General instability of dipeptides in concentrated sulfuric acid as relevant for the Venus cloud habitability.

Recent renewed interest in the possibility of life in the acidic clouds of Venus has led to new studies on organic chemistry in concentrated sulfuric acid. We have previously found that the majority of amino acids are stable in the range of Venus' cloud sulfuric acid concentrations (81% and 98% w/w, the rest being water). The natural next question is whether dipeptides, as precursors to larger peptides and proteins, could be stable in this environment. We investigated the reactivity of the peptide bond using 20 homodipeptides and find that the majority of them undergo solvolysis within a few weeks, at both sulfuric acid concentrations. Notably, a few exceptions exist. HH and GG dipeptides are stable in 98% w/w sulfuric acid for at least 4 months, while II, LL, VV, PP, RR and KK resist hydrolysis in 81% w/w sulfuric acid for at least 5 weeks. Moreover, the breakdown process of the dipeptides studied in 98% w/w concentrated sulfuric acid is different from the standard acid-catalyzed hydrolysis that releases monomeric amino acids. Despite a few exceptions at a single concentration, no homodipeptides have demonstrated stability across both acid concentrations studied. This indicates that any hypothetical life on Venus would likely require a functional substitute for the peptide bond that can maintain stability throughout the range of sulfuric acid concentrations present.