2.7. Apocyneae We sampled 36 species from 17 of 21 genera of Apocyneae. In this tribe, PAs had been previously reported from Anodendron affine Druce (Sasaki and Hirata, 1970) and Amphineurion marginatum (Roxb.) D.J. Middleton (Colegate et al., 2016). These species belong to two early diverging lineages of Apocyneae (subtribes Papuechitinae and Amphineurinae, respectively) and Livshultz et al. (2018b) suggested that PAs were potentially a chemotaxonomic character for delimiting these early diverging lineages from the rest of the radiation. No PAs were detected with high confidence in any sample of Apocyneae. Two of three accessions of Amphineurion marginatum leaves possessed a molecular ion of m/z 320 which appeared only in the PREC 120 scan with associated chromatography at a late retention time of ~15 min (> 3 to 1 S/N) (Table 2). Fragmentation of a PA with an m/z 320 via product ion scan (PIS) by Colegate et al. (2016) was consistent with our PREC 120 findings, as that compound was found to only result in m/z 120 fragment with no m/z 138 or 156 fragments. HRMS was utilized to propose a molecular formula for the m/z 320 ion, C 18 H 26 NO 4, and therefore it was tentatively identified as an open-chain diester (Colegate et al., 2016). Colegate et al. (2016) reported that leaves of Amphineurion marginatum contained the lowest percentage by dry weight of PAs (0.02%) compared to roots (0.13%) and stems (0.09%). This may account for the lack of PA structural diversity and abundance observed in our leaf samples but other factors, environmentally induced plasticity, genetic variation, cannot be ruled out in the present study. A product scan and/or HRMS would need to be completed on the m/z 320 molecular ion in our Amphineurion marginatum leaf samples to understand fully the fragmentation pattern and identity of this compound. The unusual PAs previously identified in Anodendron affine have a platynecine core (Sasaki and Hirata, 1970) that would not be detectable with our strategy. Thus, the occurrence and distribution of platynecine-derived PAs in other Apocyneae species also remains an open question.

Published as part of Barny, Lea A., Tasca, Julia A., Sanchez, Hugo A., Smith, Chelsea R., Koptur, Suzanne, Livshultz, Tatyana & Minbiole, Kevin P. C., 2021, Chemotaxonomic investigation of Apocynaceae for retronecine-type pyrrolizidine alkaloids using HPLC-MS / MS, pp. 1-15 in Phytochemistry (112662) (112662) 185 on page 9, DOI: 10.1016/j.phytochem.2021.112662, http://zenodo.org/record/8259664