*4.6. Protein Phosphatase Inhibition Assay*

Ampoules of the RM of **1**, along with the CRM of **2** (CRM-MCLR), were sent to Abraxis LLC (Warminster, PA, USA) for evaluation of toxicity. PP2A assays were performed using the microcystin-PP2A plate kit according to the kit's standard procedures [32].

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2072-6651/12/2/77/s1. Figure S1: LC–HRMS of CPCC-464 with thiol derivatization (positive mode), Figure S2: LC–HRMS of CPCC-464 (positive and negative modes), Figure S3: 1H NMR Spectrum of [Leu1]MC-LY (**1**) in CD3OH, Figure S4: COSY NMR Spectrum of [Leu1]MC-LY (**1**) in CD3OH, Figure S5: DIPSI NMR Spectrum of [Leu1]MC-LY (**1**) in CD3OH, Figure S6: HSQC NMR Spectrum of [Leu1]MC-LY (**1**) in CD3OH, Figure S7: HMBC NMR Spectrum of [Leu1]MC-LY (**1**) in CD3OH, Figure S8: 13C NMR Spectrum of [Leu1]MC-LY (**1**) in CD3OH, Figure S9: ROESY NMR Spectrum of [Leu1]MC-LY (**1**) in CD3OH, Figure S10: NOESY NMR Spectrum of [Leu1]MC-LY (**1**) in CD3OH, Figure S11: 1H and 13C chemical shifts overlaid on the structure of [Leu1]MC-LY (**1**), Figure S12: MS/MS spectra of MC-LA (**4**), MC-LY (**5**) and [Leu1]MC-LY (**1**), Figures S13–S18: MS/MS spectra of MC-LA (**4**), MC-LY (**5**) and [Leu1]MC-LY (**1**) (expanded), Figure S19: UV spectra of MC-LR (**2**), MC-LY (**5**) and [Leu1]MC-LY (**1**), Figure S20: LC–HRMS of CPCC-464 with Oxone oxidation (positive mode), Figure S21: MS/MS spectra of [Leu1]MC-M(O)R (**6**) and [Leu1]MC-M(O)R (**7**), Table S1: Comparison of the 13C chemical shift assignments for 1 in CD3OH with those reported for 3 in CD3OD.

**Author Contributions:** Conceptualization: F.R.P. and M.A.Q.; culture: N.I.L.; toxin isolation: P.L.; measurements: P.L., N.M., K.T. K.B. and S.L.R.; reference material preparation: K.T.; data analysis: N.M., K.T., C.O.M., P.M. and M.A.Q.; writing: P.L., F.R.P., P.M., C.O.M. and M.A.Q.; funding acquisition: F.R.P. and M.A.Q. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Ontario Ministry of the Environment (Best in Science projects 6729 and 78571). The helium cooled probe for the 700 MHz at the NRC (Halifax, NS) was provided by Dalhousie University through an Atlantic Canada Opportunities Agency Grant.

**Acknowledgments:** The authors thank Agriculture and Agri-Food Canada for access to their 600 MHz NMR spectrometer in Charlottetown, PE. The authors are grateful to R. Syvitski for the acquisition of the 600 MHz NMR spectra, F. Rubio (Abraxis LLC, Warminster, PA, USA) for performing the PP2A assays, and A. Foss (GreenWater Laboratories/CyanoLab, Palatka, FL, USA) for an extract of Poplar Island bloom material.

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