Table 1.

Comparison of the pK _a and pK _b values for glycine, valine and diglycine aqueous solutions as deduced from the observed intensity changes of the low frequency spectrum with literature values.

	pK a	pK b	Reference
glycine	2.34	9.58	[32]
	2.20±0.15	9.76±0.56	This work
valine	2.27	9.52	[32]
	2.93±0.94	8.98±0.72	This work
diglycine	3.13	8.10	[32]
	3.30±0.25	8.10±0.81	This work