Structure Based Descriptors for the Estimation of Colloidal Interactions and Protein Aggregation Propensities

<div><p>The control of protein aggregation is an important requirement in the development of bio-pharmaceutical formulations. Here a simple protein model is proposed that was used in molecular dynamics simulations to obtain a quantitative assessment of the relative contributions of proteins’ net-charges, dipole-moments, and the size of hydrophobic or charged surface patches to their colloidal interactions. The results demonstrate that the strength of these interactions correlate with net-charge and dipole moment. Variation of both these descriptors within ranges typical for globular proteins have a comparable effect. By comparison no clear trends can be observed upon varying the size of hydrophobic or charged patches while keeping the other parameters constant. The results are discussed in the context of experimental literature data on protein aggregation. They provide a clear guide line for the development of improved algorithms for the prediction of aggregation propensities.</p> </div

Nano-Indentation to Determine Mechanical Properties of Intraocular Lenses: Evaluating Penetration Depth, Material Stiffness, and Elastic Moduli

Abstract Introduction Intraocular lenses (IOL) should remain in the eye for life after implantation into the capsular bag during cataract surgery. The material must meet various requirements. It is crucial that the material has the best biocompatibility, and it should be flexible and soft for best possible implantation process but also sufficiently stable and stiff for good centering in the eye and posterior capsule opacification prevention. Methods In this laboratory experiment, we used nano-indentation for the mechanical assessment of three hydrophobic acrylic (A, B, C), three hydrophilic acrylic (D, E, F), and one silicone (G) intraocular lens. We wanted to determine whether some react more sensitively to touching/handling than others. The indentation elastic modulus and the creep were obtained from the force displacement curve. For measuring penetration depth and testing of possible damage to the intraocular lenses, the samples were measured at room temperature. A 200-µm-diameter ruby spherical tipped indenter was used for all the tests. Indentations were made to three different maximum loads, namely 5 mN (milli Newton), 15 mN, and 30 mN and repeated three times. Results The lowest penetration depth (12 µm) was observed with IOL B. However, IOL A, D, and F showed similar low penetration depths (20, 18, and 23 µm, respectively). Lenses C and E showed slightly higher penetration depths of 36 and 39 µm, respectively. The silicone lens (G) showed the greatest penetration depth of 54.6 µm at a maximum load of 5 mN. With higher maximal loads (15 and 30 mN) the penetration depth increased significantly. Lens C, however, showed the same results at both 15 and 30 mN with no increase of penetration depth. This seems to fit well with the material and manufacturing process of the lens (lathe-cut). During the holding time of 30 s at constant force all six acrylic lenses showed a significant increase of the creep (C IT 21–43%). Lens G showed the smallest creep with 14%. The mean indentation modulus (E IT) values ranged from 1 to 37 MPa. IOL B had the largest E IT of 37 MPa, which could be caused by the low water content. Conclusion It was found that results correlate very well with the water content of the material in the first place. The manufacturing process (molded versus lathe-cut) seems to play another important role. Since all included acrylic lenses are very similar, it was not surprising that the measured differences are marginal. Even though hydrophobic materials with lower water content showed higher relative stiffness, penetration and defects can also occur with these. The surgeon and scrub nurse should always be aware that macroscopic changes are difficult to detect but that defects could theoretically lead to clinical effects. The principle of not touching the center of the IOL optic at any time should be taken seriously

Properties of pseudo proteins used in this study.

a<p>Net-charge in elementary charge units, e.</p>b<p>Dipole moment in eÅ.</p>c<p>first energy minimum in potential of mean force in kJ/mol.</p>d<p>Error bars from boot-strap analysis.</p

Calculated potentials of mean force between pseudo proteins.

<p>A: three PPs with varying net-charge, constant hydrophobicity and dipole moment; B: the two PPs with the largest and smallest hydrophobicities () but identical net-charges and dipole moments; C: six PPs with varying dipole moment, identical net-charge and similar values.</p

Two PPs with different surface topologies.

<p>Left: PP10, right: PP08. Atoms are colored according to the net-charges they carry (blue negative, red positive, white neutral). The PPs are oriented so that the atom with the maximum lssc value, the center of the patch with the highest hydrophobicity, is in the center of each representation.</p

Models for protein solubilities based on molecular descriptors.

<p>Results from three different linear regression models for protein solubilities, combining the protein net-charge (q) with one of the three descriptors dipole-moment (p), normalized SAP-score (nSAP), or largest SAP value (SAPmax), and from the CCSol web-server. Included are the coefficients of the linear regression models (Eq.3), the correlations between experimental and calculated solubility (), and the P-value (probability that the observed correlation is coincidental). Data are given for two protein sets: 18 proteins from EColi-K12 (setA), and 20 mutations of RNAseSA (setB).</p

Effects of point directed mutagenesis on descriptors.

a<p>PDB ID.</p>b<p>Net-charge of wildtype (WT) in elementary charge units, e.</p>c<p>Net-charge, difference between mutant and WT.</p>d<p>Dipole moment of wildtype in eÅ.</p>e<p>Dipole moment, difference between mutant and WT.</p