What was the organizing principle of Melanchthon's Loci Communes? Herman Bavinck, in his Reformed Dogmatics (BakerAcademic, Vol I, Chapter 1, pp 26 - 27), claims that while Melanchthon when writing his "Common Places" drew material from the book of Romans, his organizing principle was borrowed from Lombard:


  Melanchthon's loci communes owe their existence to two lines of work he was pursuing in the same period: a series of critical comments on Lombard's Sentences and a commentary on Paul's Letter to the Romans. In 1520, as a result of this twofold activity, he conceived the plan to write Locos Communes concerning "law, sin, grace, sacraments as well as other mysteries." In other words, he sought to summarize and discuss under a number of general concepts&mdash;in the manner of rubrics, following the instruction of the rhetoricians&mdash;the scriptural material he derived from his study of the Letter to the Romans.


However, Ryan McGraw, in his 2013 review of Rick Gamble's Whole Counsel of God claims that the structure of Melanchthon's work also came from Romans:


  Philip Melanchthon (1497-1560) wrote the first Protestant Systematic Theology. Yet far from imposing an arbitrary scheme upon Scripture, he developed a loci method based upon the structure of the book of Romans.


McGraw's citation:


  Lowell C. Green, “Melanchthon’s Relation to Scholasticism,” in Carl R. Trueman and R. Scott Clark, eds., Protestant Scholasticism: Essays in Reassessment (Bletchney, UK: Paternoster, 2005), 273-288.


Did Melanchthon use Romans to structure his Loci, or only to derive doctrinal content? Was his structure actually derived from Lombard?
This answer is drawn from material I received from Chris Engelsma. One question I didn't take into account at first was which of the four editions of the Loci Bavinck is thinking of; however, if we are to go with the 1521, the answer seems to be relatively clear. If we compare the TOC of the Loci with the TOC of Book I and Book II of the Sentences, there is virtually no resemblance. However, looking at the TOC of the Loci, the influence of Romans is obvious.

Some further quotes that are of interest, also sent to me by Mr. Engelsma.


  Melancthon (died 1560.)—This reformer has left us an expository work upon the Epistle to the Romans, in two different shapes. The former appeared in 1532, under the title of Annotationes; the latter in 1532, with the name Commentarii. They consisted of his lectures, and afterwards gave rise to the Loci Communes. Melancthon delivers mere scholia, and as these are generally rather doctrinal than expository, they refer chiefly to the passages in which the doctrines are stated. Their chief excellence lies in the fine development which the author gives of the importance and nature of the doctrine of free grace without the merit of works. (Fred. Aug. Gottreu Tholuck, Exposition of St. Paul’s Epistle to the Romans, trans. Robert Menzies, Second Revised and Corrected Edinburgh Edition. (Philadelphia: Sorin and Ball, 1844), 24.)


And


  The Epistle to the Romans has been an especial object of attention among protestant commentators. It embraces the peculiar and fundamental principles of Christian doctrine, and stands directly opposed to the Catholic and Pelagian errors which so long prevailed. Hence Melancthon adopted it as the basis of his Loci Communes, and Calvin as that of his ‘Institutes.’ Melancthon’s labors extended from the year 1522 to 1532. His object being dogmatic rather than exegetical, his annotations are chiefly confined to dogmatic passages. Calvin’s remark consequently was just. The latter prized the Epistle to the Romans above all the rest, because it so entirely harmonized with his argument, and with his sublime view of God’s grandeur, of his all-comprehending power, and the nothingness of man, who lives only by the grace of God, and owes his freedom wholly to its support. “Works cannot make you righteous, but the grace of God through Jesus, who is your righteousness and your life.” (Paul Henry and Henry Stebbing, The Life and Times of John Calvin, the Great Reformer, vol. 1 (New York: Robert Carter &amp; Brothers, 1851), 220.)


A further reference confirming that both the content and the form of Melanchthon's book was based most heavily on Romans, which I received via Dr. Joel Beeke, is Timothy J. Wengert, "The Rhetorical Paul: Philip Melanchthon's Interpretation of the Pauline Epistles," in A Companion to Paul in the Reformation, ed. R. Ward Holder (Leiden: Brill, 2009), 136.