Effect of glycine, dl-alanine and dl-2-aminobutyric acid on the temperature of maximum density of water

• Effect of α-amino acids on the temperature of maximum density of water is presented.
• The addition of α-amino acids decreases the temperature of maximum density of water.
• Despretz constants suggest that the amino acids behave as water structure breakers.
• Despretz constants decrease as the number of CH2 groups of the amino acid increase.
• Solute disrupting effect becomes smaller as its hydrophobic character increases.

The effect of glycine, dl-alanine and dl-2-aminobutyric acid on the temperature of maximum density of water was determined from density measurements using a magnetic float densimeter.Densities of aqueous solutions were measured within the temperature range from T = (275.65 to 278.65) K at intervals of T = 0.50 K over the concentration range between (0.0300 and 0.1000) mol · kg−1. A linear relationship between density and concentration was obtained for all the systems in the temperature range considered.The temperature of maximum density was determined from the experimental results. The effect of the three amino acids is to decrease the temperature of maximum density of water and the decrease is proportional to molality according to Despretz equation. The effect of the amino acids on the temperature of maximum density decreases as the number of methylene groups of the alkyl chain becomes larger. The results are discussed in terms of (solute + water) interactions and the effect of amino acids on water structure.