The pandemic of the coronavirus disease (COVID-19) caused by SARS-CoV-2 affects millions of people worldwide. There are still many unknown aspects to this infection which affects the whole world. In addition, the potential impacts caused by this infection are still unclear. Amino acid metabolism, in particular, contains significant clues in terms of the development and prevention of many diseases. Therefore, this study aimed to compare amino acid profile of COVID-19 and healthy subject. In this study, the amino acid profiles of patients with asymptomatic, mild, moderate, and severe/critical SARS-CoV-2 infection were scanned with LC–MS/MS. The amino acid profile encompassing 30 amino acids in 142 people including 30 control and 112 COVID-19 patients was examined. 20 amino acids showed significant differences when compared to the control group in COVID-19 patient groups with different levels of severity in the statistical analyses conducted. It was detected that the branched-chain amino acids (BCAAs) changed in correlation with one another, and l -2-aminobutyric acid and l -phenylalanine had biomarker potential for COVID-19. Moreover, it was concluded that l -2-aminobutyric acid could provide prognostic information about the course of the disease. We believe that a new viewpoint will develop regarding the diagnosis, treatment, and prognosis as a result of the evaluation of the serum amino acid profiles of COVID-19 patients. Determining l -phenylalanine and l -2-aminobutyric levels can be used in laboratories as a COVID-19-biomarker. Also, supplementing COVID patients with taurine and BCAAs can be beneficial for treatment protocols. Supplementary Information The online version contains supplementary material available at 10.1007/s00726-021-03081-w.

Geographic conditions (altitude, climate, and local flora) lead to significant differences in the chemical composition of propolis. Therefore, more research is needed for propolis in different geographical regions. So, the aim of this study was to evaluate the phenolic profile, total phenolic content, antioxidant, and antidiabetic properties of Pülümür propolis from Turkey. Methanol (MeOH), chloroform (CHCl3), and hexane extracts of propolis were analyzed. LC‐ESI‐MS/MS analysis of the extracts showed that the most abundant phenolic compound is caffeic acid in the MeOH extract (2943.12±11.12 μg phenolics/g extract), while on the other hand, CHCl3 extract had the highest total phenolic content (125.75±1.02 mg GAE/g extract). Antioxidant activity was measured using ABTS and DPPH assays, whereas CHCl3 extract (IC50=6.35±0.11 and 28.84±0.10 μg/mL, respectively) and MeOH extracts (IC50=5.04±0.07 and 28.80±0.09 μg/mL, respectively) showed relatively high antioxidant activity. The MeOH extract showed better antidiabetic activity than the standard compound, acarbose (IC50=0.544 and 0.805 mg/mL, respectively).

An accurate, rapid and precise high performance liquid chromatographic coupled with ultraviolet detector method has been developed for the determination of solifenacin succinate in pharmaceutical formulation and human plasma. Quantitative analysis of solifenacin succinate was performed using a C18 column at wavelength at 210 nm. The mobile phase consisted of 10 mM ammonium formate buffer (pH 4.0), acetonitrile and methanol (52.5:32.5:12.5 v/v/v). The method showed linearity in the concentration range of 3-60 μg/mL and the correlation coefficient was 0.9998 and 0.9994 for the standard and human plasma solutions, respectively. The proposed method was within acceptable limits in terms of accuracy, precision, selectivity, linearity, sensitivity and recovery parameters. This method is readily applicable for the determination of solifenacin succinate in human plasma and pharmaceutical formulations.

For the quantification of flurbiprofen in rat plasma, a simple UPLC-MS/MS method with high sensitivity and short retention time for flurbiprofen was developed and validated using specific parameters. Etodolac was used as internal standard. The transitions (precursor to the product) of flurbiprofen and internal standard were obtained using the electrospray ionization in the negative ion multiple reaction monitoring mode, 243.2 → 199.2, 286.2 → 212.1, respectively. For chromatographic separation, C18 column was used for the stationary phase and gradient elution was used for the mobile phase. This mobile phase consisted of a methanol (A) and a 5 mM ammonium formate solution (B), which varied at a flow rate of 0.4 mL/min. For flurbiprofen, LLOQ was determined as 5 ng/mL. Quantification of flurbiprofen in the rat plasma with a linear calibration curve of 5–5000 ng/mL (r > 0.9991 for plasma) is possible with a retention time of 1.89 min. The total analysis time of the method was 3 min. The proposed method was validated. The intraday and inter-day precision (RSD%) and accuracy (RE%) were within 10% in all cases for flurbiprofen. The stability of flurbiprofen was evaluated under conditions such as short-term, long-term, autosampler and freeze/thaw. After method validation, flurbiprofen was succesfully quantified in real rat plasma samples.

The objective of the present study was to develop a rapid and sensitive HPLC-florescence detection method for the quantification of silodosin in pharmaceuticals using "the Analytical Quality by Design (AQbD)" approach. For this purpose, at first, spectrofluorometric measurments were conducted to determine optimum excitation and emission wavelengths for silodosin, and they were found as 226 and 456 nm, respectively. A central composite design methodology was applied for optimization of critical method parameters. The parameters that have an impact on chromatographic separation of silodosin were selected as pH, column temperature, and organic content of the mobile phase (acetonitrile %) considering previous studies in the literature. A quadratic three-factor central composite design model consisting of 20 experimental observations was used for optimization of the parameters. According to the response surface methodology, the optimized conditions for the column temperature, acetonitrile percentage and the pH of the mobile phase were found as 36.8 °C, 28.2% and 3.3, respectively. The optimized method was validated according to ICH guidelines for accuracy, precision, working range, reproducibility, the limit of detection, the limit of quantification, and robustness. The method was linear in the range of 0.1-40 µg/mL, with a high correlation coefficient (0.9991) and acceptable precision (RSD<7.8%). Using the AQbD approach has provided advantages in terms of time consumption and costs. After validation studies, the developed method was successfully applied in the analysis of silodosin-containing tablet formulation indicating that the method could be used for routine quality control analyses.