To demonstrate the generality of amine modification CASL, we next turned our attention to the reaction scope (Fig. ). Three kinds of reactions in acidic buffer that had difficulty proceeding in the absence of an amyloid catalyst (NL6 amyloid: -) were selected. CASL using NL6 amyloid and 1 in combination with 2,4-dinitrofluorobenzene (DNFB) gave the SNAr product (1-DNB) in 56% yield (condition i). In addition, the corresponding CASL in combination with 2-ethynylbenzaldehyde 20 (condition ii) accommodated the imine formation followed by 6-endo cyclization to give the N-alkylated isoquinolinium salt (1-Qu) with a yield of 33%. Moreover, the amine activated by the CASL condition was successfully alkylated to give 1-Bn2 via reductive amination using benzaldehyde and sodium cyanoborohydride (condition iii) in 68% yield. To gain insight into the reaction mechanism of CASL, we performed crystallographic analysis of the amyloid catalyst. An NL6 crystal with suitable quality for X-ray crystallography, however, was not obtained. After several attempts, we found that a derivative in which the bulky pivaloyl group was introduced instead of the acetyl group at the N-terminus (pNL) gave long, thin needle crystals via slow evaporation from the formic acid solution (Fig. ). The derivative pNL moderately maintained the amyloidogenic potential (API = 5.6 and fine needle-like fibrils were predominantly observed by microscopic analysis (Fig. ) 21 ) and catalytic activity (yield = 29% under the conditions of Fig. ). The pNL crystal was suitable for Xray crystallographic analysis to give the atomic level structure (Figs. , D and Fig. ). The crystal structure of pNL shows that seven of eight amide bonds (including the side chain of Asn) participate in the construction of the amyloid structure through the formation of hydrogen bonds along the fiber axis (Fig. and). On the other hand, in the remaining amide bond between Phe and Gly, the carbonyl oxygen atom is oriented toward the solvent-exposed region rather than toward the fiber axis, which forms a hydrogen bond with formic acid (Fig. ). A docking study of 1 into the crystal structure of pNL showed that 1 stably fits in the groove of the amyloid structure and, like in formic acid, the amino group of 1 as the reactive site is close enough to the carbonyl oxygen of Phe-Gly amide to form the hydrogen bond (Figs. ). To verify the two types of amide bonds, we conducted an amideto-ester substitution, 23 in which the hydrogen bond donor is missing. Replacement of the amide bond between Asn and Phe of NL6 with the ester bond (NO) completely abolished the amyloidogenic ability (Fig. ), supporting the idea that this hydrogen bond donor of NL6 is critical for amyloid formation. In sharp contrast, an amide-to-ester substitution between Phe and Gly (FO) maintained the amyloidogenic potential of NL6 to a large extent (API > 6 and fibrils were clearly observed by microscopic analysis (Fig. )), consistent with the observation of the crystal structure that the Phe-Gly amide bond is not involved in the amyloid-forming hydrogen bond networks. Despite the maintained amyloidogenic ability of FO, however, the catalytic activity for the N-benzoylation of 1 was markedly reduced compared with that using NL6 (78% vs. 21%, Fig. ). The carbonyl oxygen of the ester is reportedly a weaker hydrogen bond acceptor than that of amide. 24 Therefore, the low yield when using FO supports that the carbonyl oxygen of the Phe-Gly amide of NL6 plays an important role in the deprotonation of 1 that is requisite for the amine modifications, which was also suggested by the crystal structure. NO lost the catalytic activity due to the lack of amyloidogenicity. Finally, we demonstrated substrate-selective conversion in a mixed reaction system (Fig. ). Competitive N-benzoylation of 1 and 2b possessing inherently comparable reactivity of amines 25 was carried out in the CASL conditions (Fig 5A , condition 1), high pH buffer (conditions 2 and 3), and base plus solvent (conditions 4 and 5). As expected, under conditions 2-5, the ratio of N-benzoylated products 1-Bz and 2b-Bz was nearly 1:1, while under condition 1 based on CASL, the yield of 1-Bz was five times higher than that of 2b-Bz. Another competition study in a mixed reaction system of 1 and a protein, lysozyme, also offered good selectivity for the amine modification of 1 in N-arylation CASL using DNFB (Fig. ). Lysozyme has seven primary amines composed of six lysine residues and an N-terminus. The lysozyme, however, underwent almost no arylation under the CASL conditions using NL6 (3%), whereas the arylation of 1 proceeded by 38% (condition 1). This result indicates that the substrate selectivity of CASL is maintained even in the presence of large biomolecules such as lysozyme. In the conditions of a high pH buffer without NL6, the amine modification of lysozyme, presumably mostly at the highly acidic N-terminal amino group, was observed at 43% yield (condition 2 and Fig. ).