Wine Chemistry and Biochemistry

(Steven Felgate) #1

9D Influence of Phenolics on Wine Organoleptic Properties 553


the hydroxyl groups of phenolic compounds and carbonyl and amide groups of pro-


teins. Hydrophobic interaction can occur between the benzenic ring of phenolic


compounds and the apolar side chains of amino acids such as leucine, lysine or


proline in proteins (Hatano and Hemingway 1996; Jobstl et al. 2006; Oh et al. 1980;


Wroblewski et al. 2001).


The proline residue is generally considered to be a good binding site as it pro-


vides a flat, rigid, hydrophobic surface that is favourable to interactions with other


planar hydrophobic surfaces such as benzenicrings. Effectively, NMR studies per-


formed by the Haslam research team in the University of Sheffield have shown the


existence of hydrophobic stacking between the benzenic ring of several phenolic


compounds and the apolar face of the proline ring of peptides rich in this amino


acid (Baxter et al. 1997; Charlton et al. 2002a; Murray et al. 1994). On the other


hand, a carbonyl group of tertiary amides, as it occurs in a protein with proline


residues, is a hydrogen acceptor more efficient than a carbonyl in a secondary or


primary amide. Hence, hydrogen bonding involving the carbonyl group of proline


is predictable, concomitantly with hydrophobic interactions. Although studies per-


formed by other authors suggest that the driving force of protein-tannin interactions


are mainly hydrogen bonding (Frazier et al. 2006; Simon et al. 2003), it seems


more prudent to accept that both type of interactions are present, strengthening


each other, depending on the protein and tannin structure and medium conditions


(Artz et al. 1987; Hagerman et al. 1998; Oh et al. 1980).


Protein-tannin interactions are importantly affected by pH (Calderon et al. 1968;


Naczk et al. 1996). Apparently there is higher protein precipitation at pH values
close to the protein isoelectric point (pI) where protein-protein electrostatic repul-


sions are minimized (Hagerman and Butler 1981): proteins with acidic pI have


higher affinities to complex with tannins at lower pH, whereas basic proteins aggre-


gate preferentially at higher pH (Yan and Bennick 1995). An interesting study by


NMR performed by Charlton et al. (2002b) with EGCG and a basic PRP fragment


of 19 amino acids, showed that pH greatly affects precipitation of aggregates but


does not change the protein-tannin binding affinities.These authors suggested that


protein-tannin particles are in a colloidal state, stabilized by repulsions of proteins


of the same charge. When this charge is minimized (at pH=pI), there is no repul-


sion and particles aggregate together and precipitate. Therefore, although the initial


binding is due to specific protein-tannin interactions, the aggregation process seems


to be due mainly to surface charge effects.


The affinity of tannins to bind proteins is favored by their ability to work as


multidentate ligands (cross-linking) in which one tannin is able to bind to more


than one protein at one time (Fig. 9D.11b) or to bind to more than one point in


the same protein (Charlton et al. 2002a; Siebert et al. 1996). These associations


between tannin and protein could result in aggregates that precipitate depending on


the ratio of protein/tannin and also the concentration of protein (Frazier et al. 2003;


Poncet-Legrand et al. 2006).


Besides the relative concentrations, those interactions are affected by the struc-


tures of both tannin and protein.

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