Globular clusters (GCs) play an essential role in understanding the star formation history of galaxies. At the same time GCs are the best candidates to check the reliability of single stellar population (SSP) models which have broad ranging applications. On the basis of a new empirical stellar library in the K band, new SSP model predictions suggest that CO and CN molecular absorptions are insensitive to age and strongly dependent on global metallicity for old stellar populations. We aim to obtain integrated spectra in the K band for a well-defined sample of Galactic GCs which have independently derived ages and abundances. It will allow us to investigate the C, N and O abundances in GCs reported by previous work, as well as to calibrate a robust metallicity scale to be applied to extragalactic GCs and early-type galaxies.