Influence of the substituent on amide nitrogen atom of N-acetyl tyrosine on interactions with β-cyclodextrin

The influence of substituent on amide nitrogen atom on the interactions of N-acetyl tyrosine amides with β-cyclodextrin was studied by means of steady-state and time-resolved fluorescence spectroscopy, 2D 1H NMR, and microcalorimetry. In comparison with AcTyr-OH a primary amide group only in a small degree modified the binding constant with β-CD, regardless of the structure (linear or branched) and the length of n-alkyl substituent which for primary amides (methyl, ethyl, n-propyl, iso-propyl, n-butyl, and sec-butyl), as determined from the microcalorimetric titrations, is in the range from 122 M−1 to 190 M−1, except for t-butyl substituent for which the highest binding constant (over 500 M−1) was determined. Moreover, for a branched substituent binding constants are a little higher in comparison with n-alkyl ones. For secondary amides (di-methyl, di-ethyl, di-n-propyl, di-iso-propyl, and di-iso-butyl) the binding constants are higher (in the range from 270 M−1 to 410 M−1).