Galina Borz

Thin film infrared spectroscopy on planar silver halide: a new technology for water and other liquids in the mesoscopic domain

The essentially unexplored phenomenon of electrostriction on polar solids is now exploited to restrict the motion of polar liquids in infrared spectroscopic experiments. We have found a wide variety of water oligomers for which theoretical properties had previously been calculated utilizing a recently developed cell for thin film infrared spectroscopy. Short acquisition times (to detect transient species) and low signal energies (less disruption of transient species) on planar silver halide (many reflections, limited penetration depth (therefore, small sample size), restraint of molecular motion by electrostriction (thus making possible capture of transient spectra)) were utilized to yield heretofore unknown spectroscopic properties of water. The results included identification of five oligomers (cyclic hexamers (chair, boat), cyclic pentamer and bicyclic books (1, 2)). The new technique of lifetime tagging was used to track these species, leading to the selection of marker peaks for specific oligomers. Each oligomer possessed groups of peaks in the symmetric OH stretching region, for which a new type of model, isomotomers, supported its identification. Intramolecular H-bonding in books 1 and 2 provided extra support for the assignments. Other polar liquids (N-methylformamide, dichloromethane) are briefly discussed. The tetrahedral character of water appears to result from an average over many linear oligomers.

N-methylformamide, a hyperplectic model for peptides in thin film infrared spectroscopy on planar AgX.

N-methylformamide (NMF), the simplest model for peptides, exhibits hyperplectic (both simple and complex) behavior as revealed by thin film infrared spectroscopy on planar AgX [AgCl:AgBr] fiber. IR spectra (0.1 s scans) of 10 microg NMF/dichloromethane(DCM) under N(2) flow first show NMF monomer, dimers, and trimers, which then form surface-organized NMF oligomers as pseudocrystals (P(n)) of increasing length and intensity to P(12). After 4 s, P(12) decays in 1.5 to 4 s steps via P(11), P(10), P(9), P(8), P(7), P(6), and P(5) to P(4) and P(3). The nature of P(n) (n = 5-12) is explained using a model based on the crystal structure of NMF and consisting of a matrix of 7 x 7 helices, alternating R(ight) and L(eft) with TDC (transition dipole coupling) in groups with 2, 3, or 4 neighbors. The total (10) dipolar couplings are matched to the 10 maxima of P(n) and prove the value of the model. P(4) (spectrum 325) fits a 5 x 5 matrix without corners. P(3) is transformed into the very weakly absorbing cyclic hexamer, shown to be very stable and swelling in DCM with increased intensity but without wavelength changes.

N-Methyl-trimethylacetamide in Thin Films Displays Infrared Spectra of π-Helices, with Visible Static and Dynamic Growth Phases, and then a β-Sheet

The simplest (minimal) peptide model is HCONHCH3. An increase in the π-helix content with increased substitution in the acyl portion suggested the examination of N-methyl-trimethylacetamide) (NMT). NMT displays spectra, in which there is evolution of a set of helices defined by their amide I maxima near 1686 (3(10)), 1655 (first π), and, most importantly, at 1637 cm(-1) (π). Expanded thin-film infrared spectroscopy (XTFIS) shows pauses or slow stages, which are identified as static phases followed by dynamic phases with the incremental gain or loss of a helix turn. In addition, absorbance at 1637 cm(-1) suddenly increases at 82.1 s (30% over 0.3 s), indicating a phase change and crystallization of the π-helix, along with a coincidental decrease in the absorbance for the first π-helix. A sharp peak occurs at the maximum of the phase change at 82.5 s, representing a pure NMT π-helix. The spectra then undergo a decreasing general absorption loss over 150 s, with the π-helix evolving further to an antiparallel β-sheet fragment. The spectral quality arises from the immobilization of polar molecules on polar surfaces. The crystal structure is that of an antiparallel β-sheet.

N-Alkylacylamides in Thin Films Display Infrared Spectra of 310-, α-, and π-Helices with Visible Static and Dynamic Growth Phases

A peptide model is a physical system containing a CONH group, the simplest being HCONHCH3 , N-methylformamide (NMF). We have discovered that NMF and N-methylacetamide (NMA), which form hydrogen-bonded oligomers in thin films on a planar AgX fiber, display infrared (IR) spectra with peaks like those of polypeptide helices. Structures can be assigned by their amide I maxima near 1672 (3(10)), 1655 (3(10)), 1653 (α), 1655 (π), and 1635 cm(-1) (π), which are the first IR data for the π-helix. Sharp peaks are an outcome of immobilization of polar species on the polar surface of silver halides. We report the first use of expanded thin-film IR spectroscopy, in which plots of every spectrum over the amide I-II range show pauses or slow stages in the increase or decrease of absorption. These are identified as static phases followed by dynamic phases, with the incremental gain or loss of a helix turn. A general theory can be stated for such processes. Density functional calculations show that the NMA α-helix pentamer (crystal structure geometry) is transformed into a π-helix-like form. For the first time, an entire sequence (3(10)-helix, α-helix, π-helix, quasiplanar species) of spectra has been recorded for NMA.

Neutral dipole-dipole dimers: A new field in science

Dimer formation with dipole neutralization produces species such as low polarity water (LPW) compatible with hydrophobic surfaces (Phys. Chem. Chem. Phys. 2015, 17, 24895-24900) Dimerization and dipole neutralization occurs for N-methylacetamide on polyethylene, a behavior drastically different from its contortions in acetonitrile on AgBr:AgCl planar crystals (AgX) (ChemPhysChem 2014, 15, 3598-3607). The weak infrared absorption of the amide dimer on polyethylene is shown experimentally. Dimerization of palmitic acid is shown along with some of the many ramifications for intracellular systems. Polyoligomers of water are present on polyethylene surfaces. Some high resolution spectra of three of the polyoligomers of water are shown along with a mechanistic scheme for polyoligomer formation and dissolution. The structures of some of the oligomers are known from spectroscopic studies of water on AgX.