pH-Dependent X-ray Photoelectron Chemical Shifts and Surface

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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

pH-Dependent X-ray Photoelectron Chemical Shifts and Surface Distribution of Cysteine in Aqueous Solution Vincenzo Carravetta, Anderson Herbert de Abreu Gomes, Susanna Monti, Alexandra Mocellin, Ricardo R. T. Marinho, Olle Bjorneholm, Hans Agren, and Arnaldo Naves de Brito J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b00866 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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The Journal of Physical Chemistry

pH‐dependent X‐ray Photoelectron Chemical Shifts and Surface Distribution of Cysteine in Aqueous Solution Vincenzo Carravettaα, Anderson Herbert de Abreu Gomes‡1, Susanna Montib, Alexandra Mocellinc, Ricardo R. T. Marinho§,c, Olle Björneholm$, Hans Ågren&,$, Arnaldo Naves de Brito‡* α

CNR‐IPCF, Institute of Chemical and Physical Processes, via G.Moruzzi 1, I‐56124 Pisa, Italy

b

CNR‐ICCOM, Institute of Chemistry of Organometallic Compounds, via G.Moruzzi 1, I‐56124 Pisa, Italy c

Institute of Physics, Brasilia University (UnB), Box 4455, Brasília 70910‐970, Brazil

‡

Dept. of Applied Physics, Institute of Physics “Gleb Wataghin”, University of Campinas, CEP: 13083‐859 Campinas‐SP, Brazil §

Institute of Physics, Federal University of Bahia, 40.170‐115, Salvador, BA, Brazil 

$

Department of Physics and Astronomy, Uppsala University, 752 36 Uppsala, Sweden

&

Theoretical Chemistry and Biology, School of Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, SE‐10044 Stockholm, Sweden

1 Current address: Laboratório Nacional de Luz Síncrotron (LNLS)”,13084‐971 Campinas‐SP, Brazil. *Corresponding author: [email protected] ACS Paragon Plus Environment

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Abstract The distribution and protonation states of amino acids in water droplets is of considerable concern in studies on the formation of clouds in the atmosphere as well as in many biological contexts. In the present work we use the cysteine amino acid as a prototype example and explore the protonation states of this molecule in aqueous solution, which are strongly affected by the acidity of the environment and that also can show different distributions between surface and bulk. We use a combination of X‐ray photoelectron chemical shift measurements, density functional theory calculations of the shifts and reactive force field molecular dynamics simulations of the underlying structural dynamics. We explore how the photoelectron spectra distinctly reflect the different protonation states that are generated by variation of the solution acidity and how the distribution of these protonation states can differ between bulk and surface regions. At specific pH values, we find that the distribution of the cysteine species at the surface is quite different from that in bulk, in particular, for the appearance in the surface region of species which do not exist in bulk. Some ramifications of this finding are discussed.

Introduction The optical properties of clouds, and therefore their indirect effect on climate, are highly dependent on the number and concentration of cloud condensation nuclei (CCN) in the atmosphere 1 and constitute a considerable uncertainty in climatic models 2. The presence of surfactant molecules in the atmosphere is known to influence the nucleation process, in particular, its supersaturation conditions, and hence the creation of droplets that ultimately form clouds 3. Amino acids constitute water‐soluble organic compounds that can play such role 4‐6 . Originating from phytoplankton and bacteria they are injected into atmospheric droplets over oceans by bubble bursting 7. Several types of amino acids ACS Paragon Plus Environment

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have been identified in both marine and continental atmospheric samples, something that also has been confirmed by laboratory measurements 8. The abundance of sea‐salt increases the solubility of several amino acids in the atmospheric droplets and raises the possibility of cooperation in cloud formation 9. There is, however, still need for a more affirmative determination of the chemical status and distribution of amino acids in aqueous droplets. The critical supersaturation of the particles depends on their composition and their physical properties and affects nucleation and growth into larger droplets. An important factor determining the growth and therefore the climatic impact is the surface tension. In the case of amino acids it has been shown that hydrophilic amino acids tend to stay in the bulk of the droplet, while the hydrophobic amino acids are found to concentrate on the surface, as largely determined by that the hydrophilic −NH3+ and −COO− groups stay inside the droplet and the hydrophobic side‐chain directs outward 10. Thus, expectedly the perturbation of the droplet surface tension induced by hydrophilic amino acids is relatively small while the hydrophobic amino acids can increase the surface tension of droplets largely. An important technique to unravel the surface distribution of molecules in a solution is X‐ray photoelectron spectroscopy (XPS) which typically samples signals from molecules within 10 Å from the surface. In a recent letter 11 the present authors used X‐ray photoelectron spectroscopy and all‐ atom reactive molecular dynamics simulations as two independent techniques to probe bulk and surface distributions of protonated species of cysteine in aqueous solutions at different pH. The cysteine molecule, see Fig. 1, presents here an important prototype for studies of amino acids since it has three groups: carboxylic (C), amine (N) and sulfuric (S) which may or may not be protonated in aqueous solution, so it may be present in 8 different protonation forms with relative ratios dependent on pH. Furthermore, cysteine with its three elements C, N, S provides three distinct sets of chemical shifts in XPS. The experimental titration curves 12, see Fig. 1, show that the species actually present in ACS Paragon Plus Environment

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a wide pH range (6‐10), are only 4, that is, all those in which the carboxylic group is deprotonated, while one more form of cysteine, fully protonated, is present only in aqueous solutions with very low values of pH. Our recent experimental and theoretical study 11 has shown, however, that the four forms missing inside (bulk) an high pH solution, namely those containing the COOH group, are present, instead, in the surface region, understood as a layer of a few Angstroms that separates the solution from the vacuum. In thermodynamic measurements, which are carried out on a macroscopic sample, the surface constitutes, of course, a negligible fraction and the titration curve only reflects the bulk composition. The theoretical prediction allowed to interpret the photoemission spectra (XPS) measured with the micro‐jet technique on an aqueous cysteine solution at pH = 9.5. Using two different photon energies, with different penetration capacities in the sample and different emersion capacities from the photoelectron sample, it was possible to measure two emission spectra that can be assigned predominantly to the bulk (higher energy) and the surface (minor energy). The differences between the two spectra (bulk/surface) measured at the N and S K‐edges, which appeared contradictory to the indications given by the titration curves, were instead explained on the basis of the results of the theoretical modeling as due to the different compositions of the bulk and the surface of the aqueous solution. The simulations could thus predict the presence on the surface of species with COOH groups, which are not present in bulk. In the present work, we take a step further and explore the full pH dependence of the XPS chemical shifts of the three C, N, S elements of the cysteine molecule. We use a combination of X‐ray photoelectron chemical shift measurements, density functional theory calculations of the shifts and reactive force field molecular dynamics simulations of the underlying structural dynamics to obtain crucial, complementary, information about the surface bulk division and surface distribution of the species. In particular, we predict the occurrence, at the surface of any aqueous solution of cysteine, of ACS Paragon Plus Environment

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low pH species, COOH, CNH3+, and CSH, which do not exist in bulk at medium or high pH.

Figure 1 On the left: experimental titration curves for cysteine in aqueous solution from Dixon and Tripton (11): distribution of various ionic species of cysteine as a function of pH; the vertical lines indicate macroscopic pKa's. On the right structure of the cysteine molecule in the gas phase: C (dark gray), O(red), N(blue), S(yellow), H(light gray).

Experimental Section The C1s photoelectron spectra were all taken using 342 eV photons and were performed at Max‐II, Lund Sweden, at the I411 undulator beamtline 13‐14. The spectra showing the Sulfur 2p core lines were taken using ≈ 238 eV, and for the N1s we employ 464 eV photons. We performed these experiments at the plane grating monochromator (PGM) beamline at the Brazilian National Synchrotron Facility (Laboratório Nacional de Luz Síncrotron ‐ LNLS), Campinas Brazil 15. We choose the excitation energies so that the photoelectrons would have about 50 eV kinetic energy, in the case of the C1s and N1s core lines and about 80 eV for the S2p spectra. For these range of photoelectron kinetic energy, the effective attenuation length is estimated to be on the order of 0.5‐1 nm 16‐18. We have employed

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the liquid microjet technique to perform all XPS measurements on a volatile cysteine aqueous solution

. In both synchrotron facilities, the liquid microjet set‐ups were acquired from

19‐20

Microliquids GmbH 21. The microjet technique is explained in greater detail else where 19. In short, a liquid jet of 18‐20 micrometer diameter is injected into the experimental chamber with a 0.5 mL/min flow and a speed of 50 m/s. Due to surface evaporation, the temperature of the liquid at the measuring point is about 10 ºC. Linearly polarized synchrotron radiation intercepts the microjet at about 1 mm from the insertion point which is well before a distance 5 mm from the insertion where the microjet freezes and bake down into small droplets. After several centimeters, liquid nitrogen cold traps capture the jet and helps to maintain the vacuum conditions manageable, i.e., 10‐4 mbar. Scienta R4000 electron analyzers were used in both set‐ups with the difference that at LNLS (Max‐II) the analyzers’ lens axis is placed at 90º (54.7°) to the propagation direction of the X‐ray beam. Thus at Max‐II possible angular distribution effects are minimized. The total experimental resolution was better than 0.19 eV in the S2p region, 0.4 eV in the N1s region and 0.3 eV in the C1s region. These values were evaluated using the gas phase water 1b1 line. We have calibrated all spectra using the valence 1b1 spectra of liquid water situated at binding energy equal to 11.16 eV 22. Cysteine was commercially obtained from Sigma Aldrich with stated purity higher than 97% and dissolved in deionized water (Millipore Direct‐Q, R > 18.2 MΩ cm) producing an aqueous solution with 1 Molar concentration of cysteine at the SI we show absolute normalization data and N1s spectra where the concentration was 0.3 Molar. The low and high photon energy spectra are not intensity calibrated between each other therefore the relative concentration of the species containing, for example, NH3+ and S‐ between surface and bulk cannot be compared. However, the observed relative changes in concentration between NH3+/NH2 and S‐/SH at surface and bulk are not affected by photon flux, cross‐section and other experimental factors. We have also added to the solution 25 mM of NaCl to


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avoid charging produced by the photoionization process and also electrokinetics 23. All spectra were fitted by a least squared fitting procedure using Voigt line shapes. Unless stated the energy position, intensities of the corresponding Gaussian widths were kept as free parameters, however, we kept them linked in such as way that they were the same for corresponding peaks in all pHs we recorded an XPS spectrum. The Lorentzian line shape representing the lifetime broadening was kept fixed at 140 meV for the N1s lines, 100 meV for the C1s line, and 70 eV for the S2p lines 24‐25. We adjusted the pH of all sample either by adding sodium hydroxide (97 % Sigma Aldrich) of hydrogen chloride and checking continuously with a calibrated pH meter Orion 410A. Further confirmation of the pH values was performed using a titration curve aiming to determine the pKa values of aqueous cysteine solution precisely. This titration curve was performed at 4 ºC as to mimic the experimental temperature conditions.

pH_system nh2_s_cooh

nh2_sh_coo

nh2_s_coo nh2_sh_cooh

nh3_sh_cooh

nh3_sh_coo

nh3_s_coo nh3_s_cooh

1.0_bulk

0.000

0.000

0.000

0.000

1.000

0.000

0.000

0.000

6.0_bulk

0.000

0.000

0.000

0.000

0.000

1.000

0.000

0.000

9.5_bulk

0.000

0.275

0.080

0.000

0.000

0.080

0.565

0.000

12.5_bulk

0.000

0.000

1.000

0.000

0.000

0.000

0.000

0.000

1.0_surf

0.000

0.000

0.000

0.121

0.758

0.091

0.030

0.000

6.0_surf

0.000

0.262

0.071

0.190

0.190

0.238

0.024

0.024

9.5_surf

0.059

0.324

0.176

0.118

0.118

0.059

0.088

0.059

12.5_surf

0.295

0.205

0.341

0.045

0.068

0.000

0.023

0.023

Table 1 Frantions of the different eight forms of cysteine predicted by reactive molecular dynamics on system simulating an aqueous solution at different pH (1.0,6.0,9.5,12.5); the ratio values in the upper boxes refer to the bulk region, while those in the lower boxes refer to the surface region of the systems.

Computational Details Computational modeling of the cysteine core photoemission spectra has been performed by reactive ACS Paragon Plus Environment

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molecular dynamics for predicting the composition of the aqueous solution at different pH and by quantum calculations for predicting the binding energies of C and N atoms at the K‐edge and of S at the L‐edge. Molecular dynamics is mostly employed to study the evolution of matter at the atomistic level by adopting simulation models that may include up to hundreds of atoms at the quantum level or hundreds of thousands of atoms at the classic level by resorting to force fields to describe the interactions between the components (atoms, molecules) of the system. Classical molecular dynamics is usually implemented with force fields that well describe the physical processes and the softer molecular interactions, but cannot describe the chemical processes. This problem can be overcome by resorting to methods based on reactive force fields that include also parameters to describe bond breaking and forming. With the purpose of describing the dynamics of a relatively large model of the cysteine aqueous solution we performed molecular dynamics calculations adopting a reactive force field (ReaxFF) proposed by van Duin et al. 26 and specially parametrized for the description of amino acids 27. This choice is dictated by the need to have a simulation system of size large enough to distinguish a bulk and surface regions and an efficient simulation method able to describe the proton transfer among the water molecules and the three mentioned sites of the cysteine molecules in the sample. Specifically, the molecular modeling has been performed on a system consisting of 212 cysteine, 13000 water molecules and an appropriate number of Na+ or Cl‐ counter ions, contained in a box of 3

dimension 89.14 x 58.08 x 86.23 Å for the simulation of the bulk of the solution and of dimension 3

89.14 x 258.08 x 86.23 Å for the simulation of the surface. The number of cysteine molecules within the simulation box has been chosen on the basis of the experimental concentration, while the ratio of the different cysteine species at the beginning of the dynamics has been established, for each pH value of the simulated aqueous solution, by the experimental titration curves, corresponding, of ACS Paragon Plus Environment

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course, to the real bulk environment. Each NVT reactive molecular dynamics has been carried out at the temperature of 300 K with a time step of 2 fs for a total simulation time ranging from 100 to 200 ps. Tab.1 collects the ratios of the different forms of cysteine in the aqueous solution at different pH as predicted by the molecular dynamics calculations at the last frame of the trajectory. The surface region is defined as a slice orthogonal to the y axis and width of about 5 Å starting from the “contact point” between the sample and the vacuum. This quantity, in turn, is taken, dynamically during the simulation, as the level along y axis where the average local atomic density is half the average value of the atomic density in the bulk. Molecule\BE_DFT(eV) cys_nh2_s_cooh_na cys_nh2_sh_coo_na cys_nh2_s_coo_na2 cys_nh2_sh_cooh cys_nh3_sh_cooh_cl cys_nh3_sh_coo cys_nh3_s_coo_na cys_nh3_s_cooh

CN 291.90 291.64 291.25 292.96 294.26 293.24 292.40 293.88

CO 294.68 293.92 293.34 295.72 296.55 295.40 294.27 295.77

CS 290.51 290.96 289.81 291.90 292.79 292.38 290.50 291.40

N 404.34 404.64 405.27 405.73 408.58 406.51 407.25 408.78

S 166.34 168.44 165.23 169.48 170.43 170.35 165.88 166.45

Table 2 Binding energies (BE) in eV predicted by DFT calculations for the different eight forms of cysteine shown in Fig. SI. 2‐0

The binding energies have been computed by HF and DFT calculations for the different species whose structures, presented in Fig. SI2, have been energy optimized by DFT calculations. Depending on the protonation state of the cysteine molecule, positive (Na+) or negative (Cl‐) counterions have been added in order to obtain neutral systems. The B3LYP density functional and the Ahlrichs‐VTZ basis set have been employed for both the DFT and geometry optimization calculations. The binding energies predicted by the HF method are very close to those obtained by the DFT method with the exception of the O‐1s binding energies that are in better agreement with the experimental data in the ACS Paragon Plus Environment

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second case; for this reason in Tab.2, as well as in the following pictures, we present only the results obtained by the DFT approach. The computed photoemission spectra are obtained with the reasonable assumption that the photoionization cross section for a specific atomic species is constant for the considered photon energies and for non‐equivalent atoms of the same kind. As a consequence, the relative intensities will be considered merely proportional to the fractions of the cysteine forms present in the considered sample. For instance, the computed C1s photoemission spectrum of the bulk of the solution at pH 9.5 will be represented by the bar diagram in Fig. SI 3 obtained by using the binding energies in the second, third and fourth column of Tab.2 and the "intensities" provided by the fractions in the fourth row of Tab.1.

Results and Discussion According to the ESCA potential model 28, the core photoelectron chemical shifts are determined by the charge redistribution of the ground state that alters the potential at the ionization site. Thus, for instance, the electronegative O,OH ligands of the COOH species investigated below, deplete electron charge from the carbon atom, leaving it more positive, thus generating a more positive potential and thus a positive (chemical) shift of the binding energy. This is to some smaller extent compensated by the negative charge redistributed on the ligands. The ESCA model is a ground state model based on a frozen orbital picture that ignores final state relaxation. However, the relaxation effect is in general considerably smaller than the ground state charging for polar molecules, while for unpolar organics, with small total shifts, it can be an important, even the principal, contributor. According to the original ESCA model, the binding energies are ruled by the on‐site potential and the shifts by the variation of this potential between different chemical species as dictated by a core‐valence Coulomb


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interaction and the on‐site valence charge. An alternative to using models 29, as the ESCA potential model, to estimate the chemical shifts, is to use full electronic structure calculations. The correction to Koopmans, frozen orbital, theorem for the binding energy and its shifts can largely be accounted for by self‐consistent field (SCF) optimization within the one‐particle picture. This owes to the fact that core electron ionization by large preserves the valence electron structure and that the largest frozen orbital correction refers to relaxation of the (one‐particle) electron structure. Thus separate SCF optimization of ground and core hole states (the so‐called delta SCF method 30 gives binding energies and their shifts to a considerable accuracy. Further refinement through difference of dynamical correlation energies between ground and core hole states are in general well captured by density functional theory 31. Remaining corrections, viz. vibronic and relativistic corrections, are in general small (for low‐Z species) and controllable 31. Accounting for these notions we have in the present work applied DFT as a SCF method, which gives shifts to an accuracy of a few tenths of an eV, and run ground and core ionized state calculations for the three elements, C, N, S, for different protonation forms of the cysteine molecule. We note that the computed binding energies of condensed phase systems are notoriously, and rather uniformly, a couple of eVs too high; something that can be mainly ascribed to the solvent effect, in particular, the final state dielectric polarization by the core‐ionized species 32. In solution, the final state electronic relaxation induced by the solvent interaction and the reaction field generated by the solvent thus give additional stabilization in the order of 2 eV for all core hole states and is in general only minorly chemically dependent. The Born energy for dissolving a unit charge gives indeed a shift of the order of a couple of eV for molecules of the size here considered. For the finer contributions one also needs to account for the differential effect of core‐valence penetration in the solvent (thus core holes tend to shift more, ca 0.5‐0.7 eV, than valence holes 33). With specific binding in the ACS Paragon Plus Environment

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solvent, the picture evidently changes so does also the close presence of ions (see further discussion below). Nevertheless, we display the absolute binding energies as computed, without solvent shifts. Spectral data We note first that cysteine contains three titratable groups: COOH, NH2, and SH see Fig. 1. The S and N titration curves were subject to analysis in our previous paper 11. Below we first show the C1s, S2p and N1s XPS experimental spectra of cysteine in water at several pH values (Fig. 2). The spectra were recorded with an electron kinetic energy of only 50 eV, thus with small escape depth and high surface sensitivity. C1s shifts We first turn our attention to the carbon spectra and analyze the “pure” pH cases ‐ pH 1.0, pH 5.17 and pH 12.5 – i.e. those corresponding (see Fig. 1), as close as possible, to a single well‐defined bulk species in solution. It is clear that we can qualitatively as well as quantitatively understand these spectra in terms of binding energy shifts, see Fig. 2. The C1s results, showing the shifts for three symmetry independent carbon atoms contain most information. For pH 1.0 we have the bulk protonation states COOH, CNH3+, and CSH, meaning that all groups are protonated, where the highest BE band corresponds to COOH, the middle band corresponds to CNH3+, and the lowest BE is due to CSH. The spectral comparison, between experiment (lowest panel of Fig. 2) and computations (lowest panel of Fig. 3), unambiguously assigns this order of the shifts. At this point we need to discuss some minor contributions also present at this pH. According to our MD calculations, as much as 9 % of the


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species at the surface is predicted to be [(NH3+) (SH) (COO‐)]. The calculations predict also 3% belonging to the molecule [(NH3+) (S‐) (COO‐)] however our XPS 2p spectra rule out this possibility, as we will discuss further down in the text, see also Fig. S7. Our fitting is compatible with a presence of 9% C1s COO‐ in excellent agreement with the calculations. The zwitterionic form [(NH3+) (SH) (COO‐)] could in principle be able to form ion‐pair dimers, which could enhance their present at the surface over the charged form [(NH3+) (SH) (COOH)]. Our experimental result at pH 1 is compatible with this prediction but they are not conclusive since a reasonable fit can also be obtained with no COO‐ peak added. The spectrum at pH 1.91 will give us solid experimental evidence about how strong is the presence of the zwitterionic pair at the surface. At pH =1.91 the CNH3+ contribution is split in two peaks of equal areas due to the influence of the carboxylic group. The two CNH3+ peaks thus correspond to molecules with COOH and to molecules with COO‐, respectively. CSH has a similar intensity as at pH 1.0. We do not see any splitting for CSH as we do for CNH3+, which probably is due to CSH being further away from the carboxylic group, and thus less affected. The energy region 293‐290 eV shows the presence of species with the COOH and COO‐ groups. According to the known speciation in the aqueous bulk at pH=1.91, cysteine is present in the aqueous bulk both as protonated COOH form (51.6 %) and zwitterionic COO‐ (48.8%); the solution is an acidic buffer. At the surface the fitting of the XPS data shows 56 % and 44% for the COOH and COO‐ forms respectivively, see Table S1. These values however contain a somewhat larger uncertainty due to the difficulty to reach a good fit at this binding energy mantaining all the constrainst we have imposed, such as energy position compatiblility with other pH and peak width. Relaxing the binding energy constrainsts, however, improves the fitting and leads to a 3% increase (decrease) of the COOH (COO‐) peak area. The higher XPS signal from the protonated [(NH3+) (SH) (COOH)] species may be a consequence of the very small probing depths of our photoelectrons. We could argue that the ACS Paragon Plus Environment

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protonated species could sit preferencially at the surface with the COOH and SH pointing towards the vaccum and the (NH3+) being solvated and pointing towards the bulk.

Figure 2 C1s XPS spectrum of cysteine in aqueous solution at 5 different pH using as excitation energy of 342 eV; an assignment of the peaks to different protonated species is proposed. A special notation is used to label the same protonated species but present in a different chemical environment. For example, the CNH3+ may be


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connected to a COOH group; in this case we introduce the notation (COOH CNH3+). In case the CNH3+ group is connected to the deprotonated COO‐ group, we use the notation (COO‐ CNH3+).

At pH= 6.0 (5.17 in the experiment) we see that the full spectrum in Fig. 2 is slightly moved to lower energies, while the internal bands, representing the differential shifts, keep the same energetic separations. We though note a contribution of COO‐ predicted by theory at the high‐energy side of the spectrum, around 294 eV. This seems to be confirmed by the deconvoluted experimental spectrum, although not as prominent as predicted by the computation. The expansion of the COO‐ band, 88% of the area intensity from the XPS data, corresponds to a decrease of the COOH contribution to 12% as seen in both theory and experiment. At pH 9.5, we can consider the bulk protonation state: COOH and COO‐, CNH2 and CS‐, as a mixture of ”pH 12.5” species: COO‐, CNH2 and CS‐ and the ”pH 5.17” species: COO‐, CNH3+, and CSH. However, to get a good fit, also COOH needs to be included, as predicted by theory (see Fig. 3). It is clear that the results for middle values of pH can be qualitatively understood as a combination of the “pure spectra” at pH 1.0 and pH 5.17 (plus a small contribution from pH 12.5). Going to even higher pH values the whole spectrum shifts to lower BEs, with the CS‐ band building up at the low energy side at the expense of CSH and the NH2 band building up in the middle of the spectrum at the expense of NH3+. All features are qualitatively well reproduced when comparing the computed shift spectra with the deconvoluted experimental spectra. Concerning the COOH species, the effects of pH are not as dramatic, but one inevitably concludes that also these species get deprotonated. Thus the XPS spectra follow a neat trend of deprotonation of all carbon‐containing groups with the increase of pH and we can essentially understand all spectra as a mixture of the two extreme protonation conditions. When comparing the experimental spectra in Fig. 2 and the computed ones in Fig. S3 one must bear ACS Paragon Plus Environment

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in mind that the experimental spectra contain a main contribution from the surface but also a contribution from the bulk. The amount of photoelectron signal from the surface depends on two relevant factors ‐ the electron mean free path and the concentration of a particular cysteine species at the surface compared to the bulk. To an excellent approximation, the electron mean free path can be considered to be the same for all three edges, S2p C1s and N1s. The concentration of cysteine at the surface, however, can change substantially depending on the number of protonated groups the cysteine molecule possesses. Indeed charged groups such as COO‐, NH3+, and S‐ will tend to reduce the presence of these molecules at the surface, sometimes this reduction can be substantial 34. For a surface depleted from cysteine molecules the photoelectron signal may reproduce the bulk since, in practice, only that region can contribute to the signal. Opposite to this situation, if the molecules form a monolayer at the aqueous interface the amount of photoelectron signal from the surface will be maximum. However, even in this case the amount of the signal from the bulk will be different from zero and can be comparable or even higher than the signal from the surface, see for example Fig. 2 from ref. 35. In that work, the photoelectron kinetic energy was about 300 eV. However, we have another study dealing with the same mixture showing that the bulk contribution is far from negligible even for a photoelectron kinetic energy about 80 eV as in the present study. The presence of charge groups does not imply the cysteine molecules containing these groups will be depleted from the surface. The possibility of forming ion pairs such as COO‐ + NH3+ or S‐ + NH3+ can lead to an enhancement of the molecules forming the pair at the surface. Recently this enhancement has been observed which shows the plausibility of our proposal 36. Taken this into consideration one would expect the spectra taken at pH 12.5 to be the least surface representative since we have two significant groups COO‐ and S‐ and no ion pair is possible. In the tables S3, S4 and S5, we present the intensity calibration for all spectra at all edges except for S 2p at pH 12.5; these data clearly confirm


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that at pH 12.5 we find the smallest amount of cysteine at the surface. From our normalized data we can explain why the resemblance between the experimental spectrum and the bulk calculated spectrum, see Fig. S4, is the highest. The experimental spectra at pH 9.5 compare well with the surface calculated spectrum meaning that our spectrum is more surface representative and that is indeed what Table S3 shows. The comparison between the experimental and calculated surface spectrum is also good for pH 6 (5.15) and pH 1. The possibility for ionic pair formation and the increasing presence of the COOH species would support the assumption that the cysteine molecules will have its presence in the surface increased compared to that at pH 12.5, see tables S(3‐5). If we keep in mind the facts discussed in the last paragraph, we can expect that the amount of COOH species, represented by its corresponding peak at pH 9.5, is a lower bound to its relative intensity. We can give one reason for this statement: the spectrum does have a contribution from the bulk, which does not contain a COOH peak. In Fig. S4 we compare separate plots of computed “bulk” and “surface” spectra using the separation of bulk and surface according to the criteria given in the computational section. For easier comparison with the experimental spectra, each bar diagram representing the computed energies and intensities (as in Fig. S3) has been convoluted in Figs. 3‐5 with a Gaussian function in order to mimic the vibrational distribution and the limited energy resolution of the experiment.


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Figure 3 C‐1s computed photoemission spectrum of cysteine in the surface region of aqueous solutions with different pH; the black continuous profiles are obtained by convolution of the bar diagrams with a Gaussian function with FWHM=1.5 eV.

N1s shifts The experimental N1s spectra in Fig. 4 clearly show the contributions of the amino group in the two NH2 / NH3+ protonation states. The spectra for pH 1.0, 5.0 and 12.5 show one component; the two former spectra at the high energy side, the latter one at the low energy side, while for pH 9.5 two components are seen. This is, again, a remarkable confirmation that protonation close to the ionization site almost entirely decides the chemical shift, making a clear‐cut analysis possible. The computational spectra in Fig. 5 qualitative reproduce the two‐peak analysis with their pH dependence. Thus we see a prominent double peak structure at pH=9.5 and high binding energy peak at pH 1.0 and a low energy peak at pH 12.5. However, we can observe that the predicted energy ACS Paragon Plus Environment

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difference (about 4 eV) between these two peaks is overestimated (well beyond the DFT precision) in comparison with the experiment and that the position of the high energy peak varies for the three lower pH values, well beyond what is seen experimentally. We interpret this as a limit of the model system adopted for the computation of the binding energies, namely the simple addition of a counterion to neutralize the charged species of cysteine. While for the COO‐ and S‐ groups the position of the counterion is reasonably obtained by a geometry optimization in the vacuum, in the case of the NH3+ group this procedure is much less reliable in absence of solvent molecules. With the strong charge effect on the chemical shift, the positioning of the counterion can become crucial in the computation of the binding energy. We also see a rather considerable variation between bulk and surface spectra, using a layer division as described in the computational section, especially for the intermediate pH values for which the barycenter of the double peak structure tend to the low energy side for the surface spectra, while instead, the opposite is the case for the bulk spectra. The low energy spectra (recorded at 463.6 eV photon energy) and high energy (recorded at 995.1 eV photon energy), see Fig. 4, indicate some salient bulk to surface changes of the pH 9.5 double structure (see also analyses in the former paper 11. Changing the photon energy from high to low value, and so changing the sampling region from bulk to surface, a considerable transfer of intensity from the NH3+ band to the NH2 band can be observed. This trend is qualitatively well reproduced (albeit somewhat underestimated) by the simulation results presented in Fig. 5. We see that this trend is further strengthened at pH 6.0, showing only the protonated bulk peak in the simulations, and for pH 1.0 both surface and bulk spectra have collapsed into the protonated form. Experimentally this seems to occur already at a higher pH value, see Fig. 4. As we do not have a corresponding experimental separation between bulk and surface spectra available except for pH 9.5, more specific assignments cannot be made.


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Figure 4 The S2p and N1s XPS spectra presented at 4 different pH values. At pH 9.5 we show the same binding energy region taken at two photon energies. The low and high photon energy spectra are not intensity calibrated to each other therefore we are not claiming the concentration of the NH3+ and S‐ are the same at surface and bulk. In (a) at pH 9.5 the N1s experimental spectra are qualitatively reproduced by the theoretical calculations shown in Fig. S4 pH 9.5. In (b) the experimental spectra at the same pH are also qualitatively reproduced by the theoretical calculations shown in Fig. 5 pH 9.5. While the peak at lower binding energy is suppressed at surface representative XPS signal at the S2p ACS Paragon Plus Environment

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edge, the opposite takes place at N1s edge. Calculations and experiment are in agreement. See text for further discussion. S2p shifts Although the sulfur and nitrogen spectra give less information, they conform well with the contention drawn from the N spectra above. The experimental S2p spectra in Fig. 4 confirm the presence of the CS‐/CSH protonation states: the spectra for pH 1.0, 5.0 and 12.5 exhibit one component, as could be expected. There is no discernable shift between pH 1.0 and pH 5.0, meaning that the protonation of the carboxylic group does not affect the S2p BE – the localization of the protonation site with respect to the ionization site is thus crucial, as discussed above for the N spectra. This is presumably due to that CSH is too far away from the carboxylic group, in analogy with the above observation for the C1s band of the CSH group. We also note that according to the ESCA model the effect of an additional proton charge is inversely correlated to the distance between proton and photoionization site. For pH 9.5 the calculated spectra show an intensity shift in the double peak structure from the low to the high energy band when switching from bulk to surface, see Fig. 4. This indicates that the relative concentration of the protonated (SH) form increases in the surface region of this solution, in contrast with the findings in the nitrogen spectra. This aspect, apparently at odds with a naïve picture based on a different acidity of bulk and surface of an aqueous solution, has already been discussed in detail in our previous work 11. The calculated spectra at pH 9.5 reproduce well the experimental spectra from bulk and surface shown in Fig. 4. Going to lower pH, the surface structure collapses into the bulk structure, with only the protonated species. Finally, as for the N spectra, although not to the same extent, we note a salient difference between computed bulk and surface spectra, where the latter is broader and showing some double structure also for pH 5.0 and 12.5.


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The S spectra are complicated by the presence of two spin‐orbit split components. We note that in the computational plot we have assumed a constant spin‐orbit splitting (1.2 eV) of the S2p1/2 and S2p3/2 states and that their intensity assumes a degeneracy ratio of 1 to 2. The latter assumption seems to empirically hold for the here presented species, while indeed, that is not so in the general case of L shell photoionization spectra, even for second‐row species, due to the relativistic effect. Our calculations predict the COOH species to be equally present at pH 9.5 and 5.17. Experimentally the (COOH) contribution at pH 5.17 amounts to 12% while 19% at pH 9.5. This small discrepancy may be due to imprecision both in the experiment and calculation, but our experimental data provides a good reason for it. At pH 9.5 the species (1) [(NH3+) (S‐) (COOH)], (2) [(NH3+) (SH) (COOH)] and (3) [(NH2) (S‐) (COOH)] are present since the N1s and S2p spectra are compatible with the (NH3+), (NH2), SH and S‐ groups see Fig. 4. Species 1 is overall neutral and can make ion pair with themselves, which shall allow them to be surface enhanced. If both species 2 and 3 are present, they can also make ion pairs and increase their surface propensity. According to the experimental data at pH 12.5 species (1) and (2) cannot be present meaning no pair between (2) and (3) can be formed. At pH 5.17 again species (1) and (3) cannot be present see Fig.4 and S7 and not pairs between (2) and (3) can be formed. Consequently, all molecules (pairs of molecules) with the COOH group having the highest surface propensity are not present at pH12.5 and pH 5.17. Furthermore, the C1s spectrum at pH 9.5 is more surface representative, compared to the spectrum at 12.5 and pH 5.17 according to the normalized intensity of the C1s spectra at different pH, see Table S3. The C1s spectrum at pH 9.5 is the most surface representative while the spectrum at pH 5.17 is the least one. This lower experimental surface representativity can explain the small disagreement between experiment and calculation.


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Figure 5 Comparison of N‐1s (left) and S‐2p (right) computed photoemission spectra of cysteine in the bulk and surface regions of aqueous solutions with different pH, convoluted with a Gaussian function with FWHM=1.5 eV.

Comparison with Reaction Force Field Structures It is relevant to compare the present data obtained from chemical shift analysis with the results of the reactive force field calculations presented in our earlier paper 11. We observed that the values obtained for the bulk slice were quite close to the experimental ones, while for the surface slice, the percentage of the four species (all having a COO− group), that are the only components in bulk, summed up to only 64%. The remaining 36% was made up of "new" species all having the COOH group. We see that the present chemical shift analysis well conforms with the predicted changes in the distribution of protonated species from the bulk to surface in the cysteine solution at pH 9.5 11. The present reactive force field simulations confirm the presence on the surface, at any pH, of species with COOH groups, which, except for very low pH, are missing in the bulk. As in 11 it is also predicted ACS Paragon Plus Environment

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that the species of cysteine present in the bulk have concentrations in the surface region that cannot agree with any single value of pH. This was at variance with results of macroscopic titration curves analysis, which, however, as argued, cannot adequately describe the local chemical equilibrium at the surface of the solution. A microscopic description of the surface, as offered by the present molecular dynamics calculations and chemical shift analysis, predicts differences between bulk and surface This is of concern for evaluating the surface tension of growing water droplets containing cysteine and other amino acids, and thus for their impact on cloud formation and climate. Surface Acidity According to the discussion in our previous paper 11, the opposite trends observed in Fig. 4 (pH 9.5) for the spectral intensity going from bulk to surface, and well reproduced by the theoretical spectra in Fig. 5 (pH 9.5), cannot be explained by a change in surface acidity. If we, however, assume that molecules containing the ionic groups such as NH3+ or S‐ are depleted from the surface as much as similar cases reported in the literature recently 34 we can rationalize the observations. The cysteine species containing the S‐ species are depleted from the surface as compared to its neutral counterpart SH species. Fig. 4b (pH9.5) clearly shows this to be the case experimentally and reproduced by the calculations. The same reasoning can be used to the species containing the NH3+ ion which shall be depleted from the surface as compared to the neutral species NH2. Again this is what we observe in Fig. 4a. The C1s spectrum at 9.5, however, goes one step further in the sense that the cysteine molecules containing the charged group COO‐ are suppressed at the surface. This suppression is in line with the behavior of the other two charged species NH3+ or S‐; however, the neutral COOH which is not present in bulk at all shows its presence at the surface according to our experiment and theory. Such an extreme behavior was predicted theoretically in our previous paper 11 and is observed for the first time experimentally here. ACS Paragon Plus Environment

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Conclusions Motivated by the importance of amino acids solvated in atmospheric water droplets we present a study of cysteine in water solutions at different pH using a combined approach based on reactive force field molecular dynamics calculations and photoelectron spectroscopy measurements of chemical shifts. The measurements are complemented by DFT calculations for the assignment of the spectral features. For all three studied ionization centers, C1s, N1s, and S2p, we find with remarkable consistency that protonation close to the ionization site almost entirely decides the chemical shifts, making possible a clear‐cut analysis of the spectra with respect to protonation dependence. The carbon XPS spectra, although containing three chemically different carbons, could be analyzed in detail by the theoretical support; they follow a neat trend of deprotonation of the carbon‐containing groups with the increase of pH. The analysis of the shift spectra at the nitrogen and sulfur edges agrees with that of carbon in so much as that the close lying protonation site largely determines the chemical shifts. Both these spectra show a clear double peak structure, where the high energy peak refers to the protonated state and the low energy peak to the unprotonated state. For all three elements, we thus find that the XPS spectra for middle values of pH can be understood by the spectra of the two pure cases, here corresponding to the extreme pH values 1.0 and 12.5. Analyzing the intensity of the two bands at intermediate pH (9.5) for N1s and S2p, we see that the distribution of protonated versus deprotonated species comes out differently in bulk and at the surface. We can confirm, in agreement with ref. 11, the presence, in the surface region of an aqueous cysteine solution, of species with the protonated carboxylic group that are missing in the bulk. Likewise, we predict the occurrence at the surface of “low pH species”, containing NH3+ and SH groups, which do not exist in the bulk of medium/high pH solutions. We suggest that the variation of species population at the aqueous solution surface may have a direct impact on surface tension and thereby on the ACS Paragon Plus Environment

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growth of water droplets containing cysteine, and other amino acids, with an impact on cloud formation and climate.

Acknowledgment This research was supported by the Brazilian funding agencies CNPq (480967/2013‐0), FAP‐DF (193 140/95) and FINEP (1070/06). The Swedish‐Brazilian collaboration STINT‐CAPE (9805/2014‐01). V.C. is grateful for the contribution of the “Ministero della Istruzione della Universita’ e della Ricerca, Direzione Generale per la Internazionalizzazione della Ricerca" of the Republic of Italy. H.A. acknowledges the support from the Swedish Science Research Council (6212012‐3347). The authors gratefully acknowledge support from FAPESP (the Sao Paulo Research Foundation, Process 2017/11986‐5) and Shell and the strategic importance of the support given by ANP (Brazil’s National Oil, Natural Gas and Biofuels Agency) through the R&D levy regulation. Supporting Information Available: Table with experimental fitting parameters for the C1s, S2p and N1s. In addition several computed C1s spectra representing the surface and bulk regions and the structure of different

cysteine species is available free of charge via the Internet at http://pubs.acs.org.

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pH-Dependent X-ray Photoelectron Chemical Shifts and Surface

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