Riboflavin interactions with the chicken isolated carrier protein.

Riboflavin, a member of the B vitamin family, is a water-soluble vitamin that participates in energy metabolism processes via two coenzymes, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), in oxidized and reduced forms. Low levels of riboflavin have been associated with growth and developmental problems. In an effort to investigate the role of hydrogen bonding in the interactions between riboflavin and chicken riboflavin binding protein, the solid state geometry characteristics of a riboflavin derivative stripped of hydroxyl groups except the primary one, N-(6'-hydroxyhexyl)isoalloxazine, were investigated and found that π-stacking and hydrogen bonding involving the isoalloxazine rings are the primary intermolecular interactions. Subsequent comparative fluorescence studies showed that at neutral pH, in presence of the protein, quenching of N-(6'-hydroxyhexyl)isoalloxazine and riboflavin occurred similarly suggesting that the hydroxyl groups were not a key component of the vitamin protein interactions in the binding pocket.

The effect of the Covid-19 pandemic on STEM faculty: Productivity and work-life balance.

The disruption caused by the COVID-19 pandemic impacted STEM professionals in numerous ways, affecting research, teaching, publications, patents, and work-life balance. A survey was conducted to determine the changes approximately one year into the pandemic shutdown in USA. Results indicate that the quarantine, limitations, and restrictions led to decreased work productivity and increased stress, anxiety, and family obligations. There was a significant difference between male and female faculty experience with women reporting more child-care, schoolwork assistance, and care for elderly relatives.

Selectivity of labeled bromoethylamine for protein alkylation.

Alkylation of cysteine residues has been used extensively for characterization of proteins and their mode of action in biological systems, research endeavors that are at the core of proteomics. Treatment with a simple alkylating agent such as [2-(13)C] bromoethylamine would result in labeled thialysine at the ε-position. This chemical modification of proteins would allow investigations via both (13)C NMR spectroscopy and mass spectrometry. However [2-(13)C] labeled bromoethylamine is not available commercially. We investigated its synthesis at acid pH with the goal of obtaining singly labeled bromoethylamine and understanding the mechanistic details of the reaction. Based on our experimental and theoretical results, bromination of [2-(13)C] labeled ethanolamine in acidic conditions takes place via exclusive attack of the nucleophile (HBr) at the hydroxyl bearing C. Moreover, hydrogen bonding guides the nucleophilic attack, resulting in no label scrambling of the bromoethylamine product. Protein alkylation at cysteine residue with the synthesized Br(13)CH(2)CH(2)NH(2)-HBr is successful. Ab initio calculations in which CH(3)SH serves as a model for the cysteine residue suggest that in gas phase intermolecular attack by the sulfur bearing nucleophile is favored over the intramolecular substitution by the amino group by 15.4 kJ mol(-1). Solution modeling shows that the trend is preserved at basic pH, which is the experimental one, but is reversed at neutral pH.

Can hydridic-to-protonic hydrogen bonds catalyze hydride transfers in biological systems?

Catalysis of hydride transfer by hydridic-to-protonic hydrogen (HHH) bonding in α-hydroxy carbonyl isomerization reactions was examined computationally in the lithium salts of 7-substituted endo-3-hydroxybicyclo[2.2.1]hept-5-en-2-ones. The barrier for intramolecular hydride transfer in the parent system was calculated to be 17.2 kcal/mol. Traditional proton donors, such as OH and NH(3)(+), stabilized the metal cation-bridged transition state by 1.4 and 3.3 kcal/mol, respectively. Moreover, among the conformers of the OH systems, the one in which the proton donor is able to interact with the migrating hydride (H(m)) has an activation barrier lower by 1.3 and 1.7 kcal/mol than the other possible OH conformers. By contrast, the presence of an electronegative group such as F, which disfavors the migration electronically by opposing development of hydridic charge, destabilizes the hydride migration by 1.5 kcal/mol relative to the epimeric exo system. In both ground and transition states the H(m)···H distance decreased with increasing acidity of the proton donor, reaching a minimum of 1.58 Å at the transition state for NH(3)(+). Both Mulliken and NPA charges show enhancement of negative character of the migrating hydride in the cases in which HHH bonding is possible.

Structural reinvestigation of ammonium hypophosphite: was dihydrogen bonding observed long ago?

The past decade has seen the explosive emergence of "dihydrogen bonds", interactions between the electrons of M-H sigma-bonds, where M is less electronegative than H (M = Al, B, Ga, Ir, Mo, Mn, Os, Re, Ru, W) and traditional proton donors. But 70 years ago, such an interaction was proposed by Zachariasen and Mooney [J. Chem. Phys. 1934, 2, 34-37] on the basis of their single-crystal X-ray diffraction structure (heavy atoms positions only) of ammonium hypophosphite (NH(4)H(2)PO(2)). We redetermined this structure at high resolution with a focus on the hydrogen atoms, using a modern diffractometer equipped with a CCD detector. Molecular orbital calculations were performed to investigate the charge density and the bond polarity of the P-H bonds and to assess their potential for participation in dihydrogen bonds. Neither the theory nor the X-ray structure supports the original claim of H...H interactions in this salt.