The red tree frog Litoria rubella from Australia has been studied for several decades showing that their dorsal skin glands secrete a number of small peptides containing a Pro–Trp sequence, known as tryptophyllin L peptides. Although peptides from many genera of Australian frogs have been reported to possess a variety of biological activities, the bioactivities of this peptide family have remained to be discovered. In this study, we investigated the antioxidant potency of a number of tryptophyllin L peptides for the first time using a joint statistical and experimental approach in which predictions based on Gaussian three‐dimensional quantitative structure–activity relationship (3D‐QSAR) models were employed to guide an in vitro experimental investigation. Two tryptophyllin tripeptides P–W–L (OH) and P–W–L (NH2) were predicted to have the Trolox equivalent antioxidant capacity (TEAC) values of 0.80 and 0.87 μM Trolox/μM peptide, respectively. With those promising results, antioxidant capabilities of five tryptophyllin L peptides with the common core Pro–Trp–Leu were synthesized and subjected to 1,1‐diphenyl‐2‐picrylhydrazyl (DPPH), ferric reducing ability of plasma (FRAP) and 2,2′‐azino‐bis(3‐ethylbenzothiazoline‐6‐sulphonic acid) radical cation (ABTS˙+) radical scavenging assays. The tests indicated that all the tested tryptophyllin L peptides, noticeably S–P–W–L (OH) and F–P–W–L (NH2), are strong ABTS˙+ radical scavengers and moderate scavengers in the other two assays. The results, thus, suggested that the tryptophyllin L peptides are likely to be a part of the skin antioxidant system helping the frog to cope with drastic change in oxygen exposure and humidity, as they inhabit over a large area of Australia with a wide climate variation.
Discovery of natural antioxidants has been carried out for decades relying mainly on experimental approaches that are commonly associated with time and cost demanding biochemical assays. The maturation of quantitative structure activity relationship (QSAR) modelling has provided an alternative approach for searching and designing antioxidant compounds with alleviated costs. As a contribution to this approach, this work aimed to establish a fragment‐based 3D‐QSAR procedure to discover and design potential antioxidants based on tryptophyllin L structures isolated from the red tree frog Litoria rubella. A force field and a Gaussian 3D‐QSAR model were built to screen for potential antioxidants from tripeptide fragments covering all sequences of tryptophyllin L database. Among those, PWY(NH2) corresponding tryptophyllin L 4.1 was predicted to have the highest 2,2′‐azino‐bis(3‐ethylbenzothiazoline‐6‐sulphonic acid) radical cation (ABTS+·) scavenging capability. Two newly designed peptides PYW and PYW(NH2) together with PWY(NH2), tryptophyllin L 4.1, and the reference peptide PWY were synthesized and subjected to two antioxidant assays including ABTS scavenging and ferric reducing antioxidant power assays. Although the experimental TEAC values of the five peptides were roughly similar to those from predictions, the activity order was not in agreement with the predictions. The dissimilarities were accounted by the difference in the experimental procedures, the deviation of modelling regression, and the synergetic effect of structural and experimental features. The ABTS radical scavenging assays revealed that all the tested peptides were strong ABTS+· scavengers with the antioxidant capabilities approximately twice as high as trolox and higher than glutathione. The ferric reducing activities of the peptides were, on the other hand, much weaker than that of trolox suggesting different antioxidant mechanisms inserted by trolox and the peptides. This work was a demonstration that 3D‐QSAR methods can be employed in conjunction with experimental methods to effectively detect and design antioxidant peptides.
The ambient air monitoring of PCDDs/PCDFs and dl-PCBs using passive air samplers in a residential urban area in Hanoi between 2012-2020 was deteminated. The seasonal variations of PCDD/PCDF and dl-PCB levels in ambient air in three periods: between Spring 2012 and Autumn 2015, from Winter 2015 to Autumn 2018, and between Winter 2018 and Autumn 2020 are similar to a V-shape with the highest peak in winter, then decreasing gradually in spring, bottoming in summer and rising again in autumn. The upward temporal trends of PCDD/PCDF and dl-PCB pollution, and total TEQD/F&DL, total TEQD/F and total TEQDL has been confirmed over time. The concentrations of total PCDFs were dominant and were approximately 1.0 to 9.3 times higher and 2.4 times higher on average than total PCDDs. Total dl-PCBs were 5.8 to 38 times higher and 17 times on average higher than the total toxic PCDDs/PCDFs. The PCDD/PCDF congeners contributed 83% to 95% of the total TEQ value. The phenomenon of temperature inversion not only causes seasonal air pollution but also increases the concentration of PCDDs/PCDFs and dl-PCBs with a narrower range but at a higher average levels.
Passive air sampling (PAS) method has been successfully developed using isotope standards 13C-labeled PCDDs/PCDFs as surrogates on the PUF disks from the beginning of sampling to monitor PCDDs/PCDFs from different sources of pollution, with very different levels of PCDDs/PCDFs in ambient air in the actual conditions of monsoon tropical climates with hot, wet weather and high temperatures in Vietnam. The 13C-labeled PCDD/PCDF surrogates acted as quantitative standards in the seasonal PAS process (spring, summer, autumn, winter) in the North and/or every 3 months in the dry and rainy seasons in the Central. At least 96.9% of all surrogates had the retention efficiencies of 17% to 185% on PUF disks, which were equivalent to that required by US EPA 1613B method for the recovery efficiencies only at sample preparation. This development method was shown to be reliable with 91.6% of the total PCDD/PCDF congeners which their relative percent differences (RPDs) between PCDD/PCDF concentrations on the PUF disks of field duplicates were not exceed 40% and an average RPD of only 7.4% to 30.5% for both PAS process and laboratory analysis.
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