## Abstract

We perform a statistical analysis of atomic distributions as a function of the distance R from the molecular geometrical center in a nonredundant set of compact globular proteins. The number of atoms increases quadratically for small *R*, indicating a constant average density inside the core, reaches a maximum at a size-dependent distance *R _{max}*, and falls rapidly for larger

*R*. The empirical curves turn out to be consistent with the volume increase of spherical concentric solid shells and a Fermi-Dirac distribution in which the distance

*R*plays the role of an effective atomic energy

*ε(R) = R*. The effective chemical potential mu governing the distribution increases with the number of residues, reflecting the size of the protein globule, while the temperature parameter beta decreases. Interestingly, μ is not as strongly dependent on protein size and appears to be tuned to maintain approximately half of the atoms in the high density interior and the other half in the exterior region of rapidly decreasing density. A normalized size-independent distribution was obtained for the atomic probability as a function of the reduced distance,

*r = R/R*, where R

_{g}_{g}is the radius of gyration. The global normalized Fermi distribution,

*F(r)*, can be reasonably decomposed in Fermi-like subdistributions for different atomic types τ,

*F*, with

_{τ}(r)*Σ*, which depend on two additional parameters μ

_{τ}F_{τ}(r) = F(r)_{τ}and

*h*. The chemical potential μ

_{τ}_{τ}affects a scaling prefactor and depends on the overall frequency of the corresponding atomic type, while the maximum position of the subdistribution is determined by h

_{τ}, which appears in a type-dependent atomic effective energy,

*ε*, and is strongly correlated to available hydrophobicity scales. Better adjustments are obtained when the effective energy is not assumed to be necessarily linear, or

_{τ}*(r) = h_{τ}r*ε*, in which case a correlation with hydrophobicity scales is found for the product α

_{τ}*(r) = h_{τ}*r^{α}_{τ}

*h*

_{τ}*. These results indicate that compact globular proteins are consistent with a thermodynamic system governed by hydrophobic-like energy functions, with reduced distances from the geometrical center, reflecting atomic burials, and provide a conceptual framework for the eventual prediction from sequence of a few parameters from which whole atomic probability distributions and potentials of mean force can be reconstructed.

Original language | English |
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Pages (from-to) | 304-320 |

Number of pages | 17 |

Journal | Proteins |

Volume | 66 |

Issue number | 2 |

Early online date | 15 Nov 2006 |

DOIs | |

Publication status | Published - 1 Feb 2007 |

## Keywords

- Amino Acids
- Hydrophobic and Hydrophilic Interactions
- Models, Molecular
- Probability
- Protein Conformation
- Journal Article
- Research Support, N.I.H., Extramural
- Research Support, Non-U.S. Gov't