Shining a light on the photochemical stability of peptidic bioinsecticides

Peptide toxins from spider venoms are being increasingly hailed as environmentally friendly alternatives to market-dominating small-molecule chemical insecticides. While the stability of knotted spider-venom peptides towards enzymatic degradation, temperature changes and varying pH conditions has already been examined, their susceptibility to sunlight remains unclear. Field applications of insecticides demand that the insecticidal component is active for at least a few days to ensure sufficient eradication of the targeted insect pests. We therefore exposed four insecticidal spider-venom peptides (ω-Hv1a, ω/κ-Hv1a, Ta1a and Dc1a) to continuous artificial sunlight for up to 7 days. After certain incubation periods, we quantified the percentage of intact peptide and identified sites of peptide cleavage. We found that after 3 days of continuous exposure (=6 days of 12 h/d sunlight), the amount of remaining intact peptide was 16 % (Ta1a), 21 % (Dc1a), 55 % (ω-Hv1a), and 67 % (ω/κ-Hv1a), whereas bovine serum albumin was completely degraded. Even after 7 days (=14 days of 12 h/d sunlight) exposure, more than 50 % of ω/κ-Hv1a and ω-Hv1a remained intact. Peptides with lower molecular mass tended to be less susceptible to sunlight, while cleavage of peptide bonds involving proline or cysteine were most susceptible to photochemical degradation. The photochemical changes detected by mass spectrometry mainly comprised oxidations, deamidations, and cysteine-targeted modifications.