Competing ultrafast photoinduced quenching reactions in cinnamic acid : peptide blends

This report focuses on the ultrafast photoinduced excited state dynamics of cinnamic acid : peptide blends, prototypical MALDI samples. We prepared thin homogeneous layers of closely spaced microcrystals using alpha-cyano-4-hydroxycinnamic acid (alpha-CHC) and sinapinic acid (SA) as cinnamic acid (CA) derivatives and Angiotensin II (Ang II) as peptide. We applied spectrally and temporally resolved photoluminescence (PL) spectroscopy and identified in these blends different ultrafast and strongly competing PL quenching mechanisms. [2 + 2] Photodimerization is a known ultrafast solid-state photoreaction in CA crystals. We show that-after sufficient illumination times-it also governs the PL dynamics of CA: peptide blends. In addition, the peptide admixture induces a new PL quenching pathway, which proceeds most probably via photoisomerization or via energy transfer to stereoisomers at the CA: peptide interface, i.e., at disorder sites inside the microcrystals or at their surface. For a-CHC : Ang II blends, we find a photodimerization rate of (20 ps)(-1) and a quenching rate coefficient of 2.67 x 10(15) s(-1) mol(-1) cm(3) when adding the Ang II peptide. For SA : Ang II blends, the corresponding rates are even faster but the more remarkable feature is that the PL of the photoproduced stereoisomers is in turn quenched with increasing illumination time by the growing number of photodimers. Both photodimerization and peptide-induced quenching are obviously fast enough to compete with singlet-singlet annihilations even at typical MALDI fluences. We suggest that photoinduced ultrafast changes of the crystalline or molecular structure play a substantial role for the release of large molecular ions from MALDI substrates and are a common feature for UV-MALDI.