Tunable pK of amino acid residues at the air-water interface gives an L-zyme (langmuir enzyme).
Various amino acid-carrying amphiphiles were synthesized, and the pK values of the attached amino acid residues were investigated at the air-water interface and in aqueous vesicles using pi-A isotherm measurements, (1)H NMR titration, and IR spectroscopy in reflection-adsorption mode. The epsilon-amino group of the Lys residue embedded at the air-water interface displays a significant pK shift (4 or 5 unit) compared with that observed in bulk water, while the pK shift in aqueous vesicles was not prominent (ca. 1 unit). Moreover, pK values of the amino acids at the air-water interface can be tuned simply by control of the subphase ionic strength as well as by molecular design of the amphiphiles. A simple equation based on the dominant contribution by the electrostatic energy to the pK shift reproduces well the surface pressure difference between protonated and unprotonated species, suggesting a reduction in the apparent dielectric constant at the air-water interface. Hydrolysis of a p-nitrophenyl ester derivative was used as a model reaction to demonstrate the use of the Lys-functionalized monolayer. Efficient hydrolysis was observed, even at neutral pH, after tuning of pK for the Lys residue in the monolayer, which is a similar case to that occurring in biological catalysis.
Ariga
Katsuhiko
K
Supermolecules Group, Advanced Materials Laboratory, National Institute for Materials Science (NINS), 1-1 Namiki, Tsukuba 305-0044, Japan. ARIGA.Katsuhiko@nims.go.jp
Nakanishi
Takashi
T
Hill
Jonathan P
JP
Shirai
Makoto
M
Okuno
Minoru
M
Abe
Takao
T
Kikuchi
Jun-ichi
J
D000388:Air; D000596:Amino Acids / Q000737:chemistry*; D002384:Catalysis; D004798:Enzymes / Q000737:chemistry*; D004952:Esters / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D008055:Lipids / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D009596:Nitrophenols / Q000737:chemistry; D009994:Osmolar Concentration; D013055:Spectrophotometry, Infrared; D055672:Static Electricity; D013499:Surface Properties; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
D000596:Amino Acids; D004798:Enzymes; D004952:Esters; D008055:Lipids; D009596:Nitrophenols; D014867:Water