Download Supporting information This document contains a comparison

Supporting information
This document contains a comparison between the XRD patterns measured on our samples and
the reference octacalcium phosphate (OCP) pattern (S.I. 1), as well as the estimation of the
stability constant for the Glu/PO43- complex based on the results shown in reference 1,
corresponding to reference 56 in the main paper (S.I. 2).
S.I. 1. XRD pattern of precipitates synthesized after 2 days, in comparison with the XRD
reference pattern of OCP.
S.I. 1a. XRD patterns of HA-Glu collected after 2 days, measured for 2θ ranging from 2.5 to 45º. The red
lines represent the reference spectrum of OCP (PDF card n. 01-074-1301).
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S.I. 1b. XRD patterns of HA-Cont, HA-Glu, HA-Arg, and HA-Combo collected after 2 days (2θ range
between 5 and 85º), compared with reference spectra for HA (black lines, PDF card n. 00-009-0432) and
for OCP (red lines, PDF card n. 01-074-1301).
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S.I. 2. Estimate of the stability constant of Glu/PO43- complex formation
Luscombe et al [1] studied the interactions between AAs and DNA bases and backbone groups,
based on the modeling of 129 different protein-DNA complexes. In the 129 structures studied:
a) Arg was found to interact with phosphate groups extensively: H-bonds were observed
218 times, Van der Waals bonds 238 times, and water-mediated interactions were
observed 113 times.
b) Glu was found to interact to a very low extent with phosphate groups. H-bonds were
observed only 6 times, Van der Waals bonds 61 times, and water-mediated bonds 29
times.
Among these interactions, H-bonds are the strongest ones. So if we just consider the number of
H-bonds observed in these structures we can have a very rough estimate of how much stronger
the Arg/PO43- interaction is compared with Glu/ PO43-. Within these structures, the number of Hbonds observed between Arg and PO43- groups was 36 fold larger than between Glu and PO43-.
This would indicate that Arg/ PO43- complexes are approximately 36 times more likely to occur
than Glu/ PO43- complexes, which could mean that (very roughly) the stability constant of Glu/
PO43- complexes would be 36 fold smaller than that of Arg/PO43- complexes. Based on this
estimate and on the value of 1.9 reported for log K Arg/ PO43- complexes [2], we can estimate
that the stability constant of Glu/ PO43- complexes would be approximately 0.34:
K Glu/PO43- = (K Arg/PO43-)/36
Log (K Glu/PO43-) = Log (K Arg/PO43-) –Log 36
Log (K Glu/ PO43-) =1.9-1.56= 0.34
This is of course just an estimate. We are in the process of performing some calorimetric
measures in order to find an experimental value for this constant. A paper describing the results
of these measures will be published soon.
[1]
[2]
N. M. Luscombe, R. A. Laskowski, and J. M. Thornton, "Amino acid–base interactions: a
three-dimensional analysis of protein–DNA interactions at an atomic level," Nucleic
Acids Research, vol. 29, pp. 2860-2874, July 1, 2001 2001.
G. Lancelot and H. Claude, "Model studies of interactions between nucleic acids and
proteins: hydrogen bonding of amides with nucleic acid bases," Nucleic Acids Research,
vol. 6, pp. 1063-1072, March 1, 1979 1979.
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