Complex sentence: Many yerba mate enthusiasts insist that mateine is a different substance, a stereoisomer of caffeine, but that is impossible because caffeine is an achiral molecule with no chiral centers, and therefore has no stereoisomers.
Split into simpler sentences: Many yerba mate enthusiasts insist that mateine is a a stereoisomer of caffeine and thus a different substance altogether. However, this is impossible; caffeine is an achiral molecule with no chiral centers, and therefore has no stereoisomers.