Design of a local quasi-distributed tuning and matching circuit for dissolution DNP cross polarization.

Dynamic nuclear polarization (DNP) build-up times at low temperature for low-gamma nuclei can be unfavorably long and can be accelerated by transfer of polarization from protons. The efficiency of the cross polarization (CP) depends on the B1-field strengths, the pulse sequence chosen for cross polarization and the sample composition. CP experiments rely on high B1-fields, which typically lead to electrical discharge and breakdown in the circuit. This problem is particularly severe in the low pressure helium atmosphere due to easily ionized helium atoms. The purpose of this study is to identify strategies to minimize voltages across components in a tuning and matching circuit of the coil to avoid electrical discharge during CP experiments. Design equations for three tuning and matching network configurations are derived. The results of the study are then used in the design of a single coil double resonance DNP probe operating at 71.8 MHz (13C frequency) and 285.5 MHz (1H frequency). In the current setup we achieve 28% polarization on 13C in urea with a build-up time of 11.6 min with CP compared to 14% and 53 min by direct polarization using TEMPOL as the radical. Different cross polarization sequences are compared.

Optimized microwave delivery in dDNP.

Dissolution dynamic nuclear polarization (dDNP) has permitted the production of highly polarized liquid-state samples, enabling real-time imaging of metabolic processes non-invasively in vivo. The desire for higher magnetic resonance sensitivity has led to the development of multiple home-built and commercial dDNP polarizers employing solid-state microwave sources. Providing efficient microwave delivery that avoids unwanted heating of the sample is a crucial step to achieve high nuclear polarization. Consequently, a process is described to reduce waveguide attenuation due to resistive loss thereby doubling the delivered power. A mirror and reflector are designed and tested to increase the microwave field density across the sample volume resulting in a 2.3 dB increase of delivered power. Thermal considerations with regards to waveguide geometry and dDNP probe design are discussed. A thermal model of the dDNP probe is computed and experimentally verified.

Compact, low-cost NMR spectrometer and probe for dissolution DNP.

The desire for higher magnetic resonance sensitivity has led to the development of multiple home-built and commercial dissolution dynamic nuclear polarization polarizers. The emergence of polarizers capable of variable magnetic field strengths desires a versatile standalone spectrometer and NMR circuit to fulfill detection needs at different frequencies. We present a benchtop NMR spectrometer with a duplexer capable of serving high-field solid and liquid state NMR applications up to 450 MHz. A detailed view of the employed probe is discussed. Tuning and matching schemes are investigated yielding and experimentally verifying closed-form equations to estimate the nutation frequency for a remotely tuned and matched sample coil.

Identification and characterization of a new antifungal peptide in fermented milk product containing bioprotective Lactobacillus cultures.

Mold and yeast contamination constitutes a major problem in food commodities, including dairy products, hence new natural preventive measures are in high demand. The aim of the current study is to identify and characterize novel antifungal peptides produced by lactic acid bacteria (LAB) in sour cream. By the use of a newly developed image-based 96-well plate fungal growth inhibition assay targeting Debaryomyces hansenii, combined with a range of analytical tools comprising HPLC-high-resolution mass spectrometry, ultrahigh-performance liquid chromatography-Triple Quadrupole MS and nuclear magnetic resonance spectroscopy, we successfully identified a new antifungal peptide (DMPIQAFLLY; 1211 Da) in sour cream enriched with two bioprotective LAB strains. This peptide represents a fragment of casein, the most abundant protein in milk. Presumably, the proteolytic activity of these bioprotective strains results in the observed 4-fold higher concentration of the peptide during storage. Both bioprotective strains are able to generate this peptide in concentrations up to 0.4 µM, independently of the sour cream starter culture employed. The peptide attenuates the growth rate of D. hansenii at concentrations ≥35 µM, and results in smaller cells and more compact colonies. Hence, the peptide is likely contributing to the overall preserving effect of the investigated bioprotective LAB strains.