1. Introduction

2. Results and Discussion

2.1. Chemical Analyses of Propolis Samples

2.1.1. Total Polyphenols and Total Flavonoids Quantification

As reported in the reference of the Chinese Pharmacopoeia [7], but also in the cited literature [6,10,11]and again by Popova et al., 2007 [13], poplar propolis is mainly characterized by a high content of polyphenols and in particular of flavonoids; for this reason, the analysis of total polyphenols (TP) and total flavonoids (TF) represented the first analytical step of this work.

The analysis of TP and TF in propolis of different geographical Italian regions (North, Center, South, and Islands) revealed significant variations in the content of these bioactive compounds in different samples (Table 1).

Table 1. Content of total polyphenols, expressed as gallic acid equivalent, and total flavonoids expressed as galangin equivalent. Sample are grouped according to different geographical Italian areas, North (samples N1-N5), Central (C1-C13), South (S1-S7) and Islands, namely Capraia Island (Tuscany), I1 and Sicily, I2. Values are expressed as a percentage w/w (mean ± SD).

Table 1. Content of total polyphenols, expressed as gallic acid equivalent, and total flavonoids expressed as galangin equivalent. Sample are grouped according to different geographical Italian areas, North (samples N1-N5), Central (C1-C13), South (S1-S7) and Islands, namely Capraia Island (Tuscany), I1 and Sicily, I2. Values are expressed as a percentage w/w (mean ± SD).

Sample code	TP (% w/w)	TF (% w/w)	Sample code	TP (% w/w)	TF (% w/w)
N1	5.34 ± 0.64	3.38 ± 0.17	C10	23.84 ± 1.08	21.78 ± 1.78
N2	21.86 ± 3.07	18.13 ± 0.06	C11	22.70 ± 2.06	19.63 ± 2.36
N3	10.45 ± 1.88	9.26 ± 0.18	C12	27.36 ± 3.35	21.42± 0.98
N4	18.20 ± 2.09	11.04 ± 0.37	C13	16.58 ± 1.61	11.60 ± 0.14
N5	14.78 ± 3.28	12.28 ± 0.63	S1	27.88 ± 4.24	24.28 ± 3.27
C1	28.31 ± 4.61	26.64 ± 2.01	S2	21.41 ± 2.58	16.65 ± 0.05
C2	23.24 ± 4.74	21.29 ± 0.02	S3	22.53 ± 2.06	19.90 ± 0.65
C3	22.67 ± 4.28	18.15 ± 0.60	S4	8.26 ± 2.19	5.33 ± 0.01
C4	27.85 ± 6.44	20.70 ± 2.16	S5	12.10 ± 1.04	11.39 ± 0.01
C5	36.41 ± 0.30	18.33 ± 2.29	S6	20.14 ± 2.30	16.56 ± 0.90
C6	31.45 ± 4.82	25.65 ± 3.74	S7	19.64 ± 0.22	17.67 ± 0,04
C7	31.90 ± 5.78	20.50 ± 1.45	I1	3.68 ± 0.37	3.57 ± 0.12
C8	21.67 ± 2.30	18.94 ± 0.24	I2	3.04 ± 0.15	0.90 ± 0.10
C9	20.82 ± 2.01	20.43 ± 0.06

Within propolis from Northern regions, the sample collected in Piedmont N1 (Biella province) markedly differed from other samples having low TP and TF content. Within the central regions, samples C13 from Latium showed marked differences from the other samples, revealing low TP and TF content, whereas C5 from the Florence countryside showed the highest TP content but medium TF content. In general, propolis from Tuscany, C1-C10 in this work showed a very similar TP and TF content respect to samples from Umbria, C11 and C12, probably due to the similar climatic conditions of these two regions. In propolis samples from Southern regions, sample S4 and S5 from Apulia, were those with lower content of TP and TF. In agreement with [10], in propolis from Islands we found the overall lowest content of TP and in the case of I2 from Tuscany, also the lowest content of TF.

Afterwards, in order to provide a more visual and comprehensive overview of the chemical similarities and differences across regions; TP and TF content of the 27 samples, grouped by their geographical provenance, were summed. This approach allowed for a clearer and more understandable comparison of the results based on regional divisions as reported in Table 2

In propolis from Northern regions, the mean TP content was 14.13 ± 6.47% w/w. This value was lower compared to that recorded for propolis from Central and Southern regions but higher than that observed in samples from Islands (as graphically depicted in Figure 1, panel A). Propolis from Central regions exhibited the highest mean TP content, 25.75 ± 5.41% w/w, same results for TF, as depicted in Figure 1, panel A and B. Statistical analysis was performed to highlight different of TP and TF within the four Italian regions. Results showed a significant difference of samples from Central regions compared to samples from Northern regions (p < 0.05), but not compared to samples from Southern Regions. In propolis from Southern regions the mean TP content was 18.85 ± 6.60% w/w, not statistically different from the mean value of propolis from Northern regions.

Propolis samples from Tuscan and Sicilian Islands displayed the lowest polyphenol content, with an average of 3.36 ± 0.45% w/w. Statistically significant differences were observed compared to samples from all other regions. Interestingly, despite clear distinct climatic conditions that characterize the Tuscan archipelago and Sicily, the two insular propolis samples maintained a similar poor content of TP, Figure 1 panel A, as already reported in [10] for propolis from Sicily and Sardinia.

As regards TF (Table 2), these secondary metabolites composed the main phenolic subclasses in almost all samples, with the exception of I2; in many cases, TF represented more than 90% of TP. The analysis of the ratio between TF and TP in samples divided by geographical area, gave similar values, ranging from 0.63 ± 0.48 (Islands) to 0.83 ± 0.10 w/w% (Southern Regions) (Table 2).

Propolis samples collected in Northern regions displayed a content of TF of 10.82 ± 5.32% w/w. Similar to TP, TF content in propolis from the Northern regions (N) was lower than in propolis from the Center and South but higher than in the Islands (Figure 1, panel A and B). Propolis samples from the Central Regions (C) also showed the highest mean TF content at 20.39 ± 3.65% w/w; the difference compared to the North was statistically significant (p = 0.007), but the differences between propolis form Central and Southern Regions were not significant. The mean TF content in propolis from Southern Regions, in fact, was intermediate, 15.97 ± 6.09% w/w, with no significant difference found when compared also to propolis from the North. The two propolis samples collected in Italian Islands deeply differed for TF content, even if both values were very low, 3.57 ± 0.12% w/w in propolis from Tuscan Capraia Island and only 0.90 ± 0.10% w/w in the Sicilian propolis. As in the case of TP, differences were found compared to all other regions: North (p = 0.02), Center (p = 0.004), and South (p = 0.002).

In line with the aim of this work, results obtained by analyzing propolis samples collected in different geographical Italian areas, allowed us to deepen previous works and showed that results related to TP and TF content were in agreement with Gardini et al. (2018) [10] and Popova et al. (2007) [13]. Nevertheless, we found that a distinction could be made in order to distinct propolis from Northern, Central, Southern regions and Islands because of marked differences in TP content, but also in TF as in the case of the comparison North/Central and Islands with other areas.

Temperature and soil composition could be responsible for the variations in propolis composition, as well as the differing presence of Populus species and other plants in the Salicaceae family across various locations. However, regarding this latter point, it's important to note that in Italy, Populus species are primarily found in the regions of Piedmont, Lombardy, and Emilia-Romagna [14], where collected propolis samples resulted poor both in TP and TF content (with the exception of N2). The question of the harvesting method, as claimed by Papotti et al. (2012) [11], newly emerges as a supplementary factor that could affect propolis quality.

2.1.2. Quantification of Pinocembrin, Chrysin, Galangin and CAPE through HPLC-DAD

The UV methods used to quantify TP and TF have some important strengths because they are reliable, unexpensive and rapid, therefore very useful for a preliminary chemical screening of multiple samples. Nevertheless, these methods are not sufficient to provide a high-quality characterization of natural products and, in the case of propolis, they do not allow to investigate more subtle quali-quantitative differences in the flavonoid profile of samples from different Italian areas.

HPLC-DAD analysis was therefore performed to characterize the flavonoid fraction of propolis under analysis and allowed the identification of the main constituents. In accordance with what previously reported by Biagi et al. (2016) [9], we identified pinocembrin (PIN), with a retention time (RT) of 10.4 min. While for chrysin (CHR) and galangin (GAL) we were not able to perfectly separate these two peaks that partially overlapped with RT of 12.1 and 12.5 min., respectively. Therefore, we chose to quantify CHR and GAL together to avoid errors depending on different overlays recorded in samples. Caffeic acid phenethyl ester (CAPE) was identified and quantified in all samples at RT = 11.2 min. Figure 2A–D show the chromatogram of N1, C11, S6 and I2, representative of samples from different origins.

PIN was found the flavonoid at highest content in several samples, as reported in Table 3: In particular, in five samples from Central Regions its content was > 10% w/w. In propolis from Northern Regions, 2 (N2, N5) out of 5 samples PIN ranged from 5.5% m/m to 8.2% w/w. With the exception of S4, in propolis from Southern Regions. PIN ranged from 5.1% w/w to 9.9% w/w. Finally, in samples from islands, PIN was in high amount compared to TF.

Table 3. Content of pinocembrin (PIN), the sum of chrysin and galangin (CHR + GAL) and caffeic acid phenethyl ester (CAPE) in different propolis samples expressed as a percentage w/w (mean ± SD).

Table 3. Content of pinocembrin (PIN), the sum of chrysin and galangin (CHR + GAL) and caffeic acid phenethyl ester (CAPE) in different propolis samples expressed as a percentage w/w (mean ± SD).

Sample code	PIN (% w/w)	CHR and GAL (% w/w)	CAPE (% w/w)	Samples code	PIN (% w/w)	CHR and GAL (% w/w)	CAPE (% w/w)
N1	1.04 ± 0.14	0.60 ± 0.01	0.86 ± 0.41	C10	10.16 ± 0.05	8.67 ± 0.14	1.65 ± 0.05
N2	8.20 ± 0.16	2.79 ± 0.17	1.30 ± 0.04	C11	9.30 ± 0.08	10.03 ± 0.34	1.70 ±0.04
N3	2.28 ± 0.10	2.52 ± 0.07	0.96 ± 0.03	C12	11.27 ± 0.05	7.13 ± 0.13	1.62 ± 0.07
N4	*coeluition	<0.05	0.81 ± 0.03	C13	4.91 ± 0.05	3.15 ± 0.07	1.01 ± 0.05
N5	5.53± 0.03	4.87 ± 0.11	1.37 ± 0.01	S1	9.87 ± 0.32	12.22 ± 0.13	1.93 ± 0.05
C1	10.07 ± 0.12	9.38 ± 0.15	1.52 ± 0.02	S2	8.14 ± 0.15	4.55 ± 0.01	1.96 ± 0.05
C2	8.16 ± 0.15	5.68 ± 0.10	1.80 ± 0.02	S3	8.33 ± 0.03	6.24 ± 0.10	1.43 ± 0.01
C3	6.88 ± 0.09	5.38 ± 0.01	1.16 ± 0.01	S4	3.08 ± 0.01	1.20 ± 0.03	1.59 ± 0.03
C4	8.05 ± 0.09	6.11 ± 0.18	1.53 ± 0.01	S5	5.13 ± 0.22	2.63 ± 0.12	1.54 ± 0.05
C5	8.01 ± 0.24	6.58 ± 0.42	1.37 ± 0.01	S6	8.01 ± 0.84	7.53 ± 0.42	1.74 ± 0.21
C6	11.53 ± 0.12	10.02 ± 0.40	1.57 ± 0.01	S7	7.13 ± 0.13	7.70 ± 0.14	1.17 ± 0.07
C7	5.14 ± 0.17	4.31 ± 0.18	1.19 ± 0.01	I1	1.46 ± 0.10	0.28 ± 0.01	0.14 ± 0.03
C8	9.97 ± 0.03	4.57 ± 0.04	1.54 ± 0.43	I2	0.24 ± 0.11	0.07 ± 0.01	0.10 ± 0.01
C9	10.95 ± 0.13	6.47 ± 0.27	1.96 ± 0.41

*Quantification vitiated by a marked overlay: approximate value ranging from 4.5% to 6.0% .

Similarly to what previously done with regards to the TP and TF in Table 2, in order to provide a general overview of chemical differences within the four Italian geographical area of interest, quantification of the identified flavonoids and CAPE as well as the ratio of PIN and TF, CHR and GAL and TF, CAPE and TP were calculated and reported in Table 4.

The mean content of PIN in propolis from Northern regions was 4.26 ± 3.24% w/w, from Central regions was 8.80 ± 2.18% w/w, from Southern regions was 7.10 ± 2.28% w/w and in insular regions was 0.85 ± 0.86% w/w. The statistical analysis revealed a significant difference in PIN content by comparing Northern and Central Regions (p = 0.003) and in insular propolis compared to all other samples. PIN content resulted correlated to TF and the ratio between PIN and TF resulted in similar values for propolis from all geographical areas. In conclusion, with regards to PIN content, as already suggested by Gardini et al. (2018) [10] but also by Cui-Ping et al. (2015) [15] for Chinese propolis, in this work we reinforced the opinion that PIN could be considered a general marker of poplar-type propolis, but we also noticed that its high content distinguished propolis from Central and Southern provenance from Insular and Northern regions.

Regarding CHR and GAL a larger variation was recorded. In propolis from the Northern Regions CHR and GAL content ranged from < 0.05% (N4) to 4.87% w/w (N5). In propolis from the Central Regions the range was 3.15 % (C13) up to 10.03% w/w (C11), in propolis from the Southern Regions values ranged from 1.20% (S4) up to 12.22% w/w (S1). Islands (I) presented the lowest CHR and GAL content (0.28% and 0.07% in I1 and I2, respectively), reinforcing the distinctiveness of insular propolis compared to mainland samples.

The very low CHR and GAL content obtained with the sample from the Alps, in Trentino, the one collected at the highest altitude (> 500 m), was considered a point worthy of further investigation, which is currently difficult to discuss without other references from the same source.

The mean content of CHR + GAL in samples collected in Northern Regions was 2.16 ± 1.93% w/w, a value statistically different compared to the content of CHR + GAL in samples from Central Regions (6.73 ± 2.22% w/w, p = 0.002), but also compared to values recorded in samples from Southern Regions (6.01 ± 3.66% w/w, p = 0.04). On the other hand, mean content of CHR + GAL in samples from Central and Southern Regions did not differ in a significant manner. Propolis from Tuscan archipelago and Sicily once again deeply differed from the others from Central Regions and South (and North, even not in statistically significant manner). A less marked difference was noted in the ratio between CHR + GAL and TF content among samples from North, Center and South but, similar to PIN, the statistical analysis showed a significant difference between propolis from North and South (p = 0.04). Again, a clear difference was observed by comparing propolis from Islands and those from Central and Southern Regions (p < 0.001) (Table 4). Results partly confirm what reported in [10] that stated a lower CHR content in propolis from Italian temperate areas, namely areas far from the sea and Po river.

In this study we also analyzed the content of CAPE in all samples, being one of the most peculiar caffeic acid derivatives of propolis [16] but rarely taken into account in large comparative analysis. We identify and quantified CAPE in all samples.

As showed in Table 3, the range of CAPE content was narrower than in the other parameters previously considered, and in 22 of the 27 samples CAPE ranged between 1.0% and 2.0% w/w. Nevertheless, in line with previous findings here obtained, in 3 out of 5 samples from Northern Regions (N1, N3, N4) CAPE was below 1% w/w and also a very low amount of this compound was found in insular samples. As summarized in Table 4, samples from Northern Regions had a moderate CAPE content (1.06 ± 0.26% w/w), propolis from Central Regions a mean content of 1.51 ± 0.27% w/w, whereas samples from Southern Regions had the highest mean content of CAPE: 1.62 ± 0.28% w/w; in line with our previously findings, insular regions presented the lowest CAPE content (0.12 ± 0.03% w/w). The statistical analysis revealed that CAPE content in propolis from Northern Regions was significantly different from that of Central and Southern Regions (p = 0.01 and p = 0.006, respectively) and, as observed for other parameters, a great difference was recorded comparing CAPE content in propolis from Islands and that of all other samples (p < 0.001 for all comparisons).

A differential study of CAPE content respect to TP was finally performed (Table 4) and it was observed low differences in samples from Northern, Central and Southern Regions, but a lower ratio in propolis from Islands, with differences statistically significant with North and South (p = 0.03 and p = 0.006, respectively).

The Principal Component Analysis (PCA) plot (Figure 3) visually represents the differences in the chemical composition of propolis samples from various geographical regions. Each region is depicted using different colors, with the Northern area shown in blue, the Central area in green, the Southern area in red, and the Insular area in purple. The separation of these regions on the plot indicated that the chemical markers used in the analysis (such as PIN, CHR, GAL, and CAPE, TP and TF) effectively distinguished the samples based on their geographical origin. The Insular samples were notably distinct from the others, appearing clearly separated on the plot, which suggested a different chemical profile for these samples. This distinctiveness could be attributed to the specific environmental conditions or plant sources found on the islands. The Northern and Central samples, while closer to each other, still showed noticeable differences, implying some level of similarity between them, yet with distinct characteristics. The Southern samples also formed a separate group, highlighting their unique chemical profile compared to the Northern and Central regions. Overall, the PCA analysis provided a clear and effective visualization of how geographical factors influence the chemical composition of propolis. The distinct clustering of samples from different regions supported the conclusion that these regions have unique propolis profiles, particularly emphasizing the uniqueness of the Insular samples. This visualization strengthened the understanding of regional variations in propolis composition. In this work we could update and simplify the division of geographical clusters of Italian propolis underlining a marked difference in propolis from Northern Regions respect to those from Central and Southern Regions, more enriched in TP and TF, but also and more specifically in PIN, CHR + GAL and CAPE. However, the most evident difference in propolis samples was observed for those collected in Sicily and in Capraia, chemically very poor. Partly not in accordance with [10] , we couldn’t distinct in a clear way propolis collected near the seacoast and far from the sea and, more in general, we observed strong similarities in propolis from Central and Southern Regions, regardless latitude and altitude. Differently from [10], unfortunately we weren’t able to collect and analyze propolis from Po plain areas and we consider this one limit of this work that did not allow to provide a supplementary comparison. Moreover, as mentioned before, the very low content of CHR + GAL in the sample collected above 500 m in the Alps opened the interest in focusing on mountain propolis in the future to understand if the different flavonoid profile may be a peculiar characteristic.

3. Conclusions

4. Materials and Methods

References

1. Burlando, B.; Cornara, L. Honey in Dermatology and Skin Care: A Review. J Cosmet Dermatol 2013, 12, 306–313. [Google Scholar] [CrossRef] [PubMed]
2. Simone-Finstrom, M.; Spivak, M. Propolis and Bee Health: The Natural History and Significance of Resin Use by Honey Bees. Apidologie 2010, 41, 295–311. [Google Scholar] [CrossRef]
3. Bankova, V. Chemical Diversity of Propolis and the Problem of Standardization. J Ethnopharmacol 2005, 100, 114–117. [Google Scholar] [CrossRef] [PubMed]
4. Hata, T.; Tazawa, S.; Ohta, S.; Rhyu, M.-R.; Misaka, T.; Ichihara, K. Artepillin C, a Major Ingredient of Brazilian Propolis, Induces a Pungent Taste by Activating TRPA1 Channels. PLoS One 2012, 7, e48072. [Google Scholar] [CrossRef] [PubMed]
5. Valipour, M. Therapeutic Prospects of Naturally Occurring P38 MAPK Inhibitors Tanshinone IIA and Pinocembrin for the Treatment of SARS-CoV -2-induced CNS Complications. Phytotherapy Research 2023, 37, 3724–3743. [Google Scholar] [CrossRef] [PubMed]
6. Governa, P.; Cusi, M.G.; Borgonetti, V.; Sforcin, J.M.; Terrosi, C.; Baini, G.; Miraldi, E.; Biagi, M. Beyond the Biological Effect of a Chemically Characterized Poplar Propolis: Antibacterial and Antiviral Activity and Comparison with Flurbiprofen in Cytokines Release by LPS-Stimulated Human Mononuclear Cells. Biomedicines 2019, 7, 73. [Google Scholar] [CrossRef] [PubMed]
7. Chinese Pharmacopoeia Commission Pharmacopoeia of the People’s Republic of China; China Medical Science and Technology Press: Beijing, 2010.
8. European Parliament; European Council; European Commission Regulation Regulation (EU) 2015/2283.
9. Biagi, M.; Pecorari, R.; Appendino, G.; Miraldi, E.; Magnano, A.; Governa, P.; Cettolin, G.; Giachetti, D. Herbal Products in Italy: The Thin Line between Phytotherapy, Nutrition and Parapharmaceuticals; A Normative Overview of the Fastest Growing Market in Europe. Pharmaceuticals 2016, 9, 65. [Google Scholar] [CrossRef] [PubMed]
10. Gardini, S.; Bertelli, D.; Marchetti, L.; Graziosi, R.; Pinetti, D.; Plessi, M.; Marcazzan, G.L. Chemical Composition of Italian Propolis of Different Ecoregional Origin. J Apic Res 2018, 57, 639–647. [Google Scholar] [CrossRef]
11. Papotti, G.; Bertelli, D.; Bortolotti, L.; Plessi, M. Chemical and Functional Characterization of Italian Propolis Obtained by Different Harvesting Methods. J Agric Food Chem 2012, 60, 2852–2862. [Google Scholar] [CrossRef] [PubMed]
12. Aliboni, A.; D’Andrea, A.; Massanisso, P. Propolis Specimens from Different Locations of Central Italy: Chemical Profiling and Gas Chromatography−Mass Spectrometry (GC−MS) Quantitative Analysis of the Allergenic Esters Benzyl Cinnamate and Benzyl Salicylate. J Agric Food Chem 2011, 59, 282–288. [Google Scholar] [CrossRef] [PubMed]
13. Popova, M.P.; Bankova, V.S.; Bogdanov, S.; Tsvetkova, I.; Naydenski, C.; Marcazzan, G.L.; Sabatini, A.-G. Chemical Characteristics of Poplar Type Propolis of Different Geographic Origin. Apidologie 2007, 38, 306–306. [Google Scholar] [CrossRef]
14. Parco Nazionale foreste casentinesi Popolus Spp. Available online: https://www.parcoforestecasentinesi.it/sites/default/files/Quattordicesima.pdf (accessed on 2 September 2024).
15. Cui-ping, Z.; Shuai, H.; Wen-ting, W.; Shun, P.; Xiao-ge, S.; Ya-jing, L.; Fu-liang, H. Development of High-Performance Liquid Chromatographic for Quality and Authenticity Control of Chinese Propolis. J Food Sci 2014, 79. [Google Scholar] [CrossRef] [PubMed]
16. Tolba, M.F.; Azab, S.S.; Khalifa, A.E.; Abdel-Rahman, S.Z.; Abdel-Naim, A.B. Caffeic Acid Phenethyl Ester, a Promising Component of Propolis with a Plethora of Biological Activities: A Review on Its Anti-inflammatory, Neuroprotective, Hepatoprotective, and Cardioprotective Effects. IUBMB Life 2013, 65, 699–709. [Google Scholar] [CrossRef] [PubMed]
17. Bonetti, A.; Faraloni, C.; Venturini, S.; Baini, G.; Miraldi, E.; Biagi, M. Characterization of Phenolic Profile and Antioxidant Activity of the Leaves of the Forgotten Medicinal Plant Balsamita Major Grown in Tuscany, Italy, during the Growth Cycle. Plant Biosystems - An International Journal Dealing with all Aspects of Plant Biology 2021, 155, 908–913. [Google Scholar] [CrossRef]
18. Pressi, G.; Bertaiola, O.; Guarnerio, C.; Barbieri, E.; Rigillo, G.; Governa, P.; Biagi, M.; Guzzo, F.; Semenzato, A. In Vitro Cell Culture of Rhus Coriaria L.: A Standardized Phytocomplex Rich of Gallic Acid Derivatives with Antioxidant and Skin Repair Activity. Cosmetics 2022, 9, 12. [Google Scholar] [CrossRef]
19. Biagi, M.; Collodel, G.; Corsini, M.; Pascarelli, N.A.; Moretti, E. Protective Effect of Propolfenol ^® on Induced Oxidative Stress in Human Spermatozoa. Andrologia 2018, 50, e12807. [Google Scholar] [CrossRef] [PubMed]
20. Finetti, F.; Biagi, M.; Ercoli, J.; Macrì, G.; Miraldi, E.; Trabalzini, L. Phaseolus Vulgaris L. Var. Venanzio Grown in Tuscany: Chemical Composition and In Vitro Investigation of Potential Effects on Colorectal Cancer. Antioxidants 2020, 9, 1181. [Google Scholar] [CrossRef] [PubMed]
21. Governa, P.; Biagi, M. Copaifera Langsdorffii Desf.: In Vitro Investigation on Anti- Helicobacter Pylori and Anti-Inflammatory Activities of Oleoresin and Fruit Methanolic Extract. Plant Biosystems - An International Journal Dealing with all Aspects of Plant Biology 2020, 154, 117–124. [Google Scholar] [CrossRef]
22. Sberna, G.; Biagi, M.; Marafini, G.; Nardacci, R.; Biava, M.; Colavita, F.; Piselli, P.; Miraldi, E.; D’Offizi, G.; Capobianchi, M.R.; et al. In Vitro Evaluation of Antiviral Efficacy of a Standardized Hydroalcoholic Extract of Poplar Type Propolis Against SARS-CoV-2. Front Microbiol 2022, 13. [Google Scholar] [CrossRef] [PubMed]
23. Scikit Learn. Available online: https://scikit-learn.org/stable/modules/generated/sklearn.decomposition.PCA.html (accessed on 2 September 2024).
24. Matplotlib. Available online: https://matplotlib.org/ (accessed on 2 September 2024).