Structural and Energetic Effects in the Molecular Recognition of Acetylated Amino Acids by 18-Crown-6

Abstract

Absolute 18-crown-6 (18C6) binding affinities of four protonated acetylated amino acids (AcAAs) are determined using guided ion beam tandem mass spectrometry techniques. The AcAAs examined in this work include: N-terminal acetylated lysine (N α -AcLys), histidine (N α -AcHis), and arginine (N α -AcArg) as well as side chain acetylated lysine (N ε -AcLys). The kinetic-energydependent cross sections for collision-induced dissociation (CID) of the (AcAA)H + (18C6) complexes are analyzed using an empirical threshold law to extract absolute 0 and 298 K (AcAA)H + −18C6 bond dissociation energies (BDEs) after accounting for the effects of multiple collisions, kinetic and internal energy distributions of the reactants, and unimolecular dissociation lifetimes. Theoretical electronic structure calculations are performed to determine stable geometries and energetics for neutral and protonated 18C6 and the AcAAs as well as the proton bound complexes of these species, (AcAA)H + (18C6), at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31 G* and M06/6-311+G(2d,2p)//B3LYP/6-31G* levels of theory. For all four (AcAA)H + (18C6) complexes, loss of neutral 18C6 corresponds to the most favorable dissociation pathway. At elevated energies, products arising from sequential dissociation of the primary CID product, H + (AcAA), are also observed. Protonated N α -AcLys exhibits a greater 18C6 binding affinity than other protonated N α -AcAAs, suggesting that the side chains of Lys residues are the preferred binding sites for 18C6 complexation to peptides and proteins. N α -AcLys exhibits a greater 18C6 binding affinity than N ε -AcLys, suggesting that binding of 18C6 to the side chain of Lys residues is more favorable than to the N-terminal amino group of Lys.

Cationic Noncovalent Interactions: Energetics and Periodic Trends

“…The thermodynamic studies in this section were motivated by trying to understand the observations that 18-crown-6 (18C6) binds strongly to the protonated side chain of lysine (Lys), however, competitive binding of protonated side chains of arginine (Arg), histidine (His), and especially the N-terminal amino group limit the utility of using 18C6 as a method to identify the number of accessible lysines in a complicated protein . Nevertheless, this approach has been developed into the selective noncovalent adduct protein probing (SNAPP) method for structural elucidation in the gas phase. , Thermochemistry relevant to this process has been explored by Rodgers and co-workers using TCID (C) and is summarized in Table . − This work shows that protonated amines (mimics of the Lys side chain) bind to 18C6 tightly, comparably to K + (Table ), whereas the larger imidazole and 4-methylimidazole (mimics of the His side chain) and 1-methylguanidine (mimic of the Arg side chain) bind less tightly (similar to Cs + ). , Protonated glycine (Gly), which can describe interaction of the N-terminus, binds 18C6 only slightly less than the primary amines, but even addition of the small methyl side chain in alanine (Ala) reduces this interaction . (Notably this work also provided a more precise determination of the proton affinity of 18C6 …”

“…Comparison of the binding energies of several protonated amino acids to 18C6 shows the order Gly > Ala > Lys > His > Arg . Acetylation (Ac = CH 3 CO−) of the latter three amino acids at the N-terminus (N α ) shows that the protonated side chain of Lys binds more strongly than those of His and Arg . When the side chain of Lys (N ε ) is acetylated, the binding is weaker than to N α -AcLys, indicating that the protonated side chain is the preferred binding site for 18C6.…”

Complexes of Crown Ether Macrocycles with Methyl Guanidinium: Insights into the Capture of Charge in Peptides

“…The possibility of encapsulating guanidinium with crown ethers emerged in fact soon after their discovery and has been developed in subsequent studies. [13][14][15][16][17][18][19][20] The 'crowning' of cationic residues has been postulated as a means of modulating the three dimensional conformation and interactions of proteins and of activating or suppressing their biochemical function in different contexts. [21][22][23][24] In this study, we explore fundamental aspects of the interaction of the guanidinium group with crown ether hosts.…”

“…Previous experiments, based on guided ion beam tandem mass spectrometry provided a first incursion into this topic. [19,20] The broad conformational landscape resulting from this study probes the most stable coordination configurations enabled by the size and flexibility of the cyclic polyether backbones and can aid in predicting the interaction of crown ethers with guanidinium side groups of peptidic structures.…”

“…The cyclic polyether structure of crown ethers provides a semi‐rigid pre‐organized template that is particularly suitable for coordination with the protonated side groups of the basic amino acids. The possibility of encapsulating guanidinium with crown ethers emerged in fact soon after their discovery and has been developed in subsequent studies . The ‘crowning’ of cationic residues has been postulated as a means of modulating the three dimensional conformation and interactions of proteins and of activating or suppressing their biochemical function in different contexts …”