The choice of the β-anomer over the α-anomer for the reducing end in NMR studies is largely guided by the stability and relative abundance of the anomers in solution, which are influenced by anomeric effect and thermodynamics. Here's a detailed explanation:

1. Thermodynamic Stability:

In solution, the reducing sugars exist in equilibrium between their α and β anomers. The β-anomer is generally more stable and abundant because of reduced steric hindrance and better solvation effects compared to the α-anomer.

For example, in D-glucose, the β-anomer has the hydroxyl group at the C1 position (anomeric position) in the equatorial position, which is sterically favorable compared to the axial position in the α-anomer.

2. Anomeric Effect:

The anomeric effect favors the axial orientation (α-anomer) in non-aqueous polar solvents because of favorable hyperconjugation. However, in water or protic solvents used in NMR studies, this effect is minimized, making the β-anomer more dominant.

3. Structural Elucidation:

In NMR, the β-anomer often provides sharper and more distinct signals due to its higher population in solution, making it easier to analyze.

The signals corresponding to the β-anomer are more representative of the predominant structure in physiological conditions, which is important for biological relevance.

4. Biological Relevance:

Many naturally occurring oligosaccharides and glycoconjugates preferentially adopt the β-anomeric form at the reducing end. Studying the β-anomer can better mimic natural systems, particularly for interactions with enzymes, receptors, and other biomolecules.

5. Simplification of Interpretation:

In complex mixtures or glycan studies, focusing on the more abundant β-anomer reduces complexity in spectral interpretation, as the α/β ratio might not always be 1:1.

In summary, the β-anomer is typically chosen because it is more stable, abundant, and biologically relevant under typical NMR experimental conditions. This makes it the preferred choice for structural studies and simplifies spectral interpretation.