**4. Conclusions**

Contributing to the chemical study on the underexplored marine tunicate *Cnemidocarpa stolonifera*, three new alkaloids stolonines A–C (**1**–**3**) belonging to the taurine amide structure class were identified. Structures of these compounds were determined by NMR spectroscopy and confirmed by synthetic methods. Stolonines A–C (**1**–**3**) represent the 48–50th naturally occurring taurine amide compounds. However, this is the first time that the conjugates of taurine with 3-indoleglyoxylic acid, quinoline-2-carboxylic acid and β-carboline-3-carboxylic acid have been reported. Although compounds **1**–**3** displayed no cytotoxicity against the prostate cancer PC3 cells at the concentration of 20 μM, an immunofluorescence assay on PC3 cells revealed that compounds **1** and **3** increased cell size, induced mitochondrial texture elongation and affected the PC3 cell death via apoptotic pathway*.* Further investigation would be necessary to understand their mechanism of action on PC3 cells.

**Supplementary Materials:** IR, 1D and 2D NMR spectra of stolonines A–C (**1**–**3**) and other synthetic compounds (**8** and **10**); and calculated 13C and 1H NMR chemical shifts for each conformer of compounds **1-I** and **1-II**.

**Acknowledgments:** One of the authors (T.D.T.) acknowledges Griffith University for the provision of the "Griffith University International Postgraduate Research Scholarship and Griffith University Postgraduate Research Scholarship". We thank H. T. Vu at Eskitis Institute, Griffith University for acquiring the HRESIMS measurement and G. Pierens at Center for Advanced Imaging, University of Queensland for acquiring the 900 MHz NMR measurement. We acknowledge the Australian Research Council for support toward NMR and MS equipment (ARC LE0668477 and ARC LE0237908) and funding (ARC Discovery DP130102400).

**Author Contributions:** Ronald J. Quinn conceived and designed the research. Ronald J. Quinn and Ngoc B. Pham supervised the chemical experiments, structure elucidation, DFT calculations, synthesis and biological experiments. Trong D. Tran performed the natural product isolation, structural identification, DFT calculations, synthesis, cytotoxicity assays, immunofluorescence staining and cell imaging. John N. A. Hooper and Merrick Ekins collected and identified the tunicate samples. Trong D. Tran, Ngoc B. Pham and Ronald J. Quinn interpreted the results and wrote the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
