UV spectroscopy was used to determine the binding constants of ascorbic acid binding to various synthetic peptides derived from the first extracellular loop of the human beta 2 adrenergic receptor (B2AR).

This loop had been identified as being homologous to the sodium-dependent ascorbic acid transporter (Figure 13), and therefore likely to bind ascorbate. Control peptides were derived from the human insulin receptor (IR) and human dopamine D1 receptor, which lacks homology at this site for the ascorbic acid transporter. The Kd is the binding constant. The Kda is the ascorbate concentration where ½ of the peptide is bound. Rate constants were calculated from a linearized least-squares fit to an exponential decay where R2 is a measure of the fit of the calculated equation to the data. The highest affinity peptides share only one short sequences, which KMW, suggesting that this motif is the main component of the Asc binding site. Peptides that lack this motif have little or no Asc binding.*Kda is the ascorbate concentration where ½ of the peptide is bound.