Quartz Microbalance Measurements in the Liquid Phase

Quartz microbalances are mass-sensitive sensors consisting of a quartz slide with gold electrodes on each side of the slide (Fig. 10.8). To investigate cyclopeptides as new su- 

We have investigated a large variety of cyclopeplides and analytes using quartz sensors in gas and liquid phases. Here, we report on the interaction of cyclopeptides 1 and 2 (Fig. 10.9) with proteinogenic amino acids in aqueous solution. Both peptides contain three thiol groups for the covalent and conformationally restricting attachment to the gold electrodes. They differ in only one amino acid (L-Lys in 1, L-Phe in 2). 

Acidic, neutral or basic amino acids have been used as analytes dissolved in phosphate buffer (0.1 M, pH 7.4). Under these conditions, basic groups like the side chains of arginine, lysine and to some extent also histidine are protonated and therefore cationic. Acidic groups like the side chains of aspartic or glutamic acid are deprotonated and therefore anionic. Tryptophan was included in the macrocyclus because its indole system offers n-cation interactions or hydrophobic interactions. 

Sensitivity of quartz microbalances coated with cyclopeptides 1 or 2 for 15 different L-ami-acids (1 mmol/1) in phosphate buffer (0.1 mol/1, pH 7.4) [99], 

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