Neil Blough

Environmental Photochemistry and Optics:  The long-term goals of this work are to understand the factors controlling the distribution and dynamics of chromophore-containing natural dissolved organic matter (CDOM) in natural waters, as well as the structural basis of its optical properties. CDOM is a complex organic material found ubiquitously in aquatic systems where it plays a central role, not only through its impact on the aquatic light field and on the remote sensing of phytoplankton biomass by satellite ocean color sensors, but also through its photochemical reactions. Although the optical absorption and emission properties of CDOM have been known and studied extensively for over fifty years, no satisfactory explanation has yet been provided that can account for the long-wavelength absorption and emission behavior of these materials. Based on prior work, we proposed that this behavior results from intramolecular charge transfer interactions between hydroxy-aromatic donors and quinoid acceptors formed by the partial oxidation of soluble lignin precursors.1-5 The structural predictions of this charge transfer model, as well as predicted relationships between CDOM structure and its optical properties continue to be explored through the use of steady-state absorption and fluorescence spectroscopies, time-resolved fluorescence spectroscopy, ultra-high resolution Fourier Transform ion cyclotron resonance mass spectrometry (in collaboration E. Kujawinski at the Woods Hole Oceanographic Institution) and a variety of chemical analyses employing HPLC and GC.

We further proposed that the charge transfer model can largely explain the diverse suite of photochemical reactions attributed to this material. We are currently testing this hypothesis through a well-defined series of photochemical and photophysical measurements including: 1) the dependence of H2O2 quantum yields on O2 concentration and its associated wavelength dependence; 2) the dependence of the oxidation rates of selected (phenol) donors on both O2 concentration and donor concentration, as well as the associated wavelength dependencies; 3) the dependence of 1O2 yields on O2 concentrations; 4) the wavelength dependence of emission quantum yields and lifetimes.

References: 1) R. Del Vecchio, N.V. Blough, Environ. Sci. Technol. 2004, 38, 3885. 2) E.B. Kujawinski, R. Del Vecchio, N.V. Blough, G.C. Klein, A.G. Marshall, Mar. Chem. 2004, 92, 23. 3) J.V. Goldstone, R. Del Vecchio, N.V. Blough, B.M. Voelker, Photochem. Photobiol. 2004, 80, 52. 4) R. Del Vecchio, N.V. Blough, Mar. Chem. 2004, 89, 169. 5) R. Del Vecchio, N.V. Blough, Mar. Chem. 2002, 78, 231.

Fluorescence Probes for Radical Detection: Since their initial introduction by our group in the late 1980’s and early 1990’s,1 the use of prefluorescent nitroxide sensors to detect and quantify radical formation has expanded to include a wide range of applications. This approach utilizes stable nitroxide radicals as optical switches. By covalently coupling a nitroxide at a short distance from a chromophore, fluorescence emission from the chromophore can be largely quenched. Upon reaction of the nitroxide moiety with (usually carbon-centered) radicals to form diamagnetic products, the intramolecular quenching pathway is eliminated and fluorescence emission is greatly enhanced, thereby allowing radicals to be detected and quantified, either directly through changes in fluorescence intensity or indirectly through product analysis by HPLC. We continue to develop and apply these sensors to a variety of problems in chemistry and biology.2

References: 1) For example, a) N.V. Blough, D.J. Simpson, J.Am.Chem.Soc. 1988, 110, 1915. b) S.A.Green, D.J.Simpson, G.Zhou, P.S.Ho, N.V.Blough, J.Am.Chem.Soc, 1990, 112, 7337. c) D.J.Kieber, N.V.Blough, Anal.Chem. 1990, 62, 2275-2283. 2) Some recent examples, a) Q. Zhu, Y. Lian, S. Thyagarajan, S.E. Rokita, K.D. Karlin, N.V. Blough, J. Am. Chem. Soc .2008 130, 6304. b) D. Gan, M. Jia, P.P. Vaughan, D.E. Falvey, N.V. Blough, J. Phys. Chem. A  2008, 112, 2803. c) M. Alaghmand, N.V. Blough, Environ.Sci.Technol. 2007, 41, 2364.

Spectroscopic Approaches for Measuring Extra-cellular Enzymatic Activities: In collaboration with C. Arnosti at the University of North Carolina, we are examining a number of spectroscopic approaches for studying the dynamics and extra-cellular hydrolysis of macromolecules such as polysaccharides and proteins in natural waters. Both fluorescence spectroscopy1,2 and electron paramagnetic resonance spectroscopy3 have been employed to date.

References: 1) C. Arnosti, S.C. Keith, N.V. Blough, Mar. Chem. 2000, 71, 321. 2) A.D. Steen, P. Gururaj, J. Ma, N.V. Blough, C. Arnosti, 2008, submitted. 3) A.D. Steen, C. Arnosti, L. Ness, N.V. Blough, Mar. Chem. 2006, 101, 266.