.10 1.64 -1.10

-12.04 -10.59 -18.53 -12.17 -11.32

-7.08 -3.95 -11.02 -6.00 -6.68

-4.14 -3.57 -1.33 1.16 -0.54

1.11 4.32 1.38 3.95 4.07

6.02 6.43 6.12 9.77 8.13

-3.18 -0.86 -6.21 -3.27 0.46

3.61 4.15 1.94 2.14 3.45

7.46 7.82 9.34 9.53 9.84

protein

TABLE 2: Influence of the Amino Acid Sequence of Single and Twinned Coiled r-Helix Structures on the Adsorption Thermodynamics of Explosive Molecules ∆G298, kcal/mol

∆H, kcal/mol protein

TATP

TNT

NG

RDX

EGDN

TATP

TNT

NG

RDX

EGDN

KGGCGSCGCS QCSCCKPCCS SACTNSWQVD SSGCGSSCCA GGGGGGGGGG × 2 KGGCGSCGCS × 2 QCSCCKPCCS × 2 SSGCGSSCCA × 2

-5.20 -7.05 -4.93 -8.14 -8.65 -11.68 -13.24 -16.17

-2.39 -2.32 -4.72 -1.12 -7.43 -5.02 -5.11 -5.35

-9.63 -11.36 -12.06 -13.36 -12.98 -13.24 -14.84 -18.06

-2.07 -1.55 -6.99 -5.18 -7.18 -10.49 -8.37 -7.62

1.18 -2.38 1.69 -0.75 -1.84 -0.84 -4.47 -7.07

7.01 7.02 7.07 4.22 6.10 36.13 57.97 1.52

9.64 5.68 5.85 10.10 5.27 8.32 5.86 5.42

1.85 -0.92 -2.35 -3.70 -0.88 0.27 -0.42 -4.32

7.58 6.01 3.03 2.42 2.58 0.89 4.44 4.07

8.82 7.08 10.10 9.05 11.54 10.42 8.00 5.57

TABLE 3: DFT Optimized Hydrogen Bond Lengths (dO-H, Å) and Corresponding Thermodynamic Parameters (∆H, kcal/mol and ∆G, kcal/mol at 298.15 K) for TATP and NG Interactions with the Most Common Functional Groups Present on the Hair Surface as well as Atomic Polar Tensor Charges (APT QH) on the Interacting Hydrogen Atoms

a

Two hydroquinone OH groups form two hydrogen bonds with the explosive molecules.

energies obtained using the corresponding monomer models; however, because of the compensation effect, changes in the Gibbs free energies are negligible (Table 2). 3.c. Coulomb Effects (DFT Results). To obtain a more accurate description of Coulomb interactions and the separation between steric and Coulomb effects, DFT calculations were used to examine the interactions of TATP and NG with the most common amino acid sequences in the hair protein. Similar calculations were carried out to obtain the interaction strength of these two molecules with possible active melanin sites (Table 3). The exact structure of melanin in the hair has not been fully elucidated; however, most melanins are polymers based on indolequinone and dihydroxyindole carboxylic acid.23 The complexes formed due to hydrogen bonding are more stable with larger binding energy and shorter bond length for the more

acidic centers. Both the acetic and the hydroquinone groups of the melanin molecule form more stable hydrogen bonds with TATP than with all of the other functional groups tested; only these centers are thermodynamically stable at room temperature. The indolequinone group is the only one that forms a thermodynamically stable complex with NG at room temperature. These results are consistent with the generally accepted opinion that melanin presents the most active site for adsorption of drugs and other contaminates in hair. However, our results indicate that ∆G changes for TATP and NG interactions with melanin are quite small and could not be responsible for the experimentally observed high adsorption capacity of black hair.3 Since the atomic polar tensor (APT) charges on the oxygen atoms in NG are larger than those in TATP, the resulting hydrogen bonds are stronger in the former case.

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TABLE 4: MM Optimized Geometry for the Interaction between Explosive Molecules and a Single 18-MEA Molecule as well as the Corresponding ∆H and ∆G298 Values (kcal/ mol)

Figure 2. Top (left) and side (right) views of TATP molecule adsorbed within a rigid rhombic site formed by four 18-MEA molecules.

TABLE 5: Adsorption Thermodynamic Data of Explosive Molecules in the Space Formed by Four 18-MEA Molecules with Frozen Anteiso-Terminus and an Optimized Position of the Hydrocarbon Chains and the Adsorbate Positions ∆H, kcal/mol

∆G298, kcal/mol

molecule

I

II

I

II

NG RDX TATP TNT

-10.05 -10.54 -14.66 -8.74

-11.20 -1.29 -12.51 6.76

-3.79 5.30 -10.10 2.87

-1.23 8.85 1.75 16.50

Summarizing, these results indicate that neither geometric nor Coulombic effects lead to the strong adsorption of explosive molecules on protein surfaces. Among the molecules examined here, NG is the only species for which adsorption at room temperature is thermodynamically favorable. Melanin exhibits at least two good adsorption sites for explosive molecules, that are thermodynamically stable at room-temperature mainly due to hydrogen bonding. However, since most of the melanin is located inside the hair core and not on the surface, its role in explosives binding at the hair surface is unclear. 4. Adsorption in the Lipid Layer It is generally accepted that the lipid layer on hair surface is composed mainly of saturated fatty acids with a high content of 18-MEA. It is also established that the average lipids content shows large fluctuations among individuals.24,13 In order to estimate the contribution of the fatty acid layer to the adsorption of explosives, we performed simulations assuming that 18-MEA molecules are the only constituents of the lipid layer. 4.a. Interaction with a Single 18-MEA Molecule. Interaction between a single 18-MEA molecule and different explosive molecules (Table 4) explores the role of the lipid branched structure on molecular adsorption. Becasue of the nonpolar nature of the hydrocarbon chain, the interactions are attributed to dispersive forces. For all of the molecules examined, these forces are quite weak, and the interactions are thermodynamically unfavorable at room temperature. 4.b. Lipid Layer on Hair Surface. The 18-MEA molecules forming the lipid layer are attached to the polymeric substrate via a thioester linkage to sulfur groups on the protein surface. It was suggested that the lipids form a uniform monolayer with pseudohexagonal structure. In this arrangement, the anteisoterminus of each 18-MEA molecule occupies an average surface area of approximately 68.89 Å.25 The simulations employing such a uniform rigid layer of 18-MEA molecules, Figure 2, demonstrate that there is not enough space within such an arrangement to adsorb large molecules. In the same model systems with frozen anteisoterminus of the 18-MEA molecules and optimized positions of other atoms in the system, we found, for each explosive molecule, two different adsorption sites along

Figure 3. Initial “straight” lipids (left) and two low energy orientations of four 18-MEA molecules with anteiso-terminus fixed in the hexagonal arrangement at a distance of 9.36 Å between lipids. The corresponding ∆H values (kcal/mol) are calculated relative to the energy of the infinitely separated molecules.

the lipid molecules (Table 5), the first one, close to the terminus, and the second one, near the branching region of the 18-MEA molecules. Some of these sites appear to be stable at room temperature. These results demonstrate the importance of an accurate representation of the mutual orientation of 18-MEA molecules. The 18-MEA molecules, due to their high surface area, tend to form intermolecular ensembles (Figure 3), associated with decreased energy of the system. Adsorption of molecules from the gas phase disturbs the low-energy configurations of the lipid layer leading to increased destabilization of the adsorbate-lipids system. Moreover, a lipid layer on the protein surface, including its S-bound terminus, is not likely to form a uniform grid because of the following two main reasons. (i) The underlying protein structure does not satisfy the geometric requirements of a hexagonal unit cell with center-to-center separation of 9.36 Å. (ii) While R-helical geometry of the protein is expected to be quite rigid with amino acid side chains tilted toward N terminus, the attached lipid layer seems to be flexible as all C-C, and the S-C bonds could freely rotate. Therefore, in order to accurately represent the geometric structure of the lipid layer, the underlying protein layer should be included in the model explicitly. Hence, to obtain a more accurate description of the system, an R-helix structure was constructed using 10 Cys residues. Next, three (hel-M3) and four (hel-M4) 18-MEA molecules were attached to the nearest-neighboring surface S atoms. The use of such a cystein-rich model protein provides a realistic geometric arrangement of the lipid layer that is very similar to that described in ref 25. This structure is also justified by the experimental observation that the epicuticle consists of up to 30% cystein and its concentration increases then moving closer to the surface layers while deeper layers contain more acidic and basic amino acids.7 Optimized structures of TATP adsorbed to hel-M3 and hel-M4 models, together with their thermodynamic characteristics, are presented in Table 6. For a single TATP molecule, we found two possible adsorption sites with the smaller model (three lipids) and three sites using the four lipid molecules model. However, only one of these sites is stable at room temperature.

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TABLE 6: Geometry and Thermodynamics of the TATP Adsorption on the r-Helix Surface Covered by Three and Four Lipid Molecules

TABLE 7: Geometry and Thermodynamics of TATP Adsorption on the Largest Model Composed from Two r-Helixes Covered by Seven Lipid Molecules

Next, the influence on explosive adsorption of the lipid model size used was examined. To construct a reliable larger model, a model of the underlying protein using two R-helixes with 10 Cys residues in each was constructed. The two R-helixes were bound by one disulfide (Cys-Cys) linkage; this corresponds approximately to one disulfide bond per every 4 turns of the helix. Three models of lipid arrangements, containing three (2hel-M3), four (2hel-M4) and seven (2hel-M7) 18-MEA molecules attached to the double helix surface, were tested. The results indicate that increasing the number of 18-MEA molecules leads to a stronger adsorption of a TATP molecule. Three distinct adsorption sites along the lipid layer were identified: the most stable one (Site 1) is located deep in between the lipid chains at close proximity to the protein surface. The other, less stable site (Site 2) is found near the center of the lipid layer. The last site (Site 3) is located close to the upper end of the lipid molecules. The adsorption energy of this site is related to the interaction with the methyl branch of the hydrocarbon chain. The thermodynamics of the interaction between a TATP molecule and an adsorption site strongly depend on the adsorption-induced perturbation of the interaction between the 18-MEA molecules (Table 7). Very similar results were obtained for TNT adsorption. The influence of the model applied in the calculations of TATP and TNT adsorption thermodynamics in the lipid layer on the hair surface is presented in Figure 4. The only notable difference is that Site 3 is almost as stable as Site 1 for TNT adsorption.

The model of seven 18-MEA molecules may not be large enough to represent the whole lipid layer properly; however, further increase of the system is beyond our computational capabilities. Therefore, we used the 2hel-7M system to model the adsorption of all other explosive molecules. The results are summarized in Table 8, where thermodynamically stable, at room temperature, forms of adsorption for all of the molecules are presented. The nature of the adsorbate-lipid interactions determines the orientation of the adsorbed molecule in an adsorption site. The lowest energy (the most stable) orientation is characterized by the largest interaction area between the adsorbate and the lipid chains. The adsorption thermodynamics is an outcome of the compromise between the adsorption-induced distortion of the lipid layer and the interaction of the adsorbed molecule with its surrounding. Both effects depend on the size and shape of the molecule: large molecules induce larger distortion of the lipid layer. However, a larger molecular surface area results in a stronger binding to the lipid layer. The arrangement of the 18-MEA molecules on the hair surface leaves only restricted space for adsorption. Therefore, insertion of a large molecule leads to a strong distortion of the lipid molecule shape in neighboring sites making them less suitable for adsorption. Adsorption of two TATP or RDX molecules coordinated in the same site along the lipid chain is thermodynamically stable at room temperature while for the TNT molecule this arrangement appears to be nonstable. NG and EGDN adsorptions show

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Figure 4. Influence of the lipid model size on the thermodynamics of TATP (a,b) and TNT (c,d) adsorption on the hair surface.

slightly different behavior: because of the relatively small size of these molecules, they cause smaller distortion of the 18-MEA arrangement and require smaller space for their adsorption. Therefore, the most stable arrangement of these molecules corresponds to their adsorption in neighboring adsorption sites. The small size of NG and EGDN results in a high adsorption capacity of the lipid layer with respect to these molecules (Figure 5) 5. Adsorption Capacity When considering effectiveness of surface adsorption, two properties of the system are of interest: the adsorption capacity and the rate of adsorption. These properties are affected by the nature of both the substrate and the adsorbate. The estimate of the substrate adsorption capacity at equilibrium with the gas phase containing the adsorbate at 298.15 K is based on the computational results described above. The following considerations control the adsorption on the hair: (i) Adsorption Thermodynamics. Adsorption thermodynamics determines the capability of a site on the substrate to bind adsorbates. The results described above indicate that molecular adsorption on the protein forming the hair cuticle can be neglected. The calculations reported in the present study show that explosive molecules adsorb at room-temperature mainly to the lipid layer attached to the hair surface. (ii) Maximal Adsorption Capacity. The surface area of hair (assuming cylindrical shape and hair mass density of 1.3 g cm-3)26 decreases from 1710 to 171 m2/g with increasing hair diameter, D, from 18 to 180 µm, respectively. Next, it is assumed that the surface area associated with each adsorption site is approximately 68.89 Å2. As discussed above, the largest number of adsorbed molecules in an adsorption site is three; hence, the maximal (geometric) adsorption capacity is estimated to be in the range of 0.161-1.607 µmol/g. The estimated maximal adsorption capacities of hair with characteristic average diameters for oriental, brown, and blond hair10,27 are presented in Table 9 together with the measured adsorption capacities.3a,b

It is clear that the experimentally determined amounts of adsorbed EGDN and NG (on all hair types) and the amount of TATP on oriental hair are much larger than the theoretically estimated values. For all the other cases, except RDX, the calculated and measured values are in reasonable agreement. In the case of RDX, the calculated data is much larger for all three hair types than the measured values. The vapor pressures of EGDN, NG, and TATP are in the range of 5 × 10-3 to 5 × 10-2 mmHg at room temperature while that of TNT and RDX is 4 to 6 orders of magnitude smaller.3d It is quite clear from the data presented in Table 9 that for the explosives with the higher vapor pressure the measurements yield much larger quantities of adsorbates than the calculated estimation. The situation is reversed for RDX that possesses the lowest vapor pressure. A possible explanation for the discrepancy between the experimental and the theoretical data for the high vapor pressure explosive is associated with the formation of microcrystals (or droplets for the liquid explosives) of these explosives on the hair. Evidence for such phenomena has been described by Oxley and co-workers3b who demonstrated formation of ∼100 µm crystals when hair was exposed to TATP vapor. Hence, the much larger values of experimental hair adsorption capacity for EGDN, NG, and TATP may be associated with microcrystal formation at various nucleation sites on the hair surface. The nature of such nucleation sites is being investigated theoretically at present and will be discussed in a future publication. The agreement between the experimental and the theoretical results for the TNT, RDX, and two cases of TATP are within an order of magnitude. We consider these results to be in good agreement in view of the approximations and system simplifications involved in the calculations. Moreover, the theoretical simulations suggest that explosive binding is mainly to the lipid layer; the large variation in the density of 18-MEA molecules on different individual’s hair limits the agreement between experimental and theoretical data. (iii) Adsorption Kinetics. As described in the discussion above, adsorption of explosive molecules within the lipid layer

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TABLE 8: Optimized Geometries Together with Adsorption Enthalpy (∆H, kcal/mol) and Gibbs Free Energy (∆G298, kcal/mol) for the Interaction of One, Two, or Three Explosive Molecules with the Lipid Layer Consisting of Seven 18-MEA Molecules Attached to a Double Helix Structure

on the hair surface is accompanied by reorganization of the lipid molecules. This process implies that the adsorption event is associated with activation energy to adsorption. The branched structure of 18-MEA molecules yields additional steric restrictions to the introduction of foreign species into the layer. Unfortunately, the large size of the system required for the simulation did not allow us to evaluate the magnitude of Ea. These values were estimated by fitting the experimental initial rates of adsorption reported by Oxley et al.3a,b to the theoretical rate expression described below. The probability of molecule A to occupy an adsorption site when approaching the substrate from the gas phase is given by its flux FA ) pA/(2πmAkBT)1/2 multiplied by the product of the area associated with the adsorption site, a2, and the sticking

probability. The sticking probability can be expressed as w ) exp(-Ea/RT), where pA is the partial pressure of molecules A, mA is their mass, kB is the Boltzmann constant, T is the absolute temperature, and Ea is the activation energy for adsorption. Therefore, the initial rate of molecular adsorption into the lipid layer can be expressed as ka) FAa2 exp(-Ea/RT). Setting ka equal to the experimentally measured values from ref 3a,b leads to the activation energies presented in Table 10. There is a clear correlation between the adsorbate size and the magnitude of the activation barriers. In addition, for all five adsorbates, the variation in the magnitude of Ea as a function of hair type is quite small. The largest change in Ea values for the three hair origins is obtained for TATP, where Ea of oriental hair is markedly smaller than those for blond and brown hair. This

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Figure 5. Variation of Gibbs free energy as a function of adsorbate coverage in the lipid layer.

TABLE 9: Comparison of Theoretically Estimated Maximal Adsorption Capacity (µg/g) of the Various Explosives with the Measured Capacities.3a,b calculated maximal capacity, D ) 100 µm Oriental hair (experiment) calculated maximal capacity, D ) 50 µm brown hair (experiment) calculated maximal capacity, D ) 65 µm blond hair (experiment)

EGDN

NG

TATP

TNT

34

51

49

51

RDX

22143 68

721 101

1681 99

114 101

7.6 97

14030 52

721 78

92 76

61 78

2.3 75

18493

721

201

72

2.7

49

TABLE 10: Estimated Ea Values (kcal/mol) for the Adsorption of Explosives in the Lipid Layer EGDN

NG

TATP

TNT

RDX

2.2 2.3 1.5

4.2

3.9 5.8 5.5

6.8 7.3 7.1

8.7 9.5 9.0

Oriental blond brown

result is in agreement with the experimentally observed high adsorption rate in the system as compared with that of other large molecules and other hair types. Note, however, that activation energies for adsorption of TNT and RDX molecules on oriental hair are somewhat lower than those on blond and brown hairs. This may be indicative of a specific chemical composition, geometric arrangement, or both of the lipid layer on the oriental hair surface. This point will be examined in more detail in a future publication. 6. Conclusions MM and DFT calculations were used to obtain atomic-level insight into the adsorption mechanism of explosive molecules (EGDN, NG, TATP, TNT, and RDX) onto the human hair surface. The calculations show that dispersion and Coulombbased interactions of these molecules with different functional groups of amino acids present in keratin are too weak to be responsible for the explosives binding observed experimentally.3a,b The NG is the only species for which adsorption at room temperature is thermodynamically favorable on the protein surface. The hydrogen bonding of TATP and NG to acidic centers of the melanin molecule is found to be much more stable.

It is consistent with experimental data suggesting that melanin may be responsible for high adsorptive properties of black hair. This explanation of the experimental results is in contradiction to the current understanding of hair structure, namely, that melanin is found in the inner cortex of the hair shaft. Hence, it is not likely that it is responsible for the strong adsorption of molecules onto the hair surface. The theoretical results described above clearly demonstrate that adsorption of explosive molecules within the lipid layer, modeled by 18-MEA molecules attached to the protein surface, is thermodynamically favorable for all of the explosive molecules examined. The bond strength obtained is related to the total effect of the molecular interactions with the surrounding protein/lipid environment and to the energy associated with the adsorption-induced rearrangement of the lipid layer. Both interactions are found to depend on the adsorbate chemical nature, its shape, and its size. The strongest interaction was found for the most “flat” EGDN molecule while the adsorption of the bulky TNT and RDX molecules requires a large rearrangement of the lipid layer that leads to reduced interaction energies. Accept for high vapor pressure explosives, a good agreement is found between the experimentally and the theoretically determined adsorbate capacities of the hair. For high vapor pressure explosives, the experimental values are a few orders of magnitude larger than the calculated estimates. This difference is possibly due to formation of microcrystals, a process not investigated in the present study. The possibility of explosive nucleation at various sites on the hair surface will be discussed in a future publication. To summarize, the main prediction of the simulations described above is that explosive adsorption onto the human hair surface is dominated by the interaction between the explosive molecule and the lipid layer on the hair. This theoretical prediction is being examined experimentally at present. In these experiments, human hair samples are exposed to explosive vapor. Following the exposure, the adsorbate capacity on the hair sample is determined by rinsing the sample in acetonitrile until all of the adsorbed explosives are dissolved. The concentration of explosives in the acetonitrile is then evaluated using high-pressure liquid chromatography (HPLC). The role of the lipid layer in explosives adsorption is obtained by comparison between data observed for hair samples as received (after cleaning procedure) and samples that were treated (after the cleaning) with a solution of KOH and methanol. Such a treatment was shown to result in the removal of most of the lipid layer from the hair surface.3a,28 Preliminary results, corresponding to the exposure of four different human hair samples to TNT vapor at 70 °C for 48 h are shown in Figure 6 below. The results shown in Figure 6 clearly indicate that the amount of adsorbed TNT at 70 °C during 48 h exposure of hair samples exhibit a marked dependence on the hair pretreatment. The amount of TNT adsorbed onto hair samples treated by a methanol-KOH solution is approximately an order of magnitude lower than the amount obtained for untreated samples. It is not clear if the entire lipid layer is removed in the treatment by the methanol-KOH solution or if the rinsing in this mixture leads to partial lipid removal only.29 A large series of exposure experiments of treated and untreated human hair samples to the vapor of different explosives is being performed at present. The detailed analysis of the results observed in these experiments will be discussed in a future publication. However, the preliminary experimental data presented above clearly support the theoretical prediction of this study.

Explosives on Hair

Figure 6. Comparison between TNT capacities of four different hairs (treated and untreated).

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