Showing papers by "Nicholas J. Turro published in 2014"

Symmetry-breaking in the endofullerene H2O@C60 revealed in the quantum dynamics of ortho and para-water: a neutron scattering investigation

Abstract: Inelastic neutron scattering (INS) has been employed to investigate the quantum dynamics of water molecules permanently entrapped inside the cages of C60 fullerene molecules. This study of the supramolecular complex, H2O@C60, provides the unique opportunity to study isolated water molecules in a highly symmetric environment. Free from strong interactions, the water molecule has a high degree of rotational freedom enabling its nuclear spin isomers, ortho-H2O and para-H2O to be separately identified and studied. The INS technique mediates transitions between the ortho and para spin isomers and using three INS spectrometers, the rotational levels of H2O have been investigated, correlating well with the known levels in gaseous water. The slow process of nuclear spin conversion between ortho-H2O and para-H2O is revealed in the time dependence of the INS peak intensities over periods of many hours. Of particular interest to this study is the observed splitting of the ground state of ortho-H2O, raising the three-fold degeneracy into two states with degeneracy 2 and 1 respectively. This is attributed to a symmetry-breaking interaction of the water environment.

Label-free probe of HIV-1 TAT peptide binding to mimetic membranes.

Abstract: The transacting activator of transduction (TAT) protein plays a key role in the progression of AIDS. Studies have shown that a +8 charged sequence of amino acids in the protein, called the TAT peptide, enables the TAT protein to penetrate cell membranes. To probe mechanisms of binding and translocation of the TAT peptide into the cell, investigators have used phospholipid liposomes as cell membrane mimics. We have used the method of surface potential sensitive second harmonic generation (SHG), which is a label-free and interface-selective method, to study the binding of TAT to anionic 1-palmitoyl-2-oleoyl- sn -glycero-3-phospho-1′- rac -glycerol (POPG) and neutral 1-palmitoyl-2-oleoyl- sn -glycero-3-phosphocholine (POPC) liposomes. It is the SHG sensitivity to the electrostatic field generated by a charged interface that enabled us to obtain the interfacial electrostatic potential. SHG together with the Poisson–Boltzmann equation yielded the dependence of the surface potential on the density of adsorbed TAT. We obtained the dissociation constants K d for TAT binding to POPC and POPG liposomes and the maximum number of TATs that can bind to a given liposome surface. For POPC K d was found to be 7.5 ± 2 μM, and for POPG K d was 29.0 ± 4.0 μM. As TAT was added to the liposome solution the POPC surface potential changed from 0 mV to +37 mV, and for POPG it changed from −57 mV to −37 mV. A numerical calculation of K d , which included all terms obtained from application of the Poisson–Boltzmann equation to the TAT liposome SHG data, was shown to be in good agreement with an approximated solution.

Macromonomers for preparation of degradable polymers and model networks

Abstract: The present invention relates to methods for preparing degradable model networks from any monomer functionality with any degradation methodology. It is based on the use of Atom-Transfer Radical Polymerization and CLICK chemistry to form the desired product.

Photochemical studies of a fluorescent chlorophyll catabolite – source of bright blue fluorescence in plant tissue and efficient sensitizer of singlet oxygen

Abstract: Fluorescent chlorophyll catabolites (FCCs) are fleeting intermediates of chlorophyll breakdown, which is seen as an enzyme controlled detoxification process of the chlorophylls in plants. However, some plants accumulate large amounts of persistent FCCs, such as in senescent leaves and in peels of yellow bananas. The photophysical properties of such a persistent FCC (Me-sFCC) were investigated in detail. FCCs absorb in the near UV spectral region and show blue fluorescence (max at 437 nm). The Me-sFCC fluorescence had a quantum yield of 0.21 (lifetime 1.6 ns). Photoexcited Me-sFCC intersystem crosses into the triplet state (quantum yield 0.6) and generates efficiently singlet oxygen (quantum yield 0.59). The efficient generation of singlet oxygen makes fluorescent chlorophyll catabolites phototoxic, but might also be useful as a (stress) signal and for defense of the plant tissue against infection by pathogens.

DNA sequencing by synthesis using 3′-O-azidomethyl nucleotide reversible terminators and surface-enhanced Raman spectroscopic detection

Abstract: As an alternative to fluorescence-based DNA sequencing by synthesis (SBS), we report here an approach using an azido moiety (N3) that has an intense, narrow and unique Raman shift at 2125 cm−1, where virtually all biological molecules are transparent, as a label for SBS. We first demonstrated that the four 3′-O-azidomethyl nucleotide reversible terminators (3′-O-azidomethyl-dNTPs) displayed surface enhanced Raman scattering (SERS) at 2125 cm−1. Using these 4 nucleotide analogues as substrates, we then performed a complete 4-step SBS reaction. We used SERS to monitor the appearance of the azide-specific Raman peak at 2125 cm−1 as a result of polymerase extension by a single 3′-O-azidomethyl-dNTP into the growing DNA strand and disappearance of this Raman peak with cleavage of the azido label to permit the next nucleotide incorporation, thereby continuously determining the DNA sequence. Due to the small size of the azido label, the 3′-O-azidomethyl-dNTPs are efficient substrates for the DNA polymerase. In the SBS cycles, the natural nucleotides are restored after each incorporation and cleavage, producing a growing DNA strand that bears no modifications and will not impede further polymerase reactions. Thus, with further improvements in SERS for the azido moiety, this approach has the potential to provide an attractive alternative to fluorescence-based SBS.

1H NMR Study of Hydrogen Abstraction in Model Compound Mimics of Polymers

Abstract: Small molecules representing common synthetic polymers were subjected to photochemically induced hydrogen abstraction by benzophenone. Reactions were monitored using 1 H NMR to query the factors that influence preferential abstraction of protons in unique chemical environments. Differences in bond dissociation energies do not fully explain the observed hydrogen abstraction preferences. To that end, we identify contributions to abstraction from prereactive complexation, radical stability, steric effects and charge transfer effects. Using representative small molecule model compounds in lieu of polymeric materials is a novel approach to understanding photochemical reaction in polymers; however, it cannot probe the contributions of macromolecular effects—e.g. polymer rigidity or side chain and backbone mobility—to preferential hydrogen abstraction.

Photolysis of endoperoxides in the presence of nitroxides: a laser flash photolysis study with optical and ESR detection

Abstract: Time-resolved electron paramagnetic resonance spectroscopy, transient absorption, and phosphorescence spectroscopy were used to investigate the spin polarization of a nitroxide free radical induced by interaction with singlet oxygen (1O2). The latter was generated by photolysis of endoperoxides of two anthracene derivatives. Although both anthracene endoperoxides are structurally similar, opposite spin polarization of the nitroxide was observed. Photolysis of one endoperoxide leads to absorptive nitroxide spin polarization due to interaction with the generated 1O2. Photolysis of the other endoperoxide generated emissive nitroxide spin polarization, probably due to interaction of the endoperoxide triplet states with nitroxides.

Interaction of H2@C60 and Nitroxide through Conformationally Constrained Peptide Bridges

Abstract: We synthesized two molecular systems, in which an endofullerene C60, incarcerating one hydrogen molecule (H2@C60) and a nitroxide radical are connected by a folded 310-helical peptide. The difference between the two molecules is the direction of the peptide orientation. The nuclear spin relaxation rates and the para ? ortho conversion rate of the incarcerated hydrogen molecule were determined by 1 H NMR spectroscopy. The experimental results were analyzed using DFT-optimized molecular models. The relaxation rates and the conversion rates of the two peptides fall in the expected distance range. One of the two peptides is particularly rigid and thus ideal to keep the H2@C60/nitroxide separation, r ,a s large and controlled as possible, which results in particularly low relaxation and conversion rates. Despite the very similar optimized distance, however, the rates measured with the other peptide are considerably higher and thus are compatible with a shorter effective distance. The results strengthen the outcome of previous investigations that while the para ? ortho conversion rates satisfactorily obey the Wigner’s theory, the nuclear spin relaxation rates are in excellent agreement with the Solomon–Bloembergen equation predicting a 1/r 6 dependence.

Aliphatic β-nitroalcohols for therapeutic corneoscleral cross-linking: chemical stability studies using 1H-NMR spectroscopy.

Abstract: Recent studies suggest that aliphatic β-nitro alcohols may represent a useful class of compounds for use as in vivo therapeutic corneoscleral cross-linking agents with higher order nitroalcohols (HONAs) showing enhanced efficacy over the mono-nitroalcohols. The current study was undertaken in order to evaluate the chemical stability of these compounds during storage conditions. Two mono-nitroalcohols (2-nitroethanol=2ne and 2-nitro-1-propanol = 2 nprop) and two HONAs, a nitrodiol (2-methyl-2-nitro-1,3-propanediol = MNPD), and a nitrotriol (2-hydroxymethyl-2-nitro-1,3-propanediol = HNPD) were monitored for chemical stability by (1)H-NMR for up to 7 months. Each compound was studied at two concentrations (1% and 10%) either in unbuffered H2O or 0.2 m NaH2 PO4/Na2 HPO4 (pH = 5), and at 0 °C and room temperature (RT) for a total of eight conditions for each compound. The (1)H-NMR spectra for the starting material were compared to subsequent spectra. Under all four of the conditions studied, both the nitrodiol (MNPD) and nitrotriol (HNPD) were stable for the duration of 7 months. 2nprop became unstable under all conditions at 3 months. 2ne was the most unstable of all the compounds tested. HONAs exhibit excellent chemical stability under long-term storage conditions. In contrast, the nitromonols tested are significantly less stable. These findings are relevant to the translation of this technology into clinical use.