Publications

Publications

10. Halye, Jeffrey L.; Rice, Charles, V. Cadmium chelation by bacterial teichoic acid from solid-state nuclear magnetic resonance spectroscopy. Biomacromolecules, In Press. [No Link]

An effective means of studying biological metal chemistry is through the use of cadmium NMR to probe the interaction between biomolecules, such as proteins and peptides, with divalent metals, such as zinc, copper, magnesium or calcium. Gram-positive bacteria, such as S. aureus and B. subtilis, have peptidoglycan cell walls which contain teichoic acids, a poly(phosphodiester) biopolymer used for, among other things, metal chelation. Previous solid-state NMR and XAFS studies have shown that the cadmium ion binds in a bidentate manner to the phosphoryl centers of the dried teichoic acid backbone at physiological pH. However, current studies indicate that, when hydrated and at the low concentrations typically found in nature, the cadmium ions and phosphoryl sites interact through an extended solvent-separated ion pairing. These data reveal two unequal P-Cd interactions at distances of 4.2 and 4.9 Å set approximately 180° from each other in a linear arrangement.

9. Garimella, Ravindranath; Halye, Jeffrey L.; Harrison, William;, Klebba, Phillip; Rice, Charles V. Conformation of the phosphate D-alanine zwitterion in bacterial teichoic acid from nuclear magnetic resonance spectroscopy. Biochemistry, 2009, 48 (39), 9242-9249. [DOI: 10.1021/bi900503k]

The conformation of D-alanine (D-Ala) groups of bacterial teichoic acid is a central, yet untested, paradigm of microbiology. The D-Ala binds via the C-terminus, thereby allowing the amine to exist as a free cationic NH₃⁺ group with the ability to form a contact-ion-pair with the nearby anionic phosphate group. This conformation hinders metal chelation by the phosphate because the zwitterion pair is charge neutral. To the contrary, the repulsion of cationic antimicrobial peptides (CAMPs) is attributed to the presence of the D-Ala cation; thus the ion-pair does not form in this model. Solid-state nuclear magnetic resonance (NMR) spectroscopy has been used to measure the distance between amine and phosphate groups within cell wall fragments of Bacillus subtilis. The bacteria were grown on media containing ¹⁵N D-Ala and β-chloroalanine racemase inhibitor. The rotational-echo double-resonance (REDOR) pulse sequence was used to measure the internuclear dipolar coupling and the results demonstrate: 1) the metal-free amine-to-phosphate distance is 4.4 Å and 2) the amine-to-phosphate distance increases to 5.4 Å in the presence of Mg²⁺ ions. As a result, the zwitterion exists in a nitrogen-oxygen ion-pair configuration providing teichoic acid with a positive charge to repel CAMPs. Additionally, the amine of D-Ala does not prevent magnesium chelation in contradiction to the prevailing view of teichoic acids in metal binding. Thus, the NMR-based description of teichoic acid structure resolves the contradictory models, advances the basic understanding of cell wall biochemistry, and provides possible insight into the creation of new antibiotic therapies.

8. Wickham, Jason R.; Halye, Jeffrey L.; Kashtanov, Stepan;, Khandogin, Jana; Rice, Charles V. Revisiting magnesium chelation by teichoic acids with phosphorus solid-state NMR and theoretical calculations. Journal of Physical Chemistry B, 2009, 113 (7), 2177-2183. [DOI: 10.1021/jp809313j]

Teichoic acids are essential components of the Gram-positive bacterial cell wall. One of their many functions is metal binding, a vital process for bacterial growth. With the combination of phosphorus-31 solid-state NMR spectroscopy and theoretical calculations using density functional theory (DFT), we have determined that the binding mode between teichoic acids and magnesium involves bidentate coordination by the phosphate groups of teichoic acid. Measurement of chemical shift anisotropy tensors gave a reduced anisotropy (δ) of 49.25 ppm and an asymmetry (η) of 0.7. DFT calculations with diglycerol phosphate and triglycerol diphosphate model compounds were completed with Mg²⁺ in anhydrous as well as partially hydrated bidentate and fully hydrated monodentate, bidentate, and bridging binding modes. ³¹P CSA tensors were calculated from the energy-minimized model compounds using the combined DFT and GIAO methods, resulting in dramatically different tensor values for each binding mode. The anhydrous bidentate chelation mode was found to be a good approximation of the experimental data, an observation that alters the current monodentate paradigm for metal chelation by teichoic acids.

7. Wickham, Jason R.; Rice, Charles V. Solid-state NMR studies of bacterial lipoteichoic acid adsorption on different surfaces. Solid State Nuclear Magnetic Resonance, 2008, 34, 154-161. [DOI: 10.1016/j.ssnmr.2008.06.001]

Teichoic acids are important to bacteria for surface adhesion, metal ion coordination, and other biological processes crucial to bacterial survival. In particular, the surface adhesion of teichoic acids plays a crucial role in the formation of Gram-positive biofilms. Biofilms have been implicated as the major cause of various chronic infections. Biofilm formation is essentially a four-step process beginning with the adhesion of bacteria to a surface, followed by the excretion of an extracellular polymeric substance (slime), development and maturation of the biofilm architecture, and finally biofilm spreading through bacterial release. Currently, there is very little molecular level information available for the initial adhesion of bacteria to solid surfaces. Solid-state NMR is ideally suited for the study of these samples, thus we use ³¹P solid-state NMR experiments to study the initial adhesion of lipoteichoic acid (LTA) to various surfaces. ³¹P CP-MAS spectra and T₁_ρ data demonstrate that the structure of LTA changes when adhered to cellulose, cell wall peptidoglycan (PGN), or TiO₂. However, when LTA is simultaneously adhered to PGN and TiO₂ the observed structure is dependent on the amount of retained water. For LTA on TiO₂, we suggest that the alanine and glucosamine groups interact with the surface. However, during simultaneous adhesion to TiO₂ and PGN, the glucosamine groups bind to the PGN while the alanine groups bind to the surface. This arrangement traps water between the PGN and TiO₂ surface.

6. Rice, Charles V.; Giffin, Guinevere A. Quantom dots in a polymer composite: a convenient particle-in-a-box laboratory experiment. Journal of Chemical Education, 2008, 85 (6), 842-844. [Link]

Semiconductor quantum dots are at the forefront of materials science chemistry with applications in biological imaging and photovoltaic technologies. We have developed a simple laboratory experiment to measure the quantum-dot size from fluorescence spectra. A major roadblock of quantum-dot based exercises is the particle synthesis and handling; the former requires dangerous chemicals while the later can lead to sample degradation. We overcome these drawbacks by using commercially available quantum dots. The samples are composed of CdSe–ZnS core–shell nanoparticles embedded in a polymer matrix. A fluorescence spectrophotometer is used to collect photoluminescence data and the subsequent calculations are straightforward. Student feedback indicates that these experiments provide a new understanding of quantum mechanics and the particle-in-a-box model. We are able to examine the role of spectroscopy in chemical investigations and a cause of spectroscopic line broadening.

5. Burba, Christopher M.; Carter, Shawn M.; Meyer, Kevin J.; Rice, Charles V. Salt effects on poly(N-isopropylacrylamide) phase transition thermodynamics from NMR spectroscopy. Journal of Physical Chemistry B, 2008, 112 (34), 10399-10404. [DOI: 10.1021/jp8005553]

NMR spectra were collected for cross-linked poly(N-isopropylacrylamide), poly(NIPAM), hydrogels in the presence of NaCl and CaCl₂ aqueous solutions. Intensity variations in the ¹H NMR signals of the polymer provide insight into the phase transition process. These data were used to observe a two-stage phase transition process. Thermodynamic quantities were obtained from a van’t Hoff analysis of the temperature-dependent equilibrium constants, which were derived from the NMR data. The ΔH° and ΔS° values for the hydrogel in D2O are 3.4 kJ/mol and 11.2 J/mol ·K for stage I, which is attributed to the formation of hydrophobic bonds between neighboring isopropyl groups. The formation of hydrogen bonds during stage II yielded ΔH° and ΔS° values of 14.8 kJ/mol and 48.4 J/mol ·K in D2O. However, the corresponding ΔH° values in 150 mM NaCl and 150 mM CaCl2 are reduced to 1.5 and 1.8 kJ/mol for stage I of the dehydration process. This corresponds to the known effect of salts on hydrophobic bond energetics. The value of ΔS° also decreased to 4.9 and 5.9 J/mol·K in NaCl and CaCl2 solutions, respectively. However, the thermodynamic values during stage II were only slightly affected by the salts. The lower temperatures required to induce spontaneous precipitation implies that ΔG° of precipitation is reduced. With our measurement of equilibrium thermodynamics, we see that 150 mM NaCl and CaCl2 solutions have a greater effect on hydrophobic bond formation associated with the phase transition process. In this manner, these salts aid in solvent reorganization necessary to form the hydrophobic bond, and this suggests that the formation of hydrophobic bonds is a strong determining factor in the stability of poly(NIPAM) hydrogels in water.

4. Wickham, Jason R.; Mason, Rachel N.; Rice, Charles V. Solid-state NMR studies of the crystalline and amorphous domains within PEO and PEO:LiTf systems. Solid State Nuclear Magnetic Resonance, 2007, 31, 184-192. [DOI: 10.1016/j.ssnmr.2007.05.001]

Solid polymer electrolytes (SPEs) contain amorphous and crystalline regions, each of which have unique contributions to the ¹³C NMR spectrum. Understanding and assigning the ¹³C NMR signals are vital to interpreting the NMR data collected for each phase. The ¹³C CPMAS solid-state NMR spectrum of poly(ethylene oxide), a common polymer electrolyte host material, has superimposed broad and narrow components. Previously, the narrow component has been assigned to the amorphous region and the broad component to the crystalline PEO fraction. These assignments for pure PEO have been applied to various PEO:salt systems. Using lithium triflate salt dissolved in PEO, we revisit the spectral assignments and discover that the narrow component is due to crystalline PEO:LiTf component, which is reversed from the previous pure PEO assignment. This paradigm shift is based on data collected from a 100% crystalline PEO:LiTf with a 3:1 oxygen:lithium ratio sample, which exhibited only the narrow peak. For dilute electrolytes, such as 20:1 PEO:LiTf, the ¹³C CPMAS spectra contain the narrow peak superimposed on a broad peak as seen with pure PEO. As dilute electrolytes are heterogeneous with crystalline and amorphous regions of both pure PEO and PEO:LiTf complex, peak assignments for pure PEO and PEO:LiTf are important. Thus, we reexamine the previous assignment for pure PEO using samples of pure powdered PEO, thermally treated pure powdered PEO, and a thin film PEO cast from an acetonitrile solution. With these different samples, we observed the growth of the narrow peak under conditions that favor crystallization. Therefore, for pure PEO, we have reassigned the narrow peak to the crystalline region and the broad peak to the amorphous region. In light of our observations, previous NMR studies of pure PEO and PEO SPEs should be reinvestigated. We also use rotational echo double resonance (REDOR) to study the 20:1 PEO:LiTf created from 2 and 100 kDa PEO. We find that the lithium environment is similar in the respective microcrystalline domains. However, the 100 kDa samples have a larger fraction of pure crystalline PEO.

3. Wickham, Jason R.; York, Shawna S.; Rocher, Nathalie M.; Rice, Charles V. Lithium environment in dilute poly(ethylene oxide)/lithium triflate polymer electrolye from REDOR NMR spectroscopy. Journal of Physical Chemistry B, 2006, 110 (10), 4538-4541. [DOI: 10.1021/jp060643m]

The role of the lithium ion environment is of fundamental interest regarding transport and conductivity in lithium polymer electrolytes. X-ray crystallography has been used to characterize the lithium environment in completely crystalline poly(ethylene oxide) (PEO) electrolytes, but this approach cannot be used with dilute PEO electrolytes. Here, using solid-state NMR data collected with the rotational-echo double-resonance ¹³C{⁷Li} (REDOR) pulse sequence, we have been able to characterize the crystalline microdomains of a PEO-lithium triflate sample with an oxygen/lithium ratio of 20:1. Our data clearly demonstrates that the lithium crystalline microdomains are nearly identical to those of a completely crystalline 3:1 sample, for which the crystal structure is known.

2. Rice, Charles V. Phase-transition thermodynamics of N-isopropylacryliamid hydrogels. Biomacromolecules, 2006, 7 (10), 2923-2925. [DOI: 10.1021/bm060607t]

1. Rice, Charles V.; Wickham, Jason R. Heterogeneous binding of lipoteichoic acid to the surface of titanium dioxide as determined with P solid-state spectroscopy. Journal of the American Chemical Society, 2005, 127 (3), 856-857. [DOI: 10.1021/ja043195+]

The adsorption of lipoteichoic acid onto the surface of titanium dioxide is shown to be heterogeneous. ³¹P CPMAS solid-state NMR reveals two distinct phosphate species. The chemical shift anisotropy, asymmetry parameter, and rotating-frame spin−lattice relaxation suggest that 50% of the phosphates are bound to the surface. The remaining phosphates also exhibit restricted molecular motion, but do not have a direct surface bond.