This contribution by Laurencin and co-workers explores the potential of using 43Ca solid-state NMR to study the structure of synthetic OCP and a series of related hybrid phases, both at natural abundance and via isotopic enrichment. Computational methods are also used to gain a deeper understanding of the OCP-citrate hybrid. This is a really nice study that highlights the challenges, but also the rewards, of studying challenging nuclei such as 43Ca. The results are very interesting and the methods presented will be of considerable use and value to other users within the community, not only those interested in this particular class of materials. The paper is very nicely written and the data is clearly presented. Before accepting, please could the authors address the following minor comments/corrections:

Whilst the introduction is very interesting and useful, it is quite long. Can the authors reduce it at all?

Move all figure captions to be under each figure.

Please ensure there are spaces between all numbers and units, i.e., number and °C.

In Table 1 it would be useful if they could also include values for the gyromagnetic ratio, particularly as they discuss it in the context of the table and the properties shown.

Page 4, lines 33-38: the sentence here isn’t really clear. Please re-phrase.

Page 5, line 12: replace ‘quadrupole’ with ‘quadrupolar’.

Page 6, line 51/52: there is an extra bracket after ‘resonance’.

In Figure 2, please can the authors add a sentence to explain what the orange and red arrows relate to. This is discussed in the main text but there is no mention of it in the figure caption. Also, in this figure can the authors remove the ‘/’ from the ‘a’ and ‘b’. Consider replacing with (a) and (b). It is quite difficult to see the calculated lines in the top spectrum in (b). It would be useful if this could be expanded so the reader could get a sense of what the calculated lines look like.

Page 13, line 43-44: please re-phrase this sentence, as it isn’t phrased well. Also in line 53, please replace ‘quadrupole’ with ‘quadrupolar’.

Page 16, line 17: change ‘difference’ to ‘differences’.

Table 2 and Figure 5: It would be useful to have these together. Hence, consider moving either the table or the figure.

Page 19, discussion of ‘three groups of 43Ca isotropic chemical shifts’. This is not immediately obvious when looking at Figure S5. Could the authors maybe highlight the three groupings in some way, e.g., some dashed lines circling the three groups?

Page 19, discussion on underestimated shift values – can the authors provide some reasoning for this?

Page 20, line 35: Figure 1 is used to highlight the length and symmetry of the organic chains. This is clearer in Figure 4. Hence, consider referring to Figure 4 instead.

The analysis shown in Figure S6 is useful and interesting. Is there any way this can be moved into the main paper? Can it be combined with Figure 6? Can the authors comment on the intensities shown in Figure 6, how were they calculated/determined? What was the analysis carried out in Figure S6? Can the authors provide a little bit more detail regarding this.

Did the authors do any EFG tensor analysis from their DFT calculations?