Dopamine, perhaps the simplest molecule that covalently links catechol and amine, together with its derivatives, has shown impressive adhesive and coating properties with its polymers. However, the scope of the molecules is rather limited, and the polymerization mechanisms are still elusive. We designed a general synthetic scheme and successfully synthesized a series of dopamine analogues with different alkyl chain lengths between the catechol and amine. Taking these new dopamine analogues, together with the molecular systems that have separate catechol and alkyl amine, we show that having both catechol and amine in the molecular system, whether covalently linked via an alkyl chain or not, is sufficient to polymerize under a similar reaction condition to that of dopamine polymerization. However, the time-dependent UV-vis characterization of the individual polymerization indicates that the polymerization for individual molecular systems likely proceeds via different reaction intermediates, depending on the length of the alkyl chain and whether there is a covalent linkage. Interestingly, whereas the covalent linkage via an alkyl chain is not necessary for showing the adhesive property, it is required to achieve the impressive coating property. Our results offer new insights into the design and synthesis of dopamine analogues for future applications, as well as a further mechanistic understanding of the polymerization of these dopamine analogues.

This work describes a quantitative high-throughput analytical method for the simultaneous measurement of small aliphatic nitrogenous biomarkers, i.e., 1,6-hexamethylenediamine (HDA), isophoronediamine (IPDA), β-methylamino-l-alanine (BMAA), and trimethylamine N-oxide (TMAO), in human urine. Urinary aliphatic diamines, HDA and IPDA, are potential biomarkers of environmental exposure to their corresponding diisocyanates. Urinary BMAA forms as a result of human exposure to blue-green algae contaminated food. And, TMAO is excreted in urine due to the consumption of carnitine- and choline-rich diets. These urinary biomarkers represent classes of small aliphatic nitrogen-containing compounds (N-compounds) that have a high aqueous solubility, low logP, and/or high basic pK. Because of the highly polar characteristics, analysis of these compounds in complex sample matrices is often challenging. We report on the development of ion-pairing chemistry based ultra-performance liquid chromatography-electrospray ionization-tandem mass spectrometry (UPLC-ESI-MS/MS) method for the simultaneous measurement of these biomarkers in human urine. Chromatographic separation was optimized using heptafluorobutyric acid-(HFBA-) based mobile phase and a reversed-phase C18 column. All four analytes were baseline separated within 2.6 min with an overall run time of 5 min per sample injection. Sample preparation involved 4 h of acid hydrolysis followed by automated solid phase extraction (SPE) performed using strong cation exchange sorbent bed with 7 N ammonia solution in methanol as eluent. Limits of detection ranged from 0.05 ng/mL to 1.60 ng/mL. The inter-day and intra-day accuracy were within 10%, and reproducibility within 15%. The method is accurate, fast, and well-suited for biomonitoring studies within targeted groups, as well as larger population-based studies such as the U. S. National Health and Nutrition Examination Survey (NHANES).

Benzene is a known genotoxic carcinogen linked to many hematological abnormalities. S-phenylmercapturic acid (PHMA, N-Acetyl-S-(phenyl)-L-cysteine, CAS# 4775-80-8) is a urinary metabolite of benzene and is used as a biomarker to assess benzene exposure. Pre-S-phenylmercapturic acid (pre-PHMA) is a PHMA precursor that dehydrates to PHMA at acidic pH. Published analytical methods that measure urinary PHMA adjust urine samples to a wide range of pH values using several types of acid, potentially leading to highly variable results depending on the concentration of pre-PHMA in a sample. Information is lacking on the variation in sample preparation among laboratories regularly measuring PHMA and the effect of those differences on PHMA quantitation in human urine samples. To investigate the differences in PHMA quantitation, we conducted an inter-laboratory comparison that included the analysis of 50 anonymous human urine samples (25 self-identified smokers, 25 self-identified non-smokers), quality control samples, and commercially available reference samples in five laboratories using different analytical methods to determine which sample preparation methods are currently in use and compare PHMA results. Observed urinary PHMA concentrations were proportionally higher at lower pH and results for anonymous urine samples varied widely among the methods. The method with the neutral preparation pH yielded results about 60% lower than the method using the most acidic conditions. Samples spiked with PHMA showed little variation, suggesting that the variability in results in human urine samples across methods is driven by the acid-mediated conversion of pre-PHMA to PHMA.

Liquid chromatography–electrospray ionization–tandem mass spectrometry (LC–ESI–MS-MS) with stable isotope-labeled internal standards (SIL-ISs) is the gold standard for quantitative analysis of drugs and metabolites in complex biological samples. Significant isotopic effects associated with deuterium labeling often cause the deuterated IS to elute at a different retention time from the target analyte, diminishing its capability to compensate for matrix effects. In this study, we systematically compared the analytical performance of deuterated (2H) SIL-IS to non-deuterated (13C and 15N) SIL-ISs for quantifying urinary 2-methylhippuric acid (2MHA) and 4-methylhippuric acid (4MHA), biomarkers of xylenes exposure, with an LC–ESI–MS-MS assay. Analytical method comparison between ISs demonstrated a quantitative bias for urinary 2MHA results, with concentrations generated with 2MHA-[2H7] on average 59.2% lower than concentrations generated with 2MHA-[13C6]. Spike accuracy, measured by quantifying the analyte-spiked urine matrix and comparing the result to the known spike concentration, determined that 2MHA-[2H7] generated negatively biased urinary results of −38.4%, whereas no significant bias was observed for 2MHA-[13C6]. Post-column infusion demonstrated that ion suppression experienced by 2MHA and 2MHA-[13C6] was not equally experienced by 2MHA-[2H7], explaining the negatively biased 2MHA results. The quantitation of urinary 4MHA results between ISs exhibited no significant quantitative bias. These results underscore the importance of the careful selection of ISs for targeted quantitative analysis in complex biological samples.