Proteins are built of amino acid residues that differ from each other only by their side chain. The pH-dependent response of these side chains on protein surface is vital for various biological processes. Here we have investigated the aqueous interface in the presence of different amino acid side chains at varying pH (bulk) using heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy. It is observed that amine/imidazolic (e.g., lysine/histidine) and alcoholic (e.g., serine and threonine) side chains preferentially orient the interfacial water as "H-down" (i.e., the water hydrogens are pointed toward the aqueous bulk) in acidic solution (pH ∼2). At physiological pH (7.4), the interfacial water takes "H-up" orientation (i.e., the water hydrogens are pointed away from the aqueous bulk) for the alcoholic/imidazolic side chains but remains "H-down" for the amine containing side chain. On further increasing the pH (up to 12.0), the interfacial water becomes increasingly H-up oriented, revealing the charging of the interface for all side chains investigated. Because the side chains are uncharged at high pH (12.0), the charging of the aqueous interface implies the adsorption of OH − anion at the interface. Moreover, in the case of imidazolic side chain (i.e., histidine), the flip-flop of interfacial water within a narrow range of pH 6.0−7.5 shows the reversal of interfacial electric field within a little variation of [H + ] (or [OH − ]) near the physiological pH. This observation provides direct experimental evidence in support of the hypothesis that the pH-dependent oxygen affinity of hemoglobin (Bohr effect) is due to changing electrostatics of histidine at the surface of hemoglobin.
Interaction
of fluoroalkyl with water is the key parameter tailoring
the applications of organofluorines in diverse fields. Nevertheless,
how fluorination modifies an alkyl group’s interaction with
water and the associated local hydrophobicity is largely unknown.
Using Raman difference spectroscopy with simultaneous curve fitting
(RD-SCF) and MD simulation, we show that fluorination breaks down
the tetrahedral water structure that is otherwise present around the
alkyl group (hydrophobic hydration). Electrostatic perturbation (due
to fluorination) restricts the water molecules to adopt the symmetrically
hydrogen-bonded tetrahedral structure. As a result, the relative population
of strongly H-bonded water decreases, while that of the weakly interacting
dangling OH increases in the hydration shell of a fluoroalkyl group.
These structural changes of water make the accommodation of fluoroalkyl
energetically costly compared to that of its hydrogenated counterpart,
which is manifested as enhanced hydrophobicity for the former.
Ionic perturbation of water has important implications in various chemical, biological and environmental processes. Previous studies revealed the structural and dynamical perturbation of water in the presence of ions, mainly with concentrated electrolyte solutions having significant interionic interactions. These investigations highlighted the need of selective extraction of the hydration shell water from a dilute electrolyte solution that is largely free from interionic interactions. Double-difference infrared (DDIR) and Raman multivariate curve resolution (Raman-MCR), as well as MD simulation, provided valuable insight in this direction, suggesting that the perturbed water mainly resides in the immediate vicinity of the ion, called the hydration shell. Recently, we have introduced Raman difference spectroscopy with simultaneous curve fitting (Raman-DS-SCF) analysis that can quantitatively extract the vibrational response of the perturbed water pertaining to the hydration shell of fully hydrated ions/solute. The DS-SCF analysis revealed novel hydrogen-bond (H-bond) structural features of hydration water, such as the existence of extremely weakly interacting water–OH (νmax ~ 3600 cm−1) in the hydration shell of high-charge-density metal ions (Mg2+, Dy3+). In addition, Raman-DS-SCF retrieves the vibrational response of the shared water in the water–shared-ion pair (WSIP), which is different from the hydration shell water of either the interacting cation and anion. Herein, we discuss the perturbation of water H-bonding in the immediate vicinity of cation, anion, zwitterion and hydrophobes and also the inter-ionic interactions, with a focus on the recent results from our laboratory using Raman-DS-SCF spectroscopy.
The
behavior of perfluorinated persistent organic pollutants (POPs), especially
perfluoroalkyl carboxylic and sulfonic acids, at aqueous interfaces
is crucial for their transport and speciation in the environment and
subsequent immunotoxicity. Here, we investigate the surface prevalence
and interfacial interaction of a prototype perfluorinated-POP, perfluoroheptanoic
acid (PFHA), with environmentally relevant amphiphiles of varying
hydrophobicity and head groups (C
n
H2n+1–X; n: 8 vs 16; −X: −OH vs −COOH)
using interface-selective vibrational sum frequency generation (VSFG)
spectroscopy. SFG intensity spectra in the CH- and OH-stretch regions
reveal that PFHA prevails at aqueous interfaces that contain amphiphiles
of intermediate chain length such as 1-octanol (n = 8) and heptanoic acid (n = 6). PFHA partially
expels as well as increases the alkyl chain order of octanol on the
water surface. Whereas heptanoic acid, though less hydrophobic than
octanol, is retained at the water surface through hydrogen-bonding
with the PFHA head group ((PFHA)COO–···HOOC(heptanoic‑acid)). Long chain amphiphiles (n = 16) such as hexadecanol and palmitic acid expel PFHA from the
water surface regardless of the difference in their head groups. Interestingly,
the dangling OH (3710 cm–1) which is diminished
at the hydrogenated-amphiphile–water interface is preserved
at the perfluorinated-POP–water interface.
Perfluoro compounds are widely used
in various manufacturing processes,
which leads to their bioaccumulation and subsequent adverse effects
on human health. Using interface-selective vibrational spectroscopy
(heterodyne-detected vibrational sum frequency generation (HD-VSFG)),
we have elucidated the molecular mechanism of the perturbation of
lipid monolayers on the water surface using a prototype perfluorinated
persistent organic pollutant, perfluoroheptanoic acid (PFHA). PFHA
disrupts the well-ordered all-trans conformation of a cationic lipid
(1,2-dipalmitoyl-3-trimethylammonium propane (DPTAP)) monolayer and
reduces the interfacial electric field at the lipid/water interface.
In contrast, the hydrophobic packing of an anionic lipid (1,2-dipalmitoyl-sn-glycero-3-phospoglycerol (DPPG)) monolayer remains largely
unaffected in the presence of PFHA, though the interfacial electric
field is reduced. For a zwitterionic lipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC))/water interface, both
alkyl chain ordering and interfacial electric field are fairly perturbed
by PFHA. Lipid headgroup-specific interaction of PFHA and the repulsive
interaction of oleophobic fluoroalkyl chain with the lipid alkyl chains
govern these distinct perturbations of the lipid monolayers on the
water surface.
Painters experience occupational exposure through inhalation
and
skin absorption to various chemicals that are used as ingredients
of paint mixtures and other related painting trades. Although several
investigations indicated significant contribution of exposure via
skin absorption to exhibit harmful effects on health among painters,
assessment of the skin absorption hazards of the paint chemicals is
limited. Here, we evaluated the skin absorption of a number of organic
chemicals relevant to painting trades using mathematical models. For
this purpose, we estimated the skin permeability coefficient of the
chemicals using the Potts and Guy correlation equation. The estimated
permeability coefficients were further utilized to estimate the maximum
flux of the non-volatile chemicals across the skin. The skin permeability
coefficient and maximum flux of the chemicals across the skin were
compared to those of the chemicals to which the American Conference
of Governmental Industrial Hygienists (ACGIH) assigned a “skin”
notation. We critically analyzed the estimated maximum fluxes and
the acute toxicity data of the chemicals available in the literature
that helped to identify the chemicals posing a significant skin absorption
hazard. The analyses suggest that triethanolamine and m-phenylenediamine pose significant skin absorption hazards, though
these chemicals have not yet been assigned a “skin”
notation in the ACGIH TLV book. The ratio of dermal uptake directly
from air to inhalation intake of volatile solvents used in paint mixtures
was estimated for a typical occupational setting. N-Methyl-2-pyrrolidone showed significant dermal uptake fraction compared
to its intake via inhalation route.
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