For exploring effects of side chain R groups on binding strength of non-covalent complexes of β-cyclodextrin(β-CD) with α-amino acids and esters, a stoichiometry of β-CD with L-Norvaline(n-Val), L-Phenylalanine(Phe), L-Leucine(Leu) L-Aspartic acid(Asp), L-Aspartic acid-4-tert-butyl ester(Asp-4-t-butyl ester) and L-Aspartic acid-4-benzyl ester(Asp-4-benzyl ester) were mixed respectively and incubated at room temperature for 12 h to reach the equilibrium. In positive mode, electrospray ionization mass spectrometry(ESI MS) results indicated that β-CD could form non-covalent complexes with n-Val, Phe, Leu, Asp, Asp-4-t-butyl ester and Asp-4-benzyl ester, respectively. By comparison, two group competition reactions showed that the binding strength order were [β-CD∶Asp-4-benzyl ester+H]+>[β-CD∶Asp-4-t-butyl ester+H]+>[β-CD∶Asp+H]+ and [β-CD∶Phe+H]+>[β-CD∶Leu+H]+>[β-CD∶n-Val+H]+, respectively. The stability constants of the complexes of β-CD with α-amino acids and esters were determined via an improved mass spectrometric titration coupled with curve fitting methodology. The obtained lgKst values were 1.81, 2.54, 3.14, 3.26, 3.36, 3.67 for [β-CD∶n-Val+H]+, [β-CD∶Asp+H]+, [β-CD∶Asp-4-t-butyl ester+H]+, [β-CD∶Asp-4-benzyl ester+H]+, [β-CD∶Leu+H]+ and [β-CD∶Phe+H]+, respectively. The obtained lgKst values further confirmed competition reactions results. Because all α-amino acids or esters have the same C-termini(—COOH) and the N-termini(—NH2), which are both hydrophilic, and different side chain R group. It can be concluded that the guest molecules were driven to approach β-CD by the hydrophobic force in the formation of the complexes in solution. Therefore, hydrophobic effect and polarity of the side chain R group can play an important role in the formation of the inclusion complexes. Moreover, the formed inclusion complexes can be stabilized by hydrogen-bonding interactions.
