UV Exposure Study of 1-Methylnicotinamide Chloride: Self-Preservation in a Simple NAD(P)⁺ Analog

Chemical biosignatures, if present, are exposed to ultraviolet (UV) radiation at several locations in the solar system, including the Martian surface. Solid thin-layer samples are commonly used to study the effect of UV irradiation on the stability of potential organic biosignatures in the laboratory and in space. Given the limitations of the layer deposition techniques previously used in astrobiology experiments, such as sublimation-recondensation and casting, antisolvent precipitation (i.e., crystallization from solution by addition of a poor solvent) offers a promising alternative. We applied the latter technique to deposit optically opaque crystalline layers of 1-methylnicotinamide chloride ((MNA)Cl) for use in subsequent UV irradiation experiments. The 1-methylnicotinamide cation in (MNA)Cl serves as a model of hypothetical extraterrestrial nicotinamide-based redox cofactors related to NAD(P)⁺. Under UV irradiation (λ ≥ 224 nm), decomposition of (MNA)Cl occurred, but it ceased after ∼14 days, leaving a large fraction of the material intact. This behavior is attributed to the shielding effect of a photoproduct layer on the crystal surfaces and can be kinetically described by a first-order bounded decay model. Such "self-preservation" may enhance the survivability of certain organic biosignatures provided they form sufficiently thick (opaque) layers or crystals. The presence of either synthetic gypsum (CaSO₄·2H₂O) or halite (NaCl) had a minimal effect on the kinetic parameters of the UV decomposition of (MNA)Cl.