Metabolic Profile of Agropyron repens (L.) P. Beauv. Rhizome Herbal Tea by HPLC-PDA-ESI-MS/MS Analysis

Abstract

Agropyron repens (L.) P. Beauv. (couch grass) is a world-wide infesting rhizomatous plant with pharmacological applications. Chemical research is focused on its allelopathic and anti-inflammatory components, which are mainly present in the essential oil. Conversely, the aqueous extracts have been sparingly investigated, although the herbal tea is by far the most used formulation. To fill the gap, the metabolic profile of Agropyron repens rhizome herbal tea was investigated by electrospray ionization (ESI) tandem-mass spectrometry (MS/MS); the phenolic profile was investigated by HPLC-PDA-ESI-MS/MS. ESI-MS fingerprinting was provided, evidencing diagnostic ions for saccharides, organic acids and amino acids. The HPLC-PDA-ESI-MS/MS analysis evidenced at least 20 characteristic phenolic compounds, the most representative being caffeoyl and feruloyl quinic esters, followed by coumaric, caffeic and ferulic acids, and hesperidin among flavonoids. In addition, the essential amino acid tryptophan was identified for the first time. The results suggest new perspectives of applications for Agropyron repens rhizome.

Keywords: ESI-MS fingerprinting; amino acids; antioxidants; flavonoids; phenolic acids; saccharides; small organic acids; tryptophan.

Figure 1

ESI-MS/MS profile of rhizome Agropyron repens herbal tea, samples AR1, AR2, AR3, in negative ionization ESI- (a) and positive ionization ESI+ (b), with signals normalized to the highest one for a direct comparison among samples. (a) Pyr ac (pyruvic acid, [M-H]⁻ = 87 m/z); Lac ac (lactic acid, [M-H]⁻ = 89 m/z); Pho ac (phosphoric acid, [M-H]⁻ = 97 m/z); Fur ac (2-furoic acid, [M-H]⁻ = 111 m/z); Fum ac (fumaric acid, [M-H]⁻ = 115 m/z); Mal ac (malic acid, [M-H]⁻ = 133 m/z); cis-Aco ac (cis-aconitic acid, [M-H]⁻ = 173 m/z); hex (hexose, [M-H]⁻ = 179 m/z); Cit ac (citric acid, [M-H]⁻ = 191 m/z); di-hex (di-hexose, [M-H]⁻ = 341 m/z, [M+Cl]⁻ = 377 m/z, [M+Cl+2]⁻ = 379 m/z); di-hex-pen (di-hexose-pentose, [M-H]⁻ = 473 m/z). (b) GABA (γ-aminobutyric acid, [M+H]⁺ = 104 m/z); Pro (proline, [M+H]⁺ = 116 m/z); Val (valine, [M+H]⁺ = 118 m/z); Asn (asparagine, [M+H]⁺ = 133 m/z); His (histidine, [M+H]⁺ = 156 m/z); Arg (arginine, [M+H]⁺ = 175 m/z); hex ([M+Na]⁺ = 203 m/z, [M+K]⁺ = 219 m/z); di-hex ([M+Na]⁺ = 365 m/z, [M+K]⁺ = 381 m/z); di-hex-pen ([M+H]⁺ = 475 m/z, [M+Na]⁺ = 497 m/z, [M+K]⁺ = 513 m/z).

Figure 2

AR2 herbal tea diluted 1:10 with mobile phase: (a) PDA chromatogram; (b) EC at λ = 324 nm; (c) EC at λ = 279 nm. TP: tryptophan; 5-CQA: 5-caffeoylquinic acid; 3-FQA: 3-feruloylquinic acid; CA: caffeic acid; 4-FQA: 4-feruloylquinic acid; CouA: coumaric acid; FA: ferulic acid.

Scheme 1

Biosynthetic pathways of growth hormones and allelopathic molecules starting from tryptophan (TP).

Figure 3

AR3 herbal tea, diluted 1:10 with mobile phase, ESI-SIR channels at m/z 174 (a), 190 (b), 215 (c) and 231 (d), corresponding to the ion [M-H]⁻ of IAA (a), 5-HIAA (b), TβC-3-COOH (c) and 6-HTβC-3-COOH (d).

Scheme 2

Hydroxybenzoic acids (HBAs) structures.

Scheme 3

Hydroxycinnamic acids (HCAs) structures.

Scheme 4

Hydroxycinnamic acids derivatives structures.

Scheme 5

Flavonoids (Fs) structures.

Scheme 6

Allelopathic molecules DIMBOA, DIBOA and HBOA structures.

Figure 4

ESI- SIR channels at (a) 180, (b) 210 and (c) 164 m/z, corresponding to the ion [M-H]⁻ of (a) DIBOA, (b) DIMBOA and (c) HBOA, respectively. AR3 herbal tea as an example.