Multi-Analytical Approach to Characterize the Degradation of Different Types of Microplastics: Identification and Quantification of Released Organic Compounds

Microplastics and nanoplastics represent one of the major environmental issues nowadays due to their ubiquitous presence on Earth, and their high potential danger for living systems, ecosystems, and human life. The formation of both microplastics and nanoplastics strongly depends on both the type of pristine materials and the degradation processes related to biological and/or abiotic conditions. The aim of this study is to investigate the effect of two of the most relevant abiotic parameters, namely temperature and light, taken under direct control by using a Solar box, on five types of reference polymers: high density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET). A multi-analytical approach was adopted to investigate in detail the first steps of plastics degradation. Samples of plastic materials at different degradation times were analyzed by means of ¹H NMR spectroscopy and thermal desorption gas chromatography mass spectrometry (TD-GC-MS) technique. Several minor molecular species released during degradation were consistently identified by both techniques thus providing a comprehensive view of the various degradation products of these five types of microplastics.

NMR internal standard signal shifts due to cyclodextrin inclusion complexes

Nuclear magnetic resonance (NMR)-based screening of materials is a powerful tool for quality control, authenticity testing, and purity testing of compounds. However, reliance on 3-(trimethylsilyl)-propane-1-sulfonate (DSS) and 3-(trimethylsilyl)propanoic acid (TMSP) for referencing the spectra of aqueous samples is not without hazard, particularly with automated analyses. The assumption that these reference signals always represent 0 ppm is ubiquitous in NMR spectroscopy and is routinely used for spectral alignment. However, it has been found that cyclodextrins readily generate inclusion complexes with DSS and TMSP with the effect of rendering this assumption incorrect. These inclusion complexes alter the electronic shielding of the trimethylsilane functional groups on DSS and TMSP yielding a small, but significant, shift to a higher frequency in the signal relied upon for spectral referencing. As a result, samples containing traces of these compounds may be incorrectly declared fraudulent, inconsistent with standards, or adulterated. In order to maintain the viability of this screening method, vigilance and/or improved referencing of spectra is needed.

Tertiary hydrogen bonds in the solution structure of transfer RNA

The high resolution nuclear magnetic resonance (NMR) spectra of hydrogen-bonded protons in four tRNAs have been studied at 270 MHz. The relative intensity of the resonances between -11 ppm and -15 ppm of Escherichia coli tRNA1-Va1 indicate that there are 26 plus or minus 3 protons, while only 20 are expected from secondary structure Watson-Crick hydrogen bonds inthe cloverleaf structure. Several possible candidates for these extra resonances are suggested by tertiary interactions observed in recent crystallographic studies. Of the four tRNAs studied, three, e.g., E. coli tRNA1Va1, E. coli tRNA-Arg and E. coli tRNA-Phe have one "GU pair" in their cloverleaf structure, while the fourth, yeast tRNA-Asp,has three "GU pairs" and one "G pair". Correlating these with the NMR spectra in the -10 ppm to -11 ppm region allows us to conclude that the "GU pairs" are not hydrogen-bonded by tautomerization to the lactim form. At the very low field region, near -14.9 ppm, the three E. coli tRNAs show a single resonance which is attributed to the 4-thiouracil 8 to adenine 14 hydrogen bond of the tertiary structure, by analogy with the recent crystal structure of yeast tRNA-Phe. This assignment is confirmed by the disappearance of this resonance after treatment with cyanogen bromide.