Complexation of amino acids by cyclotetrachromotropylene in aqueous solution - importance of CH-π and π-π interactions

✍ Scribed by Poh Bo-Long; Ming Tan Chi

• Publisher: Elsevier Science
• Year: 1994
• Tongue: French
• Weight: 486 KB
• Volume: 50
• Category: Article
• ISSN: 0040-4020
• DOI: 10.1016/s0040-4020(01)87025-3

✦ Synopsis

The stability constants K of the 1:l host to guest complexes formed between the cyclic tetramer, cyclotetrachromotropytene, and amino acids in water atpD 7.0 and 25' C were determined by H nmr spectroscopy. The results indicate that the interactions between the aromatic u-bonds of the host and the C-H bonds (for aliphatic amino acids) and aromatic r-bonds (for aromatic amino acids) of the guests are the major factors responsible for the complexation. There is a linear relationship between log K and the number of C-H bonds interacting with the hydrophobic host cavity. K-ray crystal structures of proteins have revealed the importance of two hydrophobic interactions in stabilizing the structures of proteins and their complexes. One is the interaction between C-H and aromatic-x bonds, as found in several sugar-binding proteins.' For example, in a D-glucose-binding protein, D-glucopyranose is sandwiched between two aromatic amino acid residues (phenylalanine and tryptophan) and its C-H bonds interact with their aromatic-x bonds. The other is the interaction between aromatic-n bonds of the aromatic amino acid residues (such as phenylalanine, tyrosine and tryptophan) found im many proteins.' To gain an understanding of these two hydrophobic interactions, we studied the complexation of twelve amino acids with cyclotetrachromotropylene (1) in an aqueous solution at pD I.0 using proton nmr spectroscopy. Ten aliphatic amino acids (2-11) provide the C-H bonds, and host 1 and the aromatic-n bonds. two aromatic amino acids (12-13) provide the Y Y II 2 H 8 CHZCONHZ 3 cH3 9 CHZCOZH 4 CH(CHf)2 10 (CH212C02H 5 CH(CH~)CHZCHJ 11 (CHZI,NHZ 6 CH(OH)CH3 12 cHzC6H5 7 (CH21zSCHJ 13