Browsing by Author "Banyikwa, Andrew Toyi"

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    Anhydrous monoalkylguanidines in aprotic and nonpolar solvents: models for deprotonated arginine side chains in membrane environments
    (ACS Publications, 2017) Banyikwa, Andrew Toyi; Miller, Stephen E.; Krebs, Richard A.; Xiao, Yuewu; Carney, Jeffrey M.; Braiman, Mark S.
    In this study, the synthesis of crystalline dodecylguanidine free base and its spectroscopic characterization in nonpolar environments are described. IR as well as 1H and 15N NMR spectra of the free base dissolved in aprotic solvents are substantially different from the previously reported spectra of arginine, or other monoalkylguanidinium compounds, at high hydroxide concentrations. The current results provide improved modeling for the spectroscopic signals that would be expected from a deprotonated arginine in a nonpolar environment. On the basis of our spectra of the authentic dodecylguanidine free base, addition of large amounts of aqueous hydroxide to arginine or other monoalklyguanidinium salts does not deprotonate them. Instead, hydroxide addition leads to the formation of a guanidinium hydroxide complex, with a dissociation constant near ∼500 mM that accounts for the established arginine pK value of ∼13.7. We also report a method for synthesizing a compound containing both phenol and free-base guanidine groups, linked by a dodecyl chain that should be generalizable to other hydrocarbon linkers. Such alkyl-guanidine and phenolyl-alkyl-guanidine compounds can serve as small-molecule models for the conserved arginine–tyrosine groupings that have been observed in crystallographic structures of both microbial rhodopsins and G-protein-coupled receptors.
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    Experimental and computational modeling of H‑Bonded arginine−tyrosine groupings in aprotic environments
    (ACS Publications, 2017) Banyikwa, Andrew Toyi; Goos, Alan; Kiemle, David J.; Foulkes, Michael A. C.; Braiman, Mark S.
    H-bonds between neutral tyrosine and arginine in nonpolar environments are modeled by small-molecule phenol/guanidine complexes. From the temperature and concentration dependence of UV spectra, a value of ΔH° = −74 ± 4 kJ mol–1 is deduced for the formation of H-bonded p-cresol/dodecylguanidine in hexane. ΔE = −71 kJ mol–1 is computed with density functional theory (in vacuo). In dimethyl sulfoxide or crystals, (p-phenolyl)alkylguanidines form head-to-tail homodimers with two strong H-bonding interactions, as evidenced by UV, IR, and NMR spectral shifts, strong IR continuum absorbance bands, and short O···N distances in X-ray crystal structures. Phenol/alkylguanidine H-bonded complexes consist of polarizable rapidly interconverting tautomers, with the proton shift from phenol to guanidine increasing with increase in the polarity of the aprotic solvent. As measured by NMR, both groups in these strongly H-bonded neutral complexes can simultaneously appear to be predominantly protonated. These systems serve as models for the hypothetical hydrogen-Bonded Uncharged (aRginine + tYrosine), or “BU(RY)”, motifs in membrane proteins.
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    Geochemistry and sources of fluoride and nitrate contamination of groundwater in six districts of the Dodoma region in Tanzania.
    (Elsevier BV, 2023) Banyikwa, Andrew Toyi
    Fluoride (F−) and nitrate (NO3−) are common groundwater contaminants. In high concentrations, F− and NO3− cause skeletal fluorosis and methemoglobinemia in children respectively. Therefore protection of public health necessitates the evaluation of groundwater for F− and NO3− contamination. The goal of this study was to assess the geochemistry and spatial distribution of F− and NO3− in six districts in the Dodoma region. Results show that greater than 98% of samples were found to contain F− below 1.50 mgl−1 (WHO 2017). F− Levels exist in the gradient and concentrated to the north of the region. The highest and lowest levels of F− were found in Chemba district, and Mpwapwa district. In the Chemba district, values for F− are (max= 7.30 mgl−1, min = 0.11 mgl−1, mean and SD = 0.87±1.67). In the Mpwapwa district, values for F− are (max= 0.80 mgl−1, min = 0.06 mgl−1, mean and SD= 0.36±0.23). Positive correlation between F− and HCO3− / Ca2+ +Mg+2 and negative correlation between F− and TH, F− and Ca2++Mg2+, and F− and Ca2++Mg2+/Na++K+ suggests HCO3− promote dissolution of F containing rocks while Ca2++Mg2+ removes F− through precipitation reactions. Elevated levels of NO3− were found throughout the districts. The order of districts with NO3− levels above 45.0 mgl−1 was found to be Kongwa (94.74%) > Chemba (69.23%) > Dodoma urban (34.80%) > Chamwino (28.57%) > Mpwapwa (6.60%) and Kondoa (0.00%). Kongwa and Kondoa districts are located south and north of the region. The values in Kongwa are; (max= 196.8 mgl−1, min = 12.5 mgl−1, mean and SD= 137.02±48.47). Values in Kondoa are; (max= 28.00 mgl−1, min = 12.00 mgl−1, mean and SD= 19.97±4.61). Conclusions; groundwater in the Dodoma region safe for drinking with respect to F− and unsafe for drinking with respect to NO3− without remediation.
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    Geochemistry and sources of fluoride and nitrate contamination of groundwater in six districts of the Dodoma region in Tanzania.
    (Elsevier BV, 2023) Banyikwa, Andrew Toyi
    Fluoride (F−) and nitrate (NO3−) are common groundwater contaminants. In high concentrations, F− and NO3− cause skeletal fluorosis and methemoglobinemia in children respectively. Therefore protection of public health necessitates the evaluation of groundwater for F− and NO3− contamination. The goal of this study was to assess the geochemistry and spatial distribution of F− and NO3− in six districts in the Dodoma region. Results show that greater than 98% of samples were found to contain F− below 1.50 mgl−1 (WHO 2017). F− Levels exist in the gradient and concentrated to the north of the region. The highest and lowest levels of F− were found in Chemba district, and Mpwapwa district. In the Chemba district, values for F− are (max= 7.30 mgl−1, min = 0.11 mgl−1, mean and SD = 0.87±1.67). In the Mpwapwa district, values for F− are (max= 0.80 mgl−1, min = 0.06 mgl−1, mean and SD= 0.36±0.23). Positive correlation between F− and HCO3− / Ca2+ +Mg+2 and negative correlation between F− and TH, F− and Ca2++Mg2+, and F− and Ca2++Mg2+/Na++K+ suggests HCO3− promote dissolution of F containing rocks while Ca2++Mg2+ removes F− through precipitation reactions. Elevated levels of NO3− were found throughout the districts. The order of districts with NO3− levels above 45.0 mgl−1 was found to be Kongwa (94.74%) > Chemba (69.23%) > Dodoma urban (34.80%) > Chamwino (28.57%) > Mpwapwa (6.60%) and Kondoa (0.00%). Kongwa and Kondoa districts are located south and north of the region. The values in Kongwa are; (max= 196.8 mgl−1, min = 12.5 mgl−1, mean and SD= 137.02±48.47). Values in Kondoa are; (max= 28.00 mgl−1, min = 12.00 mgl−1, mean and SD= 19.97±4.61). Conclusions; groundwater in the Dodoma region safe for drinking with respect to F− and unsafe for drinking with respect to NO3− without remediation.